Taboada, Fernando G., Jong-Yeon Park, Barbara A Muhling, Desiree Tommasi, Kisei R Tanaka, Ryan R Rykaczewski, Charles A Stock, and Jorge L Sarmiento, March 2023: Anticipating fluctuations of bigeye tuna in the Pacific Ocean from three-dimensional ocean biogeochemistry. Journal of Applied Ecology, 60(3), DOI:10.1111/1365-2664.14346463-479. Abstract
1) Subseasonal to decadal ocean forecasting can make significant contributions to achieving effective management of living marine resources in a changing ocean. Most applications rely on indirect proxies, however, often measured at the ocean surface and lacking a direct mechanistic link to the dynamics of marine populations.
2) Here, we take advantage of three-dimensional, dynamical reconstructions and forecasts of ocean biogeochemistry based on a global Earth system model to hindcast and assess the capacity to anticipate fluctuations in the dynamics of bigeye tuna (Thunnus obesus Lowe) in the Pacific Ocean during the last six decades. We reconstructed spatial patterns in catch per unit effort (CPUE) through the combination of physiological indices capturing both habitat preferences and physiological tolerance limits in bigeye tuna.
3) Our analyses revealed a sequence of four distinct regimes characterized by changes in the zonal distribution and average CPUE of bigeye tuna in the Pacific Ocean. Habitat models accounting for basin-wide fluctuations in the thermal structure and oxygen concentration throughout the water column captured interannual fluctuations in CPUE and regime switches that models based solely on surface information were unable to reproduce. Decade-long forecast experiments further suggested that forecasts of three-dimensional biogeochemical information might enable anticipation of fluctuations in bigeye tuna several years ahead.
4) Synthesis and applications. Together, our results reveal the impact of variability of biogeochemical conditions in the ocean interior on the dynamics of bigeye tuna on the Pacific Ocean, raising concerns about the future impact of ocean warming and deoxygenation. The results also lend support to incorporating subsurface biogeochemical information into ecological forecasts to implement efficient dynamic management strategies and promote the sustainable use of marine living resources.
The deep ocean releases large amounts of old, pre-industrial carbon dioxide (CO2) to the atmosphere through upwelling in the Southern Ocean, which counters the marine carbon uptake occurring elsewhere. This Southern Ocean CO2 release is relevant to the global climate because its changes could alter atmospheric CO2 levels on long time scales, and also affects the present-day potential of the Southern Ocean to take up anthropogenic CO2. Here, year-round profiling float measurements show that this CO2 release arises from a zonal band of upwelling waters between the Subantarctic Front and wintertime sea-ice edge. This band of high CO2 subsurface water coincides with the outcropping of the 27.8 kg m−3 isoneutral density surface that characterizes Indo-Pacific Deep Water (IPDW). It has a potential partial pressure of CO2 exceeding current atmospheric CO2 levels (∆PCO2) by 175 ± 32 μatm. Ship-based measurements reveal that IPDW exhibits a distinct ∆PCO2 maximum in the ocean, which is set by remineralization of organic carbon and originates from the northern Pacific and Indian Ocean basins. Below this IPDW layer, the carbon content increases downwards, whereas ∆PCO2 decreases. Most of this vertical ∆PCO2 decline results from decreasing temperatures and increasing alkalinity due to an increased fraction of calcium carbonate dissolution. These two factors limit the CO2 outgassing from the high-carbon content deep waters on more southerly surface outcrops. Our results imply that the response of Southern Ocean CO2 fluxes to possible future changes in upwelling are sensitive to the subsurface carbon chemistry set by the vertical remineralization and dissolution profiles.
Patterns of population renewal in marine fishes are often irregular and lead to volatile fluctuations in abundance that challenge management and conservation efforts. Here, we examine the relationship between life‐history strategies and recruitment variability in exploited marine fish species using a macroecological approach.
The Southern Ocean south of 30° S represents only one-third of the total ocean area, yet absorbs half of the total ocean anthropogenic carbon and over two-thirds of ocean anthropogenic heat. In the past, the Southern Ocean has also been one of the most sparsely measured regions of the global ocean. Here we use pre-2005 ocean shipboard measurements alongside novel observations from autonomous floats with biogeochemical sensors to calculate changes in Southern Ocean temperature, salinity, pH and concentrations of nitrate, dissolved inorganic carbon and oxygen over two decades. We find local warming of over 3 °C, salinification of over 0.2 psu near the Antarctic coast, and isopycnals are found to deepen between 65° and 40° S. We find deoxygenation along the Antarctic coast, but reduced deoxygenation and nitrate concentrations where isopycnals deepen farther north. The forced response of the Earth system model ESM2M does not reproduce the observed patterns. Accounting for meltwater and poleward-intensifying winds in ESM2M improves reproduction of the observed large-scale changes, demonstrating the importance of recent changes in wind and meltwater. Future Southern Ocean biogeochemical changes are likely to be influenced by the relative strength of meltwater input and poleward-intensifying winds. The combined effect could lead to increased Southern Ocean deoxygenation and nutrient accumulation, starving the global ocean of nutrients sooner than otherwise expected.
Anthropogenically forced changes in ocean biogeochemistry are underway and critical for the ocean carbon sink and marine habitat. Detecting such changes in ocean biogeochemistry will require quantification of the magnitude of the change (anthropogenic signal) and the natural variability inherent to the climate system (noise). Here we use Large Ensemble (LE) experiments from four Earth system models (ESMs) with multiple emissions scenarios to estimate Time of Emergence (ToE) and partition projection uncertainty for anthropogenic signals in five biogeochemically important upper-ocean variables. We find ToEs are robust across ESMs for sea surface temperature and the invasion of anthropogenic carbon; emergence time scales are 20–30 yr. For the biological carbon pump, and sea surface chlorophyll and salinity, emergence time scales are longer (50+ yr), less robust across the ESMs, and more sensitive to the forcing scenario considered. We find internal variability uncertainty, and model differences in the internal variability uncertainty, can be consequential sources of uncertainty for projecting regional changes in ocean biogeochemistry over the coming decades. In combining structural, scenario, and internal variability uncertainty, this study represents the most comprehensive characterization of biogeochemical emergence time scales and uncertainty to date. Our findings delineate critical spatial and duration requirements for marine observing systems to robustly detect anthropogenic change.
Arteaga, Lionel, M Pahlow, Seth M Bushinsky, and Jorge L Sarmiento, August 2019: Nutrient controls on export production in the Southern Ocean. Global Biogeochemical Cycles, 33(8), DOI:10.1029/2019GB006236. Abstract
We use observations from novel biogeochemical profiling floats deployed by the SOCCOM program to estimate annual net community production (ANCP) (associated with carbon export) from the seasonal drawdown of mesopelagic oxygen and surface nitrate in the Southern Ocean. Our estimates agree with previous observations in showing an increase in ANCP in the vicinity of the polar front (~3 mol C m−2 y−1), compared to lower rates in the subtropical zone (≤1 mol C m−2 y−1) and the seasonal ice zone (<2 mol C m−2 y−1). Paradoxically, the increase in ANCP south of the subtropical front is associated with elevated surface nitrate and silicate concentrations, but decreasing surface iron. We hypothesize that iron limitation promotes silicification in diatoms, which is evidenced by the low silicate to nitrate ratio of surface waters around the Antarctic polar front. High diatom silicification increases the ballasting effect of POC and overall ANCP in this region. A model‐based assessment of our methods shows a good agreement between ANCP estimates based on oxygen and nitrate drawdown and the modeled downward organic carbon flux at 100 m. This agreement supports the presumption that net biological consumption is the dominant process affecting the drawdown of these chemical tracers, and that, given sufficient data, ANCP can be inferred from observations of oxygen and/or nitrate drawdown in the Southern Ocean.
Substantial interannual variability in marine fish recruitment (i.e., the number of young fish entering a fishery each year) has been hypothesized to be related to whether the timing of fish spawning matches that of seasonal plankton blooms. Environmental processes that control the phenology of blooms, such as stratification, may differ from those that influence fish spawning, such as temperature‐linked reproductive maturation. These different controlling mechanisms could cause the timing of these events to diverge under climate change with negative consequences for fisheries. We use an earth system model to examine the impact of a high‐emissions, climate‐warming scenario (RCP8.5) on the future spawning time of two classes of temperate, epipelagic fishes: “geographic spawners” whose spawning grounds are defined by fixed geographic features (e.g., rivers, estuaries, reefs) and “environmental spawners” whose spawning grounds move responding to variations in environmental properties, such as temperature. By the century's end, our results indicate that projections of increased stratification cause spring and summer phytoplankton blooms to start 16 days earlier on average (±0.05 days SE) at latitudes >40°N. The temperature‐linked phenology of geographic spawners changes at a rate twice as fast as phytoplankton, causing these fishes to spawn before the bloom starts across >85% of this region. “Extreme events,” defined here as seasonal mismatches >30 days that could lead to fish recruitment failure, increase 10‐fold for geographic spawners in many areas under the RCP8.5 scenario. Mismatches between environmental spawners and phytoplankton were smaller and less widespread, although sizable mismatches still emerged in some regions. This indicates that range shifts undertaken by environmental spawners may increase the resiliency of fishes to climate change impacts associated with phenological mismatches, potentially buffering against declines in larval fish survival, recruitment, and fisheries. Our model results are supported by empirical evidence from ecosystems with multidecadal observations of both fish and phytoplankton phenology.
Bushinsky, Seth M., P Landschützer, C Rödenbeck, Alison R Gray, D F Baker, Matthew R Mazloff, Laure Resplandy, K S Johnson, and Jorge L Sarmiento, November 2019: Reassessing Southern Ocean air‐sea CO2 flux estimates with the addition of biogeochemical float observations. Global Biogeochemical Cycles, 33(11), DOI:10.1029/2019GB006176. Abstract
New estimates of pCO2 from profiling floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project have demonstrated the importance of wintertime outgassing south of the Polar Front, challenging the accepted magnitude of Southern Ocean carbon uptake (Gray et al. 2018). Here, we put 3.5 years of SOCCOM observations into broader context with the global surface carbon dioxide database (Surface Ocean CO2 Atlas, SOCAT) by using the two interpolation methods currently used to assess the ocean models in the Global Carbon Budget (Le Quéré et al. 2018) to create a ship‐only, a float‐weighted, and a combined estimate of Southern Ocean carbon fluxes (< 35°S). In our ship‐only estimate, we calculate a mean uptake of ‐1.14 ± 0.19 Pg C yr‐1 for 2015‐2017, consistent with prior studies. The float‐weighted estimate yields a significantly lower Southern Ocean uptake of ‐0.35 ± 0.19 Pg C yr‐1. Subsampling of high‐resolution ocean biogeochemical process models indicates that some of the difference between float and ship‐only estimates of the Southern Ocean carbon flux can be explained by spatial and temporal sampling differences. The combined ship and float estimate minimizes the root mean square pCO2 difference between the mapped product and both datasets, giving a new Southern Ocean uptake of ‐0.75 ± 0.22 Pg C yr‐1, though with uncertainties that overlap the ship‐only estimate. An atmospheric inversion reveals that a shift of this magnitude in the contemporary Southern Ocean carbon flux must be compensated for by ocean or land sinks within the Southern Hemisphere.
Chen, Haidi, Adele K Morrison, Carolina O Dufour, and Jorge L Sarmiento, March 2019: Deciphering patterns and drivers of heat and carbon storage in the Southern Ocean. Geophysical Research Letters, 46(6), DOI:10.1029/2018GL080961. Abstract
The storage of anomalous heat and carbon in the Southern Ocean in response to increasing greenhouse gases greatly mitigates atmospheric warming and exerts a large impact on the marine ecosystem. However, the mechanisms driving the ocean storage patterns are uncertain. Here using recent hydrographic observations, we compare for the first time the spatial patterns of heat and carbon storage, which show substantial differences in the Southern Ocean, in contrast with the conventional view of simple passive subduction into the thermocline. Using an eddy-rich global climate model, we demonstrate that redistribution of the preindustrial temperature field is the dominant control on the heat storage pattern, whereas carbon storage largely results from passive transport of anthropogenic carbon uptake at the surface. Lastly, this study highlights the importance of realistic representation of wind and surface buoyancy flux in climate models to improve future projection of circulation change and thus heat and carbon storage.
The attribution of anthropogenically forced trends in the climate system requires an understanding of when and how such signals emerge from natural variability. We applied time-of-emergence diagnostics to a large ensemble of an Earth system model, which provides both a conceptual framework for interpreting the detectability of anthropogenic impacts in the ocean carbon cycle and observational sampling strategies required to achieve detection. We found emergence timescales that ranged from less than a decade to more than a century, a consequence of the time lag between the chemical and radiative impacts of rising atmospheric CO2 on the ocean. Processes sensitive to carbonate chemical changes emerge rapidly, such as the impacts of acidification on the calcium carbonate pump (10 years for the globally integrated signal and 9–18 years for regionally integrated signals) and the invasion flux of anthropogenic CO2 into the ocean (14 years globally and 13–26 years regionally). Processes sensitive to the ocean’s physical state, such as the soft-tissue pump, which depends on nutrients supplied through circulation, emerge decades later (23 years globally and 27–85 years regionally).
Stock, Charles A., William W L Cheung, Jorge L Sarmiento, and Elsie M Sunderland, 2019: Changing Ocean Systems: A Short Synthesis In Predicting Future Oceans: Sustainability of Ocean and Human Systems Amidst Global Environmental Change [Cisneros-Montemayor, A. M., W. W. L. Cheung, and Y. Ota (eds.)], Elsevier, DOI:10.1016/B978-0-12-817945-1.00002-219-34. Abstract
Variations in weather and climate create and interact with ocean fluctuations occurring over days to decades. In some cases these fluctuations are local. In others they stretch across ocean basins. Marine organisms respond to environmental changes in diverse and sometimes dramatic ways. Over the past century natural ocean fluctuations have been augmented by a variety of anthropogenic drivers. The ocean has absorbed vast amounts of carbon dioxide, excess heat arising from the accumulation of greenhouse gases, nutrients from fertilizers, and other pollutants. While this has moderated climate change and pollution impacts on terrestrial systems, it has had diverse consequences for the ocean. This chapter provides a brief overview of ocean changes of particular relevance for marine life, including ocean acidification, warming, melting ice, shifting ocean productivity baselines, deoxygenation, coastal development, and pollution. We highlight contributions from the Nereus Program, and attempt to provide a broad context for the more detailed discussion of select topics in other chapters in this section. Anthropogenic ocean changes pose a considerable challenge to sustaining marine resources. Continued advances in understanding and predicting ocean changes, such as those described herein, are essential for meeting this challenge.
Talley, Lynne D., I Rosso, I V Kamenkovich, Matthew R Mazloff, J Wang, E S Boss, Alison R Gray, K S Johnson, Robert M Key, S C Riser, N L Williams, and Jorge L Sarmiento, January 2019: Southern Ocean biogeochemical float deployment strategy, with example from the Greenwich Meridian line (GO‐SHIP A12). Journal of Geophysical Research: Oceans, 124(1), DOI:10.1029/2018JC014059. Abstract
Biogeochemical Argo floats, profiling to 2000 m depth, are being deployed throughout the Southern Ocean by the Southern Ocean Carbon and Climate Observations and Modeling program (SOCCOM). The goal is 200 floats by 2020, to provide the first full set of annual cycles of carbon, oxygen, nitrate and optical properties across multiple oceanographic regimes. Building from no prior coverage to a sparse array, deployments are based on prior knowledge of water mass properties, mean frontal locations, mean circulation and eddy variability, winds, air‐sea heat/freshwater/carbon exchange, prior Argo trajectories, and float simulations in the Southern Ocean State Estimate (SOSE) and Hybrid Coordinate Ocean Model (HYCOM). Twelve floats deployed from the 2014‐2015 Polarstern cruise from South Africa to Antarctica are used as a test case to evaluate the deployment strategy adopted for SOCCOM's 20 deployment cruises and 126 floats to date. After several years, these floats continue to represent the deployment zones targeted in advance: (1) Weddell Gyre sea ice zone, including the Antarctic Slope Front, Maud Rise, and the open gyre; (2) Antarctic Circumpolar Current (ACC) including the topographically‐steered Southern zone ‘chimney' where upwelling carbon/nutrient‐rich deep waters produce surprisingly large carbon dioxide outgassing; (3) Subantarctic and Subtropical zones between the ACC and Africa; and (4) Cape Basin. Argo floats and eddy‐resolving HYCOM simulations were the best predictors of individual SOCCOM float pathways, with uncertainty after 2 years on the order of 1000 km in the sea ice zone and more than double that in and north of the ACC.
Arteaga, Lionel, N Haëntjens, E S Boss, K S Johnson, and Jorge L Sarmiento, April 2018: Assessment of Export Efficiency Equations in the Southern Ocean Applied to Satellite‐Based Net Primary Production. Journal of Geophysical Research: Oceans, 123(4), DOI:10.1002/2018JC013787. Abstract
Carbon export efficiency (e‐ratio) is defined as the fraction of organic carbon fixed through net primary production (NPP) that is exported out of the surface productive layer of the ocean. Recent observations for the Southern Ocean suggest a negative e‐ratio vs. NPP relationship, and a reduced dependency of export efficiency on temperature, different than in the global domain. In this study, we complement information from a passive satellite sensor with novel space‐based lidar observations of ocean particulate backscattering to infer NPP over the entire annual cycle, and estimate Southern Ocean export rates from five different empirical models of export efficiency. Inferred Southern Ocean NPP falls within the range of previous studies, with a mean estimate of 15.8 (± 3.9) Pg C yr−1 for the region south of 30°S during the 2005–2016 period. We find that an export efficiency model that accounts for silica(Si)‐ballasting, which is constrained by observations with a negative e‐ratio vs. NPP relationship, shows the best agreement with in situ‐based estimates of annual net community production (annual export of 2.7 ± 0.6 Pg C yr−1 south of 30°). By contrast, models based on the analysis of global observations with a positive e‐ratio vs. NPP relationship predict annually integrated export rates that are ∼ 33% higher than the Si‐dependent model. Our results suggest that accounting for Si‐induced ballasting is important for the estimation of carbon export in the Southern Ocean.
In this paper we study upwelling pathways and timescales of Circumpolar Deep Water (CDW) in a hierarchy of models using a Lagrangian particle tracking method. Lagrangian timescales of CDW upwelling decrease from 87 years to 31 years to 17 years as the ocean resolution is refined from 1° to 0.25° to 0.1°. We attribute some of the differences in timescale to the strength of the eddy fields, as demonstrated by temporally degrading high resolution model velocity fields. Consistent with the timescale dependence, we find that an average Lagrangian particle completes 3.2 circumpolar loops in the 1° model in comparison to 0.9 loops in the 0.1° model. These differences suggest that advective timescales and thus inter-basin merging of upwelling CDW may be overestimated by coarse resolution models, potentially affecting the skill of centennial scale climate change projections.
Gray, Alison R., K S Johnson, Seth M Bushinsky, S C Riser, Joellen L Russell, Lynne D Talley, R Wanninkhof, N L Williams, and Jorge L Sarmiento, September 2018: Autonomous biogeochemical floats detect significant carbon dioxide outgassing in the high‐latitude Southern Ocean. Geophysical Research Letters, 45(17), DOI:10.1029/2018GL078013. Abstract
Although the Southern Ocean is thought to account for a significant portion of the contemporary oceanic uptake of carbon dioxide (CO2), flux estimates in this region are based on sparse observations that are strongly biased towards summer. Here we present new estimates of Southern Ocean air‐sea CO2 fluxes calculated with measurements from biogeochemical profiling floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project during 2014‐2017. Compared to ship‐based CO2 flux estimates, the float‐based fluxes find significantly stronger outgassing in the zone around Antarctica where carbon‐rich deep waters upwell to the surface ocean. Although interannual variability contributes, this difference principally stems from the lack of autumn and winter ship‐based observations in this high‐latitude region. These results suggest that our current understanding of the distribution of oceanic CO2 sources and sinks may need revision and underscore the need for sustained year‐round biogeochemical observations in the Southern Ocean.
To gain understanding and predict how jellyfish populations will respond to anthropogenic changes, we first need to understand the factors that influence the distribution and abundance of current and historical populations. Hence, we have developed the first bioenergetic-based population model for the ubiquitous jellyfish Aurelia spp. that incorporates both benthic and pelagic life history stages. This model tracks cohorts of both life stages with temperature- and/or consumption-driven relationships for growth, reproduction and mortality. We present herein an initial model application to test hypotheses for the environmental factors that control the onset of strobilation and inter-annual variability in bloom timing and magnitude in Gulf of Mexico jellyfish populations between 1982 and 2007. To recreate the autumnal blooms of Aurelia spp. in the Gulf of Mexico, strobilation must commence while zooplankton biomass is increasing after the annual minimum. Under this scenario, the model simulated seasonal and inter-annual variability of Aurelia spp. biomass that corresponded well with observations. Markedly larger blooms in anomalously warm, high zooplankton autumns resulted from enhanced ephyrae production compounded by enhanced medusa growth under these conditions. This model confirms the importance of the polyp-to-ephyrae transition in regulating jellyfish bloom magnitude and provides a mechanistic model framework which can examine how future jellyfish populations might respond to climate change.
Ji, Qixing, Erik T Buitenhuis, P Suntharalingam, Jorge L Sarmiento, and B B Ward, December 2018: Global nitrous oxide production determined by oxygen sensitivity of nitrification and denitrification. Global Biogeochemical Cycles, 32(12), DOI:10.1029/2018GB005887. Abstract
The ocean is estimated to contribute up to ~20% of global fluxes of atmospheric nitrous oxide (N2O), an important greenhouse gas and ozone depletion agent. Marine oxygen minimum zones (OMZs) contribute disproportionately to this flux. To further understand the partition of nitrification and denitrification and their environmental controls on marine N2O fluxes, we report new relationships between oxygen concentration and rates of N2O production from nitrification and denitrification directly measured with 15N tracers in the Eastern Tropical Pacific. Highest N2O production rates occurred near the oxic‐anoxic interface, where there is strong potential for N2O efflux to the atmosphere. The dominant N2O source in OMZs was nitrate reduction, the rates of which were one to two orders of magnitude higher than those of ammonium oxidation. The presence of oxygen significantly inhibited the production of N2O from both nitrification and denitrification. These experimental data provide new constraints to a multi‐component global ocean biogeochemical model, which yielded annual oceanic N2O efflux of 1.7 – 4.4 Tg‐N (median 2.8 Tg‐N, 1 Tg = 1012 g), with denitrification contributing 20% to the oceanic flux. Thus, denitrification should be viewed as a net N2O production pathway in the marine environment.
Russell, Joellen L., I V Kamenkovich, C M Bitz, R Ferrari, Sarah T Gille, P J Goodman, Robert Hallberg, K S Johnson, K Khazmutdinova, I Marinov, Matthew R Mazloff, S C Riser, and Jorge L Sarmiento, et al., May 2018: Metrics for the Evaluation of the Southern Ocean in Coupled Climate Models and Earth System Models. Journal of Geophysical Research: Oceans, 123(5), DOI:10.1002/2017JC013461. Abstract
The Southern Ocean is central to the global climate and the global carbon cycle, and to the climate's response to increasing levels of atmospheric greenhouse gases, as it ventilates a large fraction of the global ocean volume. Global coupled climate models and earth system models, however, vary widely in their simulations of the Southern Ocean and its role in, and response to, the ongoing anthropogenic trend. Due to the region's complex water-mass structure and dynamics, Southern Ocean carbon and heat uptake depend on a combination of winds, eddies, mixing, buoyancy fluxes, and topography. Observationally-based metrics are critical for discerning processes and mechanisms, and for validating and comparing climate and earth system models. New observations and understanding have allowed for progress in the creation of observationally-based data/model metrics for the Southern Ocean. Metrics presented here provide a means to assess multiple simulations relative to the best available observations and observational products. Climate models that perform better according to these metrics also better simulate the uptake of heat and carbon by the Southern Ocean. This report is not strictly an intercomparison, but rather a distillation of key metrics that can reliably quantify the “accuracy” of a simulation against observed, or at least observable, quantities. One overall goal is to recommend standardization of observationally-based benchmarks that the modeling community should aspire to meet in order to reduce uncertainties in climate projections, and especially uncertainties related to oceanic heat and carbon uptake.
Takeshita, Y, K S Johnson, T R Martz, J N Plant, and Jorge L Sarmiento, June 2018: Assessment of Autonomous pH Measurements for Determining Surface Seawater Partial Pressure of CO2. Journal of Geophysical Research: Oceans, 123(6), DOI:10.1029/2017JC013387. Abstract
The Southern Ocean Carbon and Climate Observations and Modelling (SOCCOM) program currently operates >80 profiling floats equipped with pH sensors in the Southern Ocean. Theoretically, these floats have the potential to provide unique year‐around estimates of pCO2 derived from pH measurements. Here, we evaluate this approach in the field by comparing pCO2 estimates from pH sensors to directly measured pCO2. We first discuss data from a ship's underway system which covered a large range in temperature (2‐30°C) and salinity (33.6‐36.5) over 43 days. This pH sensor utilizes the same sensing technology but with different packaging than those on SOCCOM floats. The mean residual varied between ‐4.6 ± 4.1 and 8.6 ± 4.0 (1σ) μatm, depending on how the sensor was calibrated. However, the standard deviation of the residual, interpreted as the ability to track spatiotemporal variability, was consistently < 5 μatm and was independent of the calibration method. Second, we assessed the temporal stability of this approach by comparing pCO2 estimated from four floats over three years to the Hawaii Ocean Time‐series. Good agreement of ‐2.1 ± 10.4 (1σ) µatm was observed, with coherent seasonal cycles. These results demonstrate that pCO2 estimates derived from profiling float pH measurements appear capable of reproducing spatiotemporal variations in surface pCO2 measurements and should provide a powerful observational tool to complement current efforts to understand the seasonal to interannual variability of surface pCO2 in under‐observed regions of the open ocean.
Ballantyne, A P., William Smith, W Anderegg, P Kauppi, Jorge L Sarmiento, P P Tans, and Elena Shevliakova, et al., February 2017: Accelerating net terrestrial carbon uptake during the warming hiatus due to reduced respiration. Nature Climate Change, 7(2), DOI:10.1038/nclimate3204. Abstract
he recent ‘warming hiatus’ presents an excellent opportunity to investigate climate sensitivity of carbon cycle processes. Here we combine satellite and atmospheric observations to show that the rate of net biome productivity (NBP) has significantly accelerated from −0.007 ± 0.065 PgC yr−2 over the warming period (1982 to 1998) to 0.119 ± 0.071 PgC yr−2 over the warming hiatus (1998–2012). This acceleration in NBP is not due to increased primary productivity, but rather reduced respiration that is correlated (r = 0.58; P = 0.0007) and sensitive (γ = 4.05 to 9.40 PgC yr−1 per °C) to land temperatures. Global land models do not fully capture this apparent reduced respiration over the warming hiatus; however, an empirical model including soil temperature and moisture observations better captures the reduced respiration.
Behrenfeld, M J., Y Hu, R T O'Malley, E S Boss, S W Hostetler, D A Siegel, and Jorge L Sarmiento, February 2017: Annual boom–bust cycles of polar phytoplankton biomass revealed by space-based lidar. Nature Geoscience, 10(2), DOI:10.1038/ngeo2861. Abstract
Polar plankton communities are among the most productive, seasonally dynamic and rapidly changing ecosystems in the global ocean. However, persistent cloud cover, periods of constant night and prevailing low solar elevations in polar regions severely limit traditional passive satellite ocean colour measurements and leave vast areas unobserved for many consecutive months each year. Consequently, our understanding of the annual cycles of polar plankton and their interannual variations is incomplete. Here we use space-borne lidar observations to overcome the limitations of historical passive sensors and report a decade of uninterrupted polar phytoplankton biomass cycles. We find that polar phytoplankton dynamics are categorized by ‘boom–bust’ cycles resulting from slight imbalances in plankton predator–prey equilibria. The observed seasonal-to-interannual variations in biomass are predicted by mathematically modelled rates of change in phytoplankton division. Furthermore, we find that changes in ice cover dominated variability in Antarctic phytoplankton stocks over the past decade, whereas ecological processes were the predominant drivers of change in the Arctic. We conclude that subtle and environmentally driven imbalances in polar food webs underlie annual phytoplankton boom–bust cycles, which vary interannually at each pole.
Bushinsky, Seth M., Alison R Gray, K S Johnson, and Jorge L Sarmiento, November 2017: Oxygen in the Southern Ocean From Argo Floats: Determination of Processes Driving Air-Sea Fluxes. Journal of Geophysical Research: Oceans, 122(11), DOI:10.1002/2017JC012923. Abstract
The Southern Ocean is of outsized significance to the global oxygen and carbon cycles with relatively poor measurement coverage due to harsh winters and seasonal ice cover. In this study, we use recent advances in the parameterization of air-sea oxygen fluxes to analyze 9 years of oxygen data from a recalibrated Argo oxygen data set and from air-calibrated oxygen floats deployed as part of the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project. From this combined data set of 150 floats, we find a total Southern Ocean oxygen sink of −183 ± 80 Tmol yr−1 (positive to the atmosphere), greater than prior estimates. The uptake occurs primarily in the Polar-Frontal Antarctic Zone (PAZ, −94 ± 30 Tmol O2 yr−1) and Seasonal Ice Zone (SIZ, −111 ± 9.3 Tmol O2 yr−1). This flux is driven by wintertime ventilation, with a large portion of the flux in the SIZ passing through regions with fractional sea ice. The Subtropical Zone (STZ) is seasonally driven by thermal fluxes and exhibits a net outgassing of 47 ± 29 Tmol O2 yr−1 that is likely driven by biological production. The Subantarctic Zone (SAZ) uptake is −25 ± 12 Tmol O2 yr−1. Total oxygen fluxes were separated into a thermal and nonthermal component. The nonthermal flux is correlated with net primary production and mixed layer depth in the STZ, SAZ, and PAZ, but not in the SIZ where seasonal sea ice slows the air-sea gas flux response to the entrainment of deep, low-oxygen waters.
Henson, Stephanie A., C Beaulieu, Tatiana Ilyina, Jasmin G John, Matthew C Long, Roland Séférian, Jerry Tjiputra, and Jorge L Sarmiento, March 2017: Rapid emergence of climate change in environmental drivers of marine ecosystems. Nature Communications, 8, 14682, DOI:10.1038/ncomms14682. Abstract
Climate change is expected to modify ecological responses in the ocean, with the potential for important effects on the ecosystem services provided to humankind. Here we address the question of how rapidly multiple drivers of marine ecosystem change develop in the future ocean. By analysing an ensemble of models we find that, within the next 15 years, the climate change-driven trends in multiple ecosystem drivers emerge from the background of natural variability in 55% of the ocean and propagate rapidly to encompass 86% of the ocean by 2050 under a ‘business-as-usual’ scenario. However, we also demonstrate that the exposure of marine ecosystems to climate change-induced stress can be drastically reduced via climate mitigation measures; with mitigation, the proportion of ocean susceptible to multiple drivers within the next 15 years is reduced to 34%. Mitigation slows the pace at which multiple drivers emerge, allowing an additional 20 years for adaptation in marine ecological and socio-economic systems alike.
Johnson, K S., J N Plant, John P Dunne, Lynne D Talley, and Jorge L Sarmiento, August 2017: Annual nitrate drawdown observed by SOCCOM profiling floats and the relationship to annual net community production. Journal of Geophysical Research: Oceans, 122(8), DOI:10.1002/2017JC012839. Abstract
Annual nitrate cycles have been measured throughout the pelagic waters of the Southern Ocean, including regions with seasonal ice cover and southern hemisphere subtropical zones. Vertically resolved nitrate measurements were made using in situ ultraviolet spectrophotometer (ISUS) and submersible ultraviolet nitrate analyzer (SUNA) optical nitrate sensors deployed on profiling floats. Thirty-one floats returned forty complete annual cycles. The mean nitrate profile from the month with the highest winter nitrate minus the mean profile from the month with the lowest nitrate yields the annual nitrate drawdown. This quantity was integrated to 200 m depth and converted to carbon using the Redfield Ratio to estimate Annual Net Community Production (ANCP) throughout the Southern Ocean south of 30° S. A well-defined, zonal mean distribution is found with highest values (3 to 4 mol C m−2 y−1) from 40 to 50° S. Lowest values are found in the subtropics and in the seasonal ice zone. The area weighted mean was 2.9 mol C m−2 y−1 for all regions south of 40° S. Cumulative ANCP south of 50° S is 1.3 Pg C y−1. This represents about 13% of global ANCP in about 14% of the global ocean area.
Mislan, K A., Curtis A Deutsch, R W Brill, John P Dunne, and Jorge L Sarmiento, October 2017: Projections of climate driven changes in tuna vertical habitat based on species-specific differences in blood oxygen affinity. Global Change Biology, 23(10), DOI:10.1111/gcb.13799. Abstract
Oxygen concentrations are hypothesized to decrease in many areas of the ocean as a result of anthropogenically-driven climate change, resulting in habitat compression for pelagic animals. The oxygen partial pressure, pO2, at which blood is 50% saturated (P50) is a measure of blood oxygen affinity and a gauge of the tolerance of animals for low ambient oxygen. Tuna species display a wide range of blood oxygen affinities (i.e., P50 values) and therefore may be differentially impacted by habitat compression as they make extensive vertical movements to forage on sub-daily time scales. To project the effects of end-of-the-century climate change on tuna habitat, we calculate tuna P50 depths (i.e., the vertical position in the water column at which ambient pO2 is equal to species-specific blood P50 values) from 21st century Earth System Model (ESM) projections included in the fifth phase of the Climate Model Intercomparison Project (CMIP5). Overall, we project P50 depths to shoal, indicating likely habitat compression for tuna species due to climate change. Tunas that will be most impacted by shoaling are Pacific and southern bluefin tunas – habitat compression is projected for the entire geographic range of Pacific bluefin tuna and for the spawning region of southern bluefin tuna. Vertical shifts in P50 depths will potentially influence resource partitioning among Pacific bluefin, bigeye, yellowfin, and skipjack tunas in the northern subtropical and eastern tropical Pacific Ocean, the Arabian Sea, and the Bay of Bengal. By establishing linkages between tuna physiology and environmental conditions, we provide a mechanistic basis to project the effects of anthropogenic climate change on tuna habitats.
Photosynthesis fuels marine food webs, yet differences in fish catch across globally distributed marine ecosystems far exceed differences in net primary production (NPP). We consider the hypothesis that ecosystem-level variations in pelagic and benthic energy flows from phytoplankton to fish, trophic transfer efficiencies, and fishing effort can quantitatively reconcile this contrast in an energetically consistent manner. To test this hypothesis, we enlist global fish catch data that include previously neglected contributions from small-scale fisheries, a synthesis of global fishing effort, and plankton food web energy flux estimates from a prototype high-resolution global earth system model (ESM). After removing a small number of lightly fished ecosystems, stark interregional differences in fish catch per unit area can be explained (r = 0.79) with an energy-based model that (i) considers dynamic interregional differences in benthic and pelagic energy pathways connecting phytoplankton and fish, (ii) depresses trophic transfer efficiencies in the tropics and, less critically, (iii) associates elevated trophic transfer efficiencies with benthic-predominant systems. Model catch estimates are generally within a factor of 2 of values spanning two orders of magnitude. Climate change projections show that the same macroecological patterns explaining dramatic regional catch differences in the contemporary ocean amplify catch trends, producing changes that may exceed 50% in some regions by the end of the 21st century under high-emissions scenarios. Models failing to resolve these trophodynamic patterns may significantly underestimate regional fisheries catch trends and hinder adaptation to climate change.
Upwelling of global deep waters to the sea surface in the Southern Ocean closes the global overturning circulation and is fundamentally important for oceanic uptake of carbon and heat, nutrient resupply for sustaining oceanic biological production, and the melt rate of ice shelves. However, the exact pathways and role of topography in Southern Ocean upwelling remain largely unknown. Here we show detailed upwelling pathways in three dimensions, using hydrographic observations and particle tracking in high-resolution models. The analysis reveals that the northern-sourced deep waters enter the Antarctic Circumpolar Current via southward flow along the boundaries of the three ocean basins, before spiraling southeastward and upward through the Antarctic Circumpolar Current. Upwelling is greatly enhanced at five major topographic features, associated with vigorous mesoscale eddy activity. Deep water reaches the upper ocean predominantly south of the Antarctic Circumpolar Current, with a spatially nonuniform distribution. The timescale for half of the deep water to upwell from 30° S to the mixed layer is ~60–90 years.
Toyama, K, Keith B Rodgers, B Blanke, D Iudicone, Masao Ishii, Olivier Aumont, and Jorge L Sarmiento, November 2017: Large Re-emergence of Anthropogenic Carbon Into the Ocean’s Surface Mixed Layer Sustained by the Ocean’s Overturning Circulation. Journal of Climate, 30(21), DOI:10.1175/JCLI-D-16-0725.1. Abstract
We evaluate the output from a widely used ocean carbon cycle model to identify the subduction and obduction (re-emergence) rates of anthropogenic carbon (Cant) for climatological conditions during the World Ocean Circulation Experiment (WOCE) era in 1995 using a new set of Lagrangian diagnostic tools. The principal scientific value of the Lagrangian diagnostics is in providing a new means to connect Cant re-emergence pathways to the relatively rapid renewal timescales of mode waters through the overturning circulation.
Our main finding is that for this model with 2.04 PgC/yr of uptake of Cant via gas exchange, the subduction and obduction rates across the base of the mixed layer (MLbase) are 4.96 PgC/yr and 4.50 PgC/yr, respectively, which are twice as large as the gas exchange at the surface. Given that there is net accumulation of 0.17 PgC/yr in the mixed layer itself, this implies the residual downward Cant transport of 1.40 PgC/yr across the MLbase is associated with diffusion. Importantly, the net patterns for subduction and obduction transports of Cant mirror the large-scale patterns for transport of water volume, thereby illustrating the processes controlling Cant uptake. Although the net transfer across the MLbase by compensating subduction and obduction is relatively smaller than the diffusion, localized pattern of Cant subduction and obduction implies significant regional impacts. The median timescale for re-emergence of obducting particles is short (less than 10 years), indicating that re-emergence should contribute to limiting future carbon uptake through its contribution to perturbing the Revelle factor for surface waters.
Williams, N L., L W Juranek, Richard A Feely, K S Johnson, and Jorge L Sarmiento, et al., March 2017: Calculating surface ocean pCO2 from biogeochemical Argo floats equipped with pH: an uncertainty analysis. Global Biogeochemical Cycles, 31(3), DOI:10.1002/2016GB005541. Abstract
More than 74 biogeochemical profiling floats that measure water column pH, oxygen, nitrate, fluorescence, and backscattering at 10-day intervals have been deployed throughout the Southern Ocean. Calculating the surface ocean partial pressure of carbon dioxide (pCO2sw) from float pH has uncertainty contributions from the pH sensor, the alkalinity estimate, and carbonate system equilibrium constants, resulting in a relative standard uncertainty in pCO2sw of 2.4% (or 10 µatm at pCO2sw of 400 µatm). The calculated pCO2sw from several floats spanning a range of oceanographic regimes are compared to existing climatologies. In some locations, such as the Subantarctic zone, the float data closely match the climatologies, but in the Polar Antarctic Zone significantly higher pCO2sw are calculated in the wintertime implying a greater air-sea CO2 efflux estimate. Our results based on four representative floats suggest that despite their uncertainty relative to direct measurements the float data can be used to improve estimates for air-sea carbon flux, as well as to increase knowledge of spatial, seasonal, and interannual variability in this flux.
Relatively rapid re-emergence of anthropogenic carbon (Cant) in the Equatorial Pacific is of potential importance for its impact on the carbonate buffering capacity of surface seawater, and thereby impeding the ocean's ability to further absorb Cant from the atmosphere. We explore the mechanisms sustaining Cant re-emergence (upwelling) from the thermocline to surface layers by applying water mass transformation diagnostics to a global ocean/sea-ice/biogeochemistry model. We find that the upwelling rate of Cant (0.4 PgC yr-1) from the thermocline to the surface layer is almost twice as large as air-sea Cant fluxes (0.203 PgC yr-1). The upwelling of Cant from the thermocline to the surface layer can be understood as a two-step process: the first being due to diapycnal diffusive transformation fluxes and the second due to surface buoyancy fluxes. We also find that this re-emergence of Cant decreases dramatically during the 1982/1983 and 1997/1998 El Niño events.
Buermann, W, C Beaulieu, B Parida, David Medvigy, G J Collatz, Justin Sheffield, and Jorge L Sarmiento, March 2016: Climate-driven shifts in continental net primary production implicated as a driver of a recent abrupt increase in the land carbon sink. Biogeosciences, DOI:10.5194/bg-13-1597-2016. Abstract
The World's ocean and land ecosystems act as sinks for anthropogenic CO2, and over the last half century their combined sink strength grew steadily with increasing CO2 emissions. Recent analyses of the global carbon budget, however, uncovered an abrupt, substantial (~ 1 PgC yr−1) and sustained increase in the land sink in the late 1980s whose origin remains unclear. In the absence of this prominent shift in the land sink, increases in atmospheric CO2 concentrations since the late 1980s would have been ~ 30 % larger than observed (or ~ 12 ppm above current levels). Global data analyses are limited in regards to attributing causes to changes in the land sink because different regions are likely responding to different drivers. Here, we address this challenge by using terrestrial biosphere models constrained by observations to determine if there is independent evidence for the abrupt strengthening of the land sink. We find that net primary production has significantly increased in the late 1980s (more so than heterotrophic respiration) consistent with the inferred increase in the global land sink, and that large-scale climate anomalies are responsible for this shift. We identify two key regions in which climatic constraints on plant growth have eased: northern Eurasia experienced warming, and northern Africa received increased precipitation. Whether these changes in continental climates are connected is uncertain, but North Atlantic climate variability is important. Our findings suggest that improved understanding of climate variability in the North Atlantic may be essential for more credible projections of the land sink under climate change.
We use a large initial condition suite of simulations (30 runs) with an Earth system model to assess the detectability of biogeochemical impacts of ocean acidification (OA) on the marine alkalinity distribution from decadally repeated hydrographic measurements such as those produced by the Global Ship-Based Hydrographic Investigations Program (GO-SHIP). Detection of these impacts is complicated by alkalinity changes from variability and long-term trends in freshwater and organic matter cycling and ocean circulation. In our ensemble simulation, variability in freshwater cycling generates large changes in alkalinity that obscure the changes of interest and prevent the attribution of observed alkalinity redistribution to OA. These complications from freshwater cycling can be mostly avoided through salinity normalization of alkalinity. With the salinity-normalized alkalinity, modeled OA impacts are broadly detectable in the surface of the subtropical gyres by 2030. Discrepancies between this finding and the finding of an earlier analysis suggest that these estimates are strongly sensitive to the patterns of calcium carbonate export simulated by the model. OA impacts are detectable later in the subpolar and equatorial regions due to slower responses of alkalinity to OA in these regions and greater seasonal equatorial alkalinity variability. OA impacts are detectable later at depth despite lower variability due to smaller rates of change and consistent measurement uncertainty.
Cheung, William W., Thomas L Frölicher, R G Asch, M C Jones, Malin L Pinsky, Gabriel Reygondeau, Keith B Rodgers, Ryan R Rykaczewski, Jorge L Sarmiento, Charles A Stock, and James R Watson, May 2016: Building confidence in projections of the responses of living marine resources to climate change. ICES Journal of Marine Science, 73(5), DOI:10.1093/icesjms/fsv250. Abstract
The Fifth Assessment Report of the Intergovernmental Panel on Climate Change highlights that climate change and ocean acidification are challenging the sustainable management of living marine resources (LMRs). Formal and systematic treatment of uncertainty in existing LMR projections, however, is lacking. We synthesize knowledge of how to address different sources of uncertainty by drawing from climate model intercomparison efforts. We suggest an ensemble of available models and projections, informed by observations, as a starting point to quantify uncertainties. Such an ensemble must be paired with analysis of the dominant uncertainties over different spatial scales, time horizons, and metrics. We use two examples: (i) global and regional projections of Sea Surface Temperature and (ii) projection of changes in potential catch of sablefish (Anoplopoma fimbria) in the 21st century, to illustrate this ensemble model approach to explore different types of uncertainties. Further effort should prioritize understanding dominant, undersampled dimensions of uncertainty, as well as the strategic collection of observations to quantify, and ultimately reduce, uncertainties. Our proposed framework will improve our understanding of future changes in LMR and the resulting risk of impacts to ecosystems and the societies under changing ocean conditions.
Marine species ranging in size from microscopic zooplankton to large predatory fish move vertically in the ocean water column to forage for food and avoid predators. Oxygen and temperature decrease, often rapidly, from shallow to deeper depths, restricting the ability of species to use the vertical habitat. One physiological trait that determines the tolerance of organisms to low oxygen is the oxygen affinity of oxygen carrier proteins, hemoglobin and hemocyanin, in the blood. To quantify the range of oxygen affinities for marine organisms, we surveyed the literature for measurements of oxygen binding to blood at multiple temperatures to account for its temperature sensitivity. Oxygen affinity is mapped within the ocean environment using the depth at which oxygen pressure decreases to the point at which the blood is 50% oxygenated (P50 depth) as organisms move from the surface to depth in the ocean water column. We find that vertical gradients in both temperature and oxygen impact the vertical position and areal extent of P50 depths. Shifts in P50 due to temperature cause physiological types with the same P50 in the surface ocean to have different P50 depths and physiological types with different P50’s in the surface ocean to have the same P50 depth. The vertical distances between P50 depths are spatially variable, which may determine the frequency of ecological interactions, such as competition and predation. In summary, P50 depth, which represents a key physiological transition point between dexoxygenated and oxygenated blood, provides mechanistic insight into organism function within the water column of the global ocean.
The Southern Ocean plays a dominant role in anthropogenic oceanic heat uptake. Strong northward transport of the heat content anomaly limits warming of the sea surface temperature in the uptake region and allows the heat uptake to be sustained. Using an eddy-rich global climate model, the processes controlling the northward transport and convergence of the heat anomaly in the mid-latitude Southern Ocean are investigated in an idealized 1% yr−1 increasing CO2 simulation. Heat budget analyses reveal that different processes dominate to the north and south of the main convergence region. The heat transport northward from the uptake region in the south is driven primarily by passive advection of the heat content anomaly by the existing time mean circulation, with a smaller 20% contribution from enhanced upwelling. The heat anomaly converges in the mid-latitude deep mixed layers, because there is not a corresponding increase in the mean heat transport out of the deep mixed layers northward into the mode waters. To the north of the deep mixed layers, eddy processes drive the warming and account for nearly 80% of the northward heat transport anomaly. The eddy transport mechanism results from a reduction in both the diffusive and advective southward eddy heat transports, driven by decreasing isopycnal slopes and decreasing along-isopycnal temperature gradients on the northern edge of the peak warming.
Westberry, T, Patrick Schultz, John P Dunne, M R Hiscock, S Maritorena, Jorge L Sarmiento, D A Siegel, and M J Behrenfeld, February 2016: Annual cycles of phytoplankton biomass in the Subarctic Atlantic and Pacific Ocean. Global Biogeochemical Cycles, 30(2), DOI:10.1002/2015GB005276. Abstract
High latitude phytoplankton blooms support productive fisheries and play an important role in oceanic uptake of atmospheric carbon dioxide. In the subarctic North Atlantic Ocean, blooms are a recurrent feature each year, while in the eastern subarctic Pacific only small changes in chlorophyll (Chl) are seen over the annual cycle. Here, we show that when evaluated using phytoplankton carbon biomass (Cphyto) rather than Chl, an annual bloom in the North Pacific is evident and can even rival blooms observed in the North Atlantic. The annual increase in subarctic Pacific phytoplankton biomass is not readily observed in the Chl record because it is paralleled by light- and nutrient-driven decreases in cellular pigment levels (Cphyto:Chl). Specifically, photoacclimation and iron stress effects on Cphyto:Chl oppose the biomass increase, leading to only modest changes in bulk Chl. The magnitude of the photoacclimation effect is quantified using descriptors of the near-surface light environment and a photophysiological model. Iron-stress effects are diagnosed from satellite chlorophyll fluorescence data. Last, we show that biomass accumulation in the Pacific is slower than the Atlantic, but is closely tied to similar levels of seasonal nutrient uptake in both basins. Annual cycles of satellite-derived Chl and Cphyto are reproduced by in situ autonomous profiling floats. These results contradict the long-standing paradigm that environmental conditions prevent phytoplankton accumulation in the subarctic Northeast Pacific and suggest a greater seasonal decoupling between phytoplankton growth and losses than traditionally implied. Further, our results highlight the role of physiological processes in shaping bulk properties, such as Chl, and their interpretation in studies of ocean ecosystem dynamics and climate change.
Williams, N L., L W Juranek, K S Johnson, Richard A Feely, S C Riser, Lynne D Talley, Joellen L Russell, Jorge L Sarmiento, and R Wanninkhof, April 2016: Empirical algorithms to estimate water column pH in the Southern Ocean. Geophysical Research Letters, 43(7), DOI:10.1002/2016GL068539. Abstract
Empirical algorithms are developed using high-quality GO-SHIP hydrographic measurements of commonly measured parameters (temperature, salinity, pressure, nitrate, and oxygen) that estimate pH in the Pacific sector of the Southern Ocean. The coefficients of determination, R2, are 0.98 for pH from nitrate (pHN) and 0.97 for pH from oxygen (pHOx) with RMS errors of 0.010 and 0.008, respectively. These algorithms are applied to Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) biogeochemical profiling floats, which include novel sensors (pH, nitrate, oxygen, fluorescence, and backscatter). These algorithms are used to estimate pH on floats with no pH sensors and to validate and adjust pH sensor data from floats with pH sensors. The adjusted float data provide, for the first time, seasonal cycles in surface pH on weekly resolution that range from 0.05 to 0.08 on weekly resolution for the Pacific sector of the Southern Ocean.
Anderegg, W, A P Ballantyne, W Kolby Smith, J D Majkut, S Rabin, C Beaulieu, R A Birdsey, John P Dunne, R A Houghton, R B Myneni, Yude Pan, Jorge L Sarmiento, N Serota, and Elena Shevliakova, et al., December 2015: Tropical nighttime warming as a dominant driver of variability in the terrestrial carbon sink. Proceedings of the National Academy of Sciences, 112(51), DOI:10.1073/pnas.1521479112. Abstract
The terrestrial biosphere is currently a strong carbon (C) sink but may switch to a source in the 21st century as climate-driven losses exceed CO2-driven C gains, thereby accelerating global warming. Although it has long been recognized that tropical climate plays a critical role in regulating interannual climate variability, the causal link between changes in temperature and precipitation and terrestrial processes remains uncertain. Here, we combine atmospheric mass balance, remote sensing-modeled datasets of vegetation C uptake, and climate datasets to characterize the temporal variability of the terrestrial C sink and determine the dominant climate drivers of this variability. We show that the interannual variability of global land C sink has grown by 50–100% over the past 50 y. We further find that interannual land C sink variability is most strongly linked to tropical nighttime warming, likely through respiration. This apparent sensitivity of respiration to nighttime temperatures, which are projected to increase faster than global average temperatures, suggests that C stored in tropical forests may be vulnerable to future warming.
This study examines the role of processes transporting tracers across the Polar Front (PF) in the depth interval between the surface and major topographic sills, which we refer to as the “PF core”. A preindustrial control simulation of an eddying climate model coupled to a biogeochemical model (CM2.6-miniBLING, 0.1° ocean model) is used to investigate the transport of heat, carbon, oxygen and phosphate across the PF core, with a particular focus on the role of mesoscale eddies. We find that the total transport across the PF core results from an ubiquitous Ekman transport that drives the upwelled tracers to the north, and a localized opposing eddy transport that induces tracer leakages to the south at major topographic obstacles. In the Ekman layer, the southward eddy transport only partially compensates the northward Ekman transport, while below the Ekman layer, the southward eddy transport dominates the total transport but remains much smaller in magnitude than the near-surface northward transport. Most of the southward branch of the total transport is achieved below the PF core, mainly through geostrophic currents. We find that the eddy diffusive transport reinforces the southward eddy advective transport for carbon and heat, and opposes it for oxygen and phosphate. Eddy advective transport is likely to be the leading-order component of eddy-induced transport for all four tracers. However, eddy diffusive transport may provide a significant contribution to the southward eddy heat transport due to strong along-isopycnal temperature gradients.
We assess the uptake, transport and storage of oceanic anthropogenic carbon and
heat over the period 1861 to 2005 in a new set of coupled carbon-climate Earth
System models conducted for the fifth Coupled Model Intercomparison Project
(CMIP5), with a particular focus on the Southern Ocean. Simulations show the
Southern Ocean south of 30°S, occupying 30% of global surface ocean area, accounts
for 43 ± 3% (42 ± 5 Pg C) of anthropogenic CO2 and 75 ± 22% (23 ± 9 *1022J) of heat
uptake by the ocean over the historical period. Northward transport out of the Southern
Ocean is vigorous, reducing the storage to 33 ± 6 Pg anthropogenic carbon and 12 ± 7
*1022J heat in the region. The CMIP5 models as a class tend to underestimate the
observational-based global anthropogenic carbon storage, but simulate trends in global
ocean heat storage over the last fifty years within uncertainties of observation-based
estimates. CMIP5 models suggest global and Southern Ocean CO2 uptake have been
largely unaffected by recent climate variability and change. Anthropogenic carbon and
heat storage show a common broad-scale pattern of change, but ocean heat storage is
more structured than ocean carbon storage. Our results highlight the significance of
the Southern Ocean for the global climate and as the region where models differ the
most in representation of anthropogenic CO2 and in particular heat uptake.
Galbraith, Eric D., Eun Young Kwon, Daniele Bianchi, M P Hain, and Jorge L Sarmiento, March 2015: The impact of atmospheric pCO2 on carbon isotope ratios of the atmosphere and ocean. Global Biogeochemical Cycles, 29(3), DOI:10.1002/2014GB004929. Abstract
It is well known that the equilibration timescale for the isotopic ratios 13C/12C and 14C/12C in the ocean mixed layer is on the order of a decade, two orders of magnitude slower than for oxygen. Less widely-appreciated is the factthat the equilibration timescale is quite sensitive to the speciation of Dissolved Inorganic Carbon (DIC) in the mixed layer, scaling linearly with the ratio DIC/CO 2, which varies inversely with atmospheric pCO 2. Although this effect is included in models that resolve the role of carbon speciation in air-sea exchange, its role is often unrecognized, and it is not commonly considered in the interpretation of carbon isotope observations. Here, we use a global 3-dimensional ocean model to estimate the redistribution of the carbon isotopic ratios between the atmosphere and ocean due solely to variations in atmospheric pCO 2. Under Last Glacial Maximum (LGM) pCO 2, atmospheric Δ14 C is increased by ≈ 30 due to the speciation change, all else being equal, raising the surface reservoir age by about 250 years throughout most of the ocean. For 13 C, enhanced surface disequilibrium under LGM pCO2 causes the upper ocean, atmosphere and North Atlantic Deep Water δ13C to become at least 0.2 higher relative to deep waters ventilated by the Southern Ocean. Conversely, under high pCO2, rapid equilibration greatly decreases isotopic disequilibrium. As a result, during geological periods of high pCO2, vertical δ13C gradients may have been greatly weakened as a direct chemical consequence of the high pCO2, masquerading as very well-ventilated or biologically-deadÔStrangeloveÕ oceans. The ongoing anthropogenic rise of pCO2 is accelerating the equilibration of the carbon isotopes in the ocean, lowering atmospheric Δ14C and weakening δ13C gradients within the ocean to a degree that is similar to the traditional fossil fuel ’Suess’ effect.
Galbraith, Eric D., John P Dunne, Anand Gnanadesikan, Richard D Slater, Jorge L Sarmiento, Carolina O Dufour, Gregory F de Souza, Daniele Bianchi, M Claret, Keith B Rodgers, and S Sedigh Marvasti, December 2015: Complex functionality with minimal computation: Promise and pitfalls of reduced-tracer ocean biogeochemistry models. Journal of Advances in Modeling Earth Systems, 7(4), DOI:10.1002/2015MS000463. Abstract
Earth System Models increasingly include ocean biogeochemistry models in order to predict changes in ocean carbon storage, hypoxia and biological productivity under climate change. However, state-of-the-art ocean biogeochemical models include many advected tracers, that significantly increase the computational resources required, forcing a tradeoff with spatial resolution. Here, we compare a state-of-the art model with 30 prognostic tracers (TOPAZ) with two reduced-tracer models, one with 6 tracers (BLING), the other with 3 tracers (miniBLING). The reduced-tracer models employ parameterized, implicit biological functions, that nonetheless capture many of the most important processes resolved by TOPAZ. All three are embedded in the same coupled climate model. Despite the large difference in tracer number, the absence of tracers for living organic matter is shown to have a minimal impact on the transport of nutrient elements, and the three models produce similar mean annual pre-industrial distributions of macronutrients, oxygen and carbon. Significant differences do exist amongst the models, in particular the seasonal cycle of biomass and export production, but it does not appear that these are necessary consequences of the reduced tracer number. With increasing CO2, changes in dissolved oxygen and anthropogenic carbon uptake are very similar across the different models. Thus, while the reduced-tracer models do not explicitly resolve the diversity and internal dynamics of marine ecosystems, we demonstrate that such models are applicable to a broad suite of major biogeochemical concerns, including anthropogenic change. These results are very promising for the further development and application of reduced-tracer biogeochemical models that incorporate ‘sub-ecosystem-scale' parameterizations.
Watson, James R., Charles A Stock, and Jorge L Sarmiento, November 2015: Exploring the role of movement in determining the global distribution of marine biomass using a coupled hydrodynamic – size-based ecosystem model. Progress in Oceanography, 138, Part B, DOI:10.1016/j.pocean.2014.09.001. Abstract
Modeling the dynamics of marine populations at a global scale - from phytoplankton to fish - is necessary if we are to quantify how climate change and other broad-scale anthropogenic actions affect the supply of marine-based food. Here, we estimate the abundance and distribution of fish biomass using a simple size-based food web model coupled to simulations of global ocean physics and biogeochemistry. We focus on the spatial distribution of biomass, identifying highly productive regions - shelf seas, western boundary currents and major upwelling zones. In the absence of fishing, we estimate the total ocean fish biomass to be View the MathML source∼2.84×109tonnes, similar to previous estimates. However, this value is sensitive to the choice of parameters, and further, allowing fish to move had a profound impact on the spatial distribution of fish biomass and the structure of marine communities. In particular, when movement is implemented the viable range of large predators is greatly increased, and stunted biomass spectra characterizing large ocean regions in simulations without movement, are replaced with expanded spectra that include large predators. These results highlight the importance of considering movement in global-scale ecological models.
Zanowski, Hannah, Robert Hallberg, and Jorge L Sarmiento, November 2015: Abyssal Ocean Warming and Salinification after Weddell Polynyas in the GFDL CM2G Coupled Climate Model. Journal of Physical Oceanography, 45(11), DOI:10.1175/JPO-D-15-0109.1. Abstract
The role of Weddell Sea polynyas in establishing deep ocean properties is explored in the NOAA Geophysical Fluid Dynamics Laboratory’s (GFDL) coupled climate model, CM2G. Using statistical composite analysis of over 30 polynya events that occur in a 2,000-year-long preindustrial control run, the temperature, salinity, and water mass changes associated with the composite event are quantified. For the time period following the composite polynya cessation, termed the ‘recovery,’ warming between 0.002°C decade-1 and 0.019°C decade-1 occurs below 4200 m in the Southern Ocean basins. Temperature and salinity changes are strongest in the Southern Ocean and the South Atlantic near the polynya formation region. Comparison of the model results with abyssal temperature observations reveals that the composite polynya recovery signal could account for 10±8% of the recent warming in the abyssal Southern Ocean. For individual Southern Ocean basins, this percentage is as little as 6±11% or as much as 34±13%.
Bernardello, Raffaele, I Marinov, J B Palter, Jorge L Sarmiento, Eric D Galbraith, and Richard D Slater, March 2014: Response of the Ocean Natural Carbon Storage to Projected 21st Century Climate Change. Journal of Climate, 27(5), DOI:10.1175/JCLI-D-13-00343.1. Abstract
The separate impacts of wind stress, buoyancy fluxes, and CO2 solubility on the oceanic storage of natural carbon are assessed in an ensemble of 20th to 21st century simulations, using a coupled atmosphere-ocean-carbon cycle model. Time varying perturbations for surface wind stress, temperature and salinity are calculated from the difference between climate change and preindustrial control simulations, and are imposed on the ocean in separate simulations. The response of the natural carbon storage to each perturbation is assessed with novel prognostic biogeochemical tracers, which can explicitly decompose dissolved inorganic carbon into biological, preformed, equilibrium and disequilibrium components. Strong responses of these components to changes in buoyancy and winds are seen at high latitude, reflecting the critical role of intermediate and deep waters. Overall, circulation-driven changes in carbon storage are mainly due to changes in buoyancy fluxes, with wind-driven changes playing an opposite but smaller role. Results suggest that climate-driven perturbations to the ocean natural carbon cycle will contribute 20 Pg C to the reduction of the ocean accumulated total carbon uptake over the period 1860-2100. This reflects a strong compensation between a buildup of remineralized organic matter associated with reduced deep water formation (+96 Pg C) and a decrease of preformed carbon (-116 Pg C). The latter is due to a warming-induced decrease in CO2 solubility (-52 Pg C), and a circulation-induced decrease in disequilibrium carbon storage (-64 Pg C). Climate change gives rise to a large spatial redistribution of ocean carbon, with increasing concentrations at high latitude and stronger vertical gradients at low latitude.
Bernardello, Raffaele, I Marinov, J B Palter, Eric D Galbraith, and Jorge L Sarmiento, October 2014: Impact of Weddell Sea deep convection on natural and anthropogenic carbon in a climate model. Geophysical Research Letters, 41(20), DOI:10.1002/2014GL061313. Abstract
A climate model is used to investigate the influence of Weddell Sea open ocean deep convection on anthropogenic and natural carbon uptake for the period 1860-2100. In a three-member ensemble climate change simulation, convection ceases on average by year 1981, weakening the net oceanic cumulative uptake of atmospheric CO2 by year 2100 (-4.3 Pg C) relative to an ocean that has continued convection. This net weakening results from a decrease in anthropogenic carbon uptake (-10.1 Pg C), partly offset by an increase in natural carbon storage (+5.8 Pg C). Despite representing only 4% of its area, the Weddell Sea is responsible for 22% of the Southern Ocean decrease in total climate-driven carbon uptake and 52% of the decrease in the anthropogenic component of oceanic uptake. Although this is a model-specific result, it illustrates the potential of deep convection to produce an inter-model spread in future projections of ocean carbon uptake.
We introduce a composite tracer, Alk*, that has a global distribution primarily determined by CaCO3 precipitation and dissolution. Alk* also highlights riverine alkalinity plumes that are due to dissolved calcium carbonate from land. We estimate the Arctic receives approximately twice the riverine alkalinity per unit area as the Atlantic, and 8 times that of the other oceans. Riverine inputs broadly elevate Alk* in the Arctic surface and particularly near river mouths. Strong net carbonate precipitation lowers basin mean Indian and Atlantic Alk*, while upwelling of dissolved CaCO3 rich deep waters elevates Northern Pacific and Southern Ocean Alk*. We use the Alk* distribution to estimate the carbonate saturation variability resulting from CaCO3 cycling and other processes. We show regional variations in surface carbonate saturation are due to temperature changes driving CO2 fluxes and, to a lesser extent, freshwater cycling. Calcium carbonate cycling plays a tertiary role. Monitoring the Alk* distribution would allow us to isolate the impact of acidification on biological calcification and remineralization.
We trace the marine biogeochemical silicon (Si) cycle using the stable isotope composition of Si dissolved in seawater (expressed as image). Open ocean image observations indicate a surprisingly strong influence of the physical circulation on the large-scale marine Si distribution. Here, we present an ocean general circulation model simulation that deconvolves the physical and biogeochemical controls on the image distribution in the deep oceanic interior. By parsing dissolved Si into its preformed and regenerated components, we separate the influence of deep water formation and circulation from the effects of biogeochemical cycling related to opal dissolution at depth. We show that the systematic meridional image gradient observed in the deep Atlantic Ocean is primarily determined by the preformed component of Si, whose distribution in the interior is controlled solely by the circulation. We also demonstrate that the image value of the regenerated component of Si in the global deep ocean is dominantly set by oceanic regions where opal export fluxes to the deep ocean are large, i.e. primarily in the Southern Ocean's opal belt. The global importance of this regionally dynamic Si cycling helps explain the observed strong physical control on the oceanic image distribution, since most of the regenerated Si present within the deep Atlantic and Indo-Pacific Oceans is in fact transported into these basins by deep waters flowing northward from the Southern Ocean. Our results thus provide a mechanistic explanation for the observed image distribution that emphasizes the dominant importance of the Southern Ocean in the marine Si cycle.
Recent studies have suggested that global mean surface temperature would remain approximately constant on multi-century timescales after CO2 emissions are stopped. Here we use Earth system model simulations of such a stoppage to demonstrate that in some models, surface temperature may actually increase on multi-century timescales after an initial century-long decrease. This occurs in spite of a decline in radiative forcing that exceeds the decline in ocean heat uptake—a circumstance that would otherwise be expected to lead to a decline in global temperature. The reason is that the warming effect of decreasing ocean heat uptake together with feedback effects arising in response to the geographic structure of ocean heat uptake overcompensates the cooling effect of decreasing atmospheric CO2 on multi-century timescales. Our study also reveals that equilibrium climate sensitivity estimates based on a widely used method of regressing the Earth’s energy imbalance against surface temperature change are biased. Uncertainty in the magnitude of the feedback effects associated with the magnitude and geographic distribution of ocean heat uptake therefore contributes substantially to the uncertainty in allowable carbon emissions for a given multi-century warming target.
Kwon, Eun Young, G Kim, François W Primeau, W S Moore, H-M Cho, T DeVries, and Jorge L Sarmiento, et al., December 2014: Global Estimate of Submarine Groundwater Discharge Based on an Observationally Constrained Radium Isotope Model. Geophysical Research Letters, 41(23), DOI:10.1002/2014GL061574. Abstract
Along the continental margins, rivers and submarine groundwater supply nutrients, trace elements, and radionuclides to the coastal ocean, supporting coastal ecosystems and, increasingly, causing harmful algal blooms and eutrophication. While the global magnitude of gauged riverine water discharge is well known, the magnitude of submarine groundwater discharge (SGD) is poorly constrained. Using an inverse model combined with a global compilation of 228Ra observations, we show that the SGD integrated over the Atlantic and Indo-Pacific Oceans between 60°S and 70°N is (12 ± 3) x 1013 m3 yr-1, which is 3 to 4 times greater than the freshwater fluxes into the oceans by rivers. Unlike the rivers, where more than half of the total flux is discharged into the Atlantic, about 70% of SGD flows into the Indo-Pacific Oceans. We suggest that SGD is the dominant pathway for dissolved terrestrial materials to the global ocean, and this necessitates revisions for the budgets of chemical elements including carbon.
Efforts to test and improve terrestrial biosphere models (TBMs) using a variety of data sources have become increasingly common. However, geographically extensive forest inventories have been under-exploited in previous model-data fusion efforts. Inventory observations of forest growth, mortality, and biomass integrate processes across a range of time scales, including slow time-scale processes such as species turnover, that are likely to have important effects on ecosystem responses to environmental variation. However, the large number (thousands) of inventory plots precludes detailed measurements at each location, so that uncertainty in climate, soil properties, and other environmental drivers may be large. Errors in driver variables, if ignored, introduce bias into model-data fusion. We estimated errors in climate and soil drivers at U.S. Forest Inventory and Analysis (FIA) plots, and we explored the effects of these errors on model-data fusion with the Geophysical Fluid Dynamics Laboratory LM3V dynamic global vegetation model. When driver errors were ignored or assumed small at FIA plots, responses of biomass production in LM3V to precipitation and soil available water capacity appeared steeper than the corresponding responses estimated from FIA data. These differences became non-significant if driver errors at FIA plots were assumed large. Ignoring driver errors when optimizing LM3V parameter values yielded estimates for fine-root allocation that were larger than biometric estimates, which is consistent with the expected direction of bias. To explore if complications posed by driver errors could be circumvented by relying on intensive study sites where driver errors are small, we performed a power analysis. To accurately quantify the response of biomass production to spatial variation in mean annual precipitation within the eastern U.S. would require at least 40 intensive study sites, which is larger than the number of sites typically available for individual biomes in existing plot networks. Driver errors may be accommodated by several existing model-data fusion approaches, including hierarchical Bayesian methods and ensemble filtering methods; however, these methods are computationally expensive. We propose a new approach, in which the TBM functional response is fit directly to the driver-error-corrected functional response estimated from data, rather than to the raw observations.
Majkut, J D., Jorge L Sarmiento, and Keith B Rodgers, April 2014: A Growing Oceanic Carbon Uptake: Results from an inversion study of surface pCO2 data. Global Biogeochemical Cycles, 28(4), DOI:10.1002/2013GB004585. Abstract
Concerted community efforts have been devoted to producing an authoritative climatology of air-sea CO 2 fluxes [Takahashi et al., 2009], but identifying decadal trends in CO 2 fluxes has proven to be more challenging. The available surface pCO 2 estimates are too sparse to separate long-term trends from decadal and seasonal variability using simple linear models. We introduce Markov Chain Monte Carlo [MCMC] sampling as a novel technique for estimating the historical pCO 2 at the ocean surface. The result is a plausible history of surface pCO 2 based on available measurements and variability inferred from model simulations. Applying the method to a modern database of pCO 2 data, we find that two thirds of the ocean surface is trending toward increasing uptake of CO 2, with a mean (year 2000) uptake of 2.3 ± 0.5 PgC yr − 1 of anthropogenic carbon and an increase in the global annual uptake over the 30-year time period of 0.4 ± 0.1 PgC yr − 1 decade − 1. The results are particularly interesting in the Southern Ocean, where we find increasing uptake of carbon over this time period, in contrast to previous studies. We find evidence for increased ventilation of deep ocean carbon, in response to increased winds, which is more than offset by an associated surface cooling.
Organic particles sinking from the sunlit surface are oases of food for heterotrophic bacteria living
in the deep ocean. Particle-attached bacteria need to solubilize particles, so they produce exoenzymes
that cleave bonds to make molecules small enough to be transported through bacterial cell walls.
Releasing exoenzymes, which have an energetic cost, to the external environment is risky because there
is no guarantee that products of exoenzyme activity, called hydrolysate, will diffuse to the particleattached
bacterium that produced the exoenzymes. Strategies used by particle-attached bacteria to
counteract diffusive losses of exoenzymes and hydrolysate are investigated in a water column model.
We find that production of exoenzymes by particle-attached bacteria is only energetically worthwhile
at high bacterial abundances. Quorum sensing provides the means to determine local abundances,
and thus the model results support lab and field studies which found that particle-attached bacteria
have the ability to use quorum sensing. Additional model results are that particle-attached bacterial
production is sensitive to diffusion of hydrolysate from the particle and is enhanced by as much as
15 times when diffusion of exoenzymes and hydrolysate from particles is reduced by barriers of
biofilms and particle-attached bacteria. Bacterial colonization rates and activities on particles in both
the euphotic and mesopelagic zones impact remineralization length scales. Shoaling or deepening of
the remineralization depth has been shown to exert significant influence on the residence time and
concentration of carbon in the atmosphere and ocean. By linking variability in remineralization depths
to mechanisms governing bacterial colonization of particles and group coordination of exoenzyme
production using a model, we quantitatively connect microscale bacteria-particle interactions to the
carbon cycle and provide new insights for future observations.
Raupach, M R., M Gloor, Jorge L Sarmiento, Josep G Canadell, and Thomas L Frölicher, et al., July 2014: The declining uptake rate of atmospheric CO2 by land and ocean sinks. Biogeosciences, 11(13), DOI:10.5194/bg-11-3453-2014. Abstract
Through 1959–2012, an airborne fraction (AF) of 0.44 of total anthropogenic CO2 emissions remained in the atmosphere, with the rest being taken up by land and ocean CO2 sinks. Understanding of this uptake is critical because it greatly alleviates the emissions reductions required for climate mitigation, and also reduces the risks and damages that adaptation has to embrace. An observable quantity that reflects sink properties more directly than the AF is the CO2 sink rate (kS), the combined land–ocean CO2 sink flux per unit excess atmospheric CO2 above preindustrial levels. Here we show from observations that kS declined over 1959–2012 by a factor of about 1 / 3, implying that CO2 sinks increased more slowly than excess CO2. Using a carbon–climate model, we attribute the decline in kS to four mechanisms: slower-than-exponential CO2 emissions growth (~ 35% of the trend), volcanic eruptions (~ 25%), sink responses to climate change (~ 20%), and nonlinear responses to increasing CO2, mainly oceanic (~ 20%). The first of these mechanisms is associated purely with the trajectory of extrinsic forcing, and the last two with intrinsic, feedback responses of sink processes to changes in climate and atmospheric CO2. Our results suggest that the effects of these intrinsic, nonlinear responses are already detectable in the global carbon cycle. Although continuing future decreases in kS will occur under all plausible CO2 emission scenarios, the rate of decline varies between scenarios in non-intuitive ways because extrinsic and intrinsic mechanisms respond in opposite ways to changes in emissions: extrinsic mechanisms cause kS to decline more strongly with increasing mitigation, while intrinsic mechanisms cause kS to decline more strongly under high-emission, low-mitigation scenarios as the carbon–climate system is perturbed further from a near-linear regime.
Global climate change is expected to affect the ocean's biological productivity. The most comprehensive information available about the global distribution of contemporary ocean primary productivity is derived from satellite data. Large spatial patchiness and interannual to multidecadal variability in chlorophyll a concentration challenges efforts to distinguish a global, secular trend given satellite records which are limited in duration and continuity. The longest ocean color satellite record comes from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), which failed in December 2010. The Moderate Resolution Imaging Spectroradiometer (MODIS) ocean color sensors are beyond their originally planned operational lifetime. Successful retrieval of a quality signal from the current Visible Infrared Imager Radiometer Suite (VIIRS) instrument, or successful launch of the Ocean and Land Colour Instrument (OLCI) expected in 2014 will hopefully extend the ocean color time series and increase the potential for detecting trends in ocean productivity in the future. Alternatively, a potential discontinuity in the time series of ocean chlorophyll a, introduced by a change of instrument without overlap and opportunity for cross-calibration, would make trend detection even more challenging. In this paper, we demonstrate that there are a few regions with statistically significant trends over the ten years of SeaWiFS data, but at a global scale the trend is not large enough to be distinguished from noise. We quantify the degree to which red noise (autocorrelation) especially challenges trend detection in these observational time series. We further demonstrate how discontinuities in the time series at various points would affect our ability to detect trends in ocean chlorophyll a. We highlight the importance of maintaining continuous, climate-quality satellite data records for climate-change detection and attribution studies.
Bianchi, Daniele, Charles A Stock, Eric D Galbraith, and Jorge L Sarmiento, May 2013: Diel vertical migration: ecological controls and impacts on the biological pump in a one-dimensional ocean model. Global Biogeochemical Cycles, 27, DOI:10.1002/gbc.20031. Abstract
Diel vertical migration (DVM) of zooplankton and micronekton is widespread in the ocean and forms a fundamental component of the biological pump, but is generally overlooked in global models of the Earth System. We develop a parameterization of DVM in the ocean and integrate it with a size-structured NPZD model. We assess the model's ability to recreate ecosystem and DVM patterns at three well observed Pacific sites, ALOHA, K2 and EQPAC, and use it to estimate the impact of DVM on marine ecosystems and biogeochemical dynamics. Our model includes: (1) a representation of migration dynamics in response to food availability and light intensity, (2) a representation of the digestive and metabolic processes that decouple zooplankton feeding from excretion, egestion and respiration, and (3) a light-dependent parameterization of visual predation on zooplankton. The model captures the first order patterns in plankton biomass and productivity across the biomes, including the biomass of migrating organisms. We estimate that realistic migratory populations sustain active fluxes to the mesopelagic zone equivalent to between 15 and 40 % the particle export, and contribute up to half of the total respiration within the layers affected by migration. The localized active transport has important consequences for the cycling of oxygen, nutrients and carbon. We highlight the importance of decoupling zooplankton feeding and respiration and excretion with depth for capturing the impact of migration on the redistribution of carbon and nutrients in the upper ocean.
Cheung, William W., Jorge L Sarmiento, John P Dunne, and Thomas L Frölicher, et al., March 2013: Shrinking of fishes exacerbates impacts of global ocean changes on marine ecosystems. Nature Climate Change, 3(3), DOI:10.1038/NCLIMATE1691. Abstract
Changes in temperature, oxygen content and other ocean
biogeochemical properties directly affect the ecophysiology
of marine water-breathing organisms1–3. Previous studies
suggest that the most prominent biological responses are
changes in distribution4–6, phenology7,8 and productivity9.
Both theory and empirical observations also support the
hypothesis that warming and reduced oxygen will reduce
body size of marine fishes10–12. However, the extent to
which such changes would exacerbate the impacts of climate
and ocean changes on global marine ecosystems remains
unexplored. Here,we employ a model to examine the integrated
biological responses of over 600 species of marine fishes
due to changes in distribution, abundance and body size.
The model has an explicit representation of ecophysiology,
dispersal, distribution, and population dynamics3. We show
that assemblage-averaged maximum body weight is expected
to shrink by 14–24% globally from 2000 to 2050 under
a high-emission scenario. About half of this shrinkage is
due to change in distribution and abundance, the remainder
to changes in physiology. The tropical and intermediate
latitudinal areas will be heavily impacted, with an average
reduction of more than 20%. Our results provide a new
dimension to understanding the integrated impacts of climate
change on marine ecosystems.
Cheung, William W., D J Pauly, and Jorge L Sarmiento, September 2013: How to make progress in projecting climate change impacts. ICES Journal of Marine Science, 70(6), DOI:10.1093/icesjms/fst133. Abstract
Scientific modelling has become a crucial tool for assessing climate change impacts on marine resources. Brander et al. criticize the treatment of reliability and uncertainty of such models, with specific reference to Cheung et al. (2013, Nature Climate Change, 3: 254–258) and their projections of a decrease in maximum body size of marine fish under climate change. Here, we use the specific criticisms of Brander et al. (2013, ICES Journal of Marine Science) on Cheung et al. (2013) as examples to discuss ways to make progress in scientific modelling in marine science. We address the technical criticisms by Brander et al., then their more general comments on uncertainty. The growth of fish is controlled and limited by oxygen, as documented in a vast body of peer-reviewed literature that elaborates on a robust theory based on abundant data. The results from Cheung et al. were obtained using published, reproducible and peer-reviewed methods, and the results agree with the empirical data; the key assumptions and uncertainties of the analysis were stated. These findings can serve as a step towards improving our understanding of climate change impacts on marine ecosystems. We suggest that, as in other fields of science, it is important to develop incrementally (or radically) new approaches and analyses that extend, and ultimately improve, our understanding and projections of climate change effects on marine ecosystems.
Frölicher, Thomas L., Fortunat Joos, C C Raible, and Jorge L Sarmiento, April 2013: Atmospheric CO2 response to volcanic eruptions: the role of ENSO, season, and variability. Global Biogeochemical Cycles, 27(1), DOI:10.1002/gbc.20028. Abstract
Tropical explosive volcanism is one of the most important natural factors that significantly impact the climate system and the carbon cycle on annual to multi-decadal time scales. The three largest explosive eruptions in the last 50 years - Agung, El Chichón, and Pinatubo - occurred in spring/summer in conjunction with El Niño events and left distinct negative signals in the observational temperature and CO2 records. However, confounding factors such as seasonal variability and El Niño-Southern Oscillation (ENSO) may obscure the forcing-response relationship. We determine for the first time the extent to which initial conditions, i.e. season and phase of the ENSO, and internal variability influence the coupled climate and carbon cycle response to volcanic forcing and how this affects estimates of the terrestrial and oceanic carbon sinks. Ensemble simulations with the Earth System Model CSM1.4-carbon predict that the atmospheric CO2 response is ~60% larger when a volcanic eruption occurs during El Niño and in winter than during La Niña conditions. Our simulations suggest that the Pinatubo eruption contributed 11 ± 6% to the 25 Pg terrestrial carbon sink inferred over the decade 1990-1999 and -2 ± 1% to the 22 Pg oceanic carbon sink. In contrast to recent claims, trends in the airborne fraction of anthropogenic carbon cannot be detected when accounting for the decadal-scale influence of explosive volcanism and related uncertainties. Our results highlight the importance of considering the role of natural variability in the carbon cycle for interpretation of observations and for data-model intercomparison.
We used an end-to-end ecosystem
model that incorporates physics, biogeochemistry,
and predator−prey dynamics for the Eastern Subarctic
Pacific ecosystem to investigate the factors controlling
propagation of changes in primary production
to higher trophic levels. We found that lower
trophic levels respond to increased primary production
in unexpected ways due to complex predatory
interactions, with small phytoplankton increasing
more than large phytoplankton due to relief from
predation by microzooplankton, which are kept in
check by the more abundant mesozooplankton. We
also found that the propagation of production to
upper trophic levels depends critically on how nonpredatory
mortality is structured in the model, with
much greater propagation occurring with linear mortality
and much less with quadratic mortality, both of
which functional forms are in common use in eco -
system models. We used an ensemble simulation
approach to examine how uncertainties in model
parameters affect these results. When considering
the full range of potential responses to enhanced productivity,
the effect of uncertainties related to the
functional form of non-predatory mortality was often
masked by uncertainties in the food-web parameterization.
The predicted responses of several commercially
important species, however, were significantly
altered by non-predatory mortality assumptions.
Khatiwala, S, T Tanhua, Sara E Mikaloff-Fletcher, M Gerber, Scott C Doney, H D Graven, Nicolas Gruber, Galen McKinley, A Murata, A F Rios, C L Sabine, and Jorge L Sarmiento, April 2013: Global ocean storage of anthropogenic carbon. Biogeosciences Discussions, 10(4), DOI:10.5194/bgd-9-8931-2012. Abstract
The global ocean is a significant sink for anthropogenic carbon (Cant), absorbing roughly a third of human CO2 emitted over the industrial period. Robust estimates of the magnitude and variability of the storage and distribution of Cant in the ocean are therefore important for understanding the human impact on climate. In this synthesis we review observational and model-based estimates of the storage and transport of Cant in the ocean. We pay particular attention to the uncertainties and potential biases inherent in different inference schemes. On a global scale, three data based estimates of the distribution and inventory of Cant are now available. While the inventories are found to agree within their uncertainty, there are considerable differences in the spatial distribution. We also present a review of the progress made in the application of inverse and data-assimilation techniques which combine ocean interior estimates of Cant with numerical ocean circulation models. Such methods are especially useful for estimating the air-sea flux and interior transport of Cant, quantities that are otherwise difficult to observe directly. However, the results are found to be highly dependent on modeled circulation, with the spread due to different ocean models at least as large as that from the different observational methods used to estimate Cant. Our review also highlights the importance of repeat measurements of hydrographic and biogeochemical parameters to estimate the storage of Cant on decadal timescales in the presence of the variability in circulation that is neglected by other approaches. Data-based Cant estimates provide important constraints on ocean forward models, which exhibit both broad similarities and regional errors relative to the observational fields. A compilation of inventories of Cant gives us a "best" estimate of the global ocean inventory of anthropogenic carbon in 2010 of 155 Pg C with an uncertainty of ±20%. This estimate includes a broad range of values suggesting that a combination of approaches is necessary in order to achieve a robust quantification of the ocean sink of anthropogenic CO2.
Kwon, Eun Young, S M Downes, Jorge L Sarmiento, Riccardo Farneti, and Curtis A Deutsch, June 2013: Role of the Seasonal Cycle in the Subduction Rates of Upper–Southern Ocean Waters. Journal of Physical Oceanography, 43(6), DOI:10.1175/JPO-D-12-060.1. Abstract
A kinematic approach is used to diagnose the subduction rates of upper–Southern Ocean waters across seasonally migrating density outcrops at the base of the mixed layer. From an Eulerian viewpoint, the term representing the temporal change in the mixed layer depth (which is labeled as the temporal induction in this study; i.e., Stemp = ∂h/∂t where h is the mixed layer thickness, and t is time) vanishes over several annual cycles. Following seasonally migrating density outcrops, however, the temporal induction is attributed partly to the temporal change in the mixed layer thickness averaged over a density outcrop following its seasonally varying position and partly to the lateral movement of the outcrop position intersecting the sloping mixed layer base. Neither the temporal induction following an outcrop nor its integral over the outcrop area vanishes over several annual cycles. Instead, the seasonal eddy subduction, which arises primarily because of the subannual correlations between the seasonal cycles of the mixed layer depth and the outcrop area, explains the key mechanism by which mode waters are transferred from the mixed layer to the underlying pycnocline. The time-mean exchange rate of waters across the base of the mixed layer is substantially different from the exchange rate of waters across the fixed winter mixed layer base in mode water density classes. Nearly 40% of the newly formed Southern Ocean mode waters appear to be diapycnally transformed within the seasonal pycnocline before either being subducted into the main pycnocline or entrained back to the mixed layer through lighter density classes.
Pinsky, Malin L., Boris Worm, Michael J Fogarty, Jorge L Sarmiento, and Simon A Levin, September 2013: Marine Taxa Track Local Climate Velocities. Science, 341(6151), DOI:10.1126/science.1239352. Abstract
Organisms are expected to adapt or move in response to climate change, but observed distribution shifts span a wide range of directions and rates. Explanations often emphasize biological distinctions among species, but general mechanisms have been elusive. We tested an alternative hypothesis: that differences in climate velocity—the rate and direction that climate shifts across the landscape—can explain observed species shifts. We compiled a database of coastal surveys around North America from 1968 to 2011, sampling 128 million individuals across 360 marine taxa. Climate velocity explained the magnitude and direction of shifts in latitude and depth much more effectively than did species characteristics. Our results demonstrate that marine species shift at different rates and directions because they closely track the complex mosaic of local climate velocities.
Plancherel, Y, Keith B Rodgers, Robert M Key, A R Jacobson, and Jorge L Sarmiento, July 2013: Role of regression model selection and station distribution on the estimation of oceanic anthropogenic carbon change by eMLR. Biogeosciences, 10(7), DOI:10.5194/bg-10-4801-2013. Abstract
Differencing predictions of linear regression models generated from hydrographic data collected at different times (the eMLR method) was proposed as a means of quantifying the dominant patterns of change in oceanic anthropogenic carbon in the context of sparse data sets subject to natural variability. The ability of eMLR to recover the anthropogenic carbon signal in the North Atlantic was tested using a global circulation and biogeochemistry model. Basin-scale applications of eMLR on horizontal layers can estimate the change in anthropogenic carbon inventory with an accuracy typically better than 10%. Regression model selection influences the distribution of the recovered anthropogenic carbon change signal. The systematic use of statistically optimum regression formulae does not produce the best estimates of anthropogenic carbon change if the distribution of the station locations emphasizes hydrographic features differently in time. Additional factors, such as a balanced station distribution and vertical continuity of the regression formulae should be considered to guide model selection. Accurate results are obtained when multiple formulae are used throughout the water column. Different formulae can yield results of similar quality. The fact that good results are obtained in the hydrographically complex North Atlantic suggests that eMLR can produce accurate estimates in other basins.
The influence of changing ocean currents on climate change is evaluated by comparing an earth system model’s response to increased CO2 with and without an ocean circulation response. Inhibiting the ocean circulation response, by specifying a seasonally-varying preindustrial climatology of currents, has a much larger influence on the heat storage pattern than on the carbon storage pattern. The heat storage pattern without circulation changes resembles carbon storage (either with or without circulation changes) more than it resembles the heat storage when currents are allowed to respond. This is shown to be due to the larger magnitude of the redistribution transport – the change in transport due to circulation anomalies acting on control climate gradients – for heat than for carbon. The net ocean heat and carbon uptake are slightly reduced when currents are allowed to respond. Hence, ocean circulation changes potentially act to warm the surface climate. However, the impact of the reduced carbon uptake on radiative forcing is estimated to be small while the redistribution heat transport shifts ocean heat uptake from low to high latitudes increasing its cooling power. Consequently, global surface warming is significantly reduced by circulation changes. Circulation changes also shift the pattern of warming from broad northern hemisphere amplification to a more structured pattern with reduced warming at subpolar latitudes in both hemispheres and enhanced warming near the equator.
Beaulieu, C, Jorge L Sarmiento, Sara E Mikaloff-Fletcher, Jie Chen, and David Medvigy, January 2012: Identification and characterization of abrupt changes in the land uptake of carbon. Global Biogeochemical Cycles, 26, GB1007, DOI:10.1029/2010GB004024. Abstract
A recent study of the net land carbon sink estimated using the Mauna Loa, Hawaii atmospheric CO2 record, fossil fuel estimates, and a suite of ocean models suggests that the mean of the net land carbon uptake remained approximately constant for three decades and increased after 1988/1989. Due to the large variability in the net land uptake, it is not possible to determine the exact timing and nature of the increase robustly by visual inspection. Here, we develop a general methodology to objectively determine the nature and timing of the shift in the net land uptake based on the Schwarz Information Criterion. We confirm that it is likely that an abrupt shift in the mean net land carbon uptake occurred between 1986-1993 (with a probability between 55-61%), with 1988 being the most likely time for the shift. After taking into account the variability in the net land uptake due to the influence of volcanic aerosols and the El Niño Southern Oscillation, we find that it is most likely that there is a remaining step increase of about 1 Pg C/yr at the same time (with a probability of 55-60%). Thus, we conclude that neither the effect of volcanic eruptions nor the El Niño Southern Oscillation are the causes of the sudden increase of the land carbon sink. By also applying our methodology to the atmospheric growth rate of CO2, we demonstrate that it is likely that the atmospheric growth rate of CO2 exhibits a step decrease between two fitted lines in 1988-1989, which is most likely due to the shift in the net land uptake of carbon.
Bianchi, Daniele, John P Dunne, Jorge L Sarmiento, and Eric D Galbraith, May 2012: Data-based estimates of suboxia, denitrification and N2O production in the ocean, and their sensitivities to dissolved O2. Global Biogeochemical Cycles, 26, GB2009, DOI:10.1029/2011GB004209. Abstract
Oxygen minimum zones (OMZs) are major sites of fixed nitrogen removal from the open ocean. However, commonly-used gridded data sets such as the World Ocean Atlas (WOA) tend to overestimate the concentration of O2 compared to measurements in grids where O2 falls in the suboxic range (O2 < 2 - 10 mmol/m3), thereby underestimating the extent of O2 depletion in OMZs. We evaluate the distribution of the OMZs by (1) mapping high-quality oxygen measurements from the WOCE program, and (2) by applying an empirical correction to the gridded WOA based on in situ observations. The resulting suboxic volumes are a factor 3 larger than in the uncorrected gridded WOA. We combine the new oxygen data sets with estimates of global export and simple models of remineralization to estimate global denitrification and N2O production. We obtain a removal of fixed nitrogen of 70 {plus minus} 50 Tg/year in the open ocean and 198 {plus minus} 64 Tg/year in the sediments, and a global N2O production of 6.2 {plus minus} 3.2 Tg/year. Our results (1) reconcile water column denitrification rates based on global oxygen distributions with previous estimates based on nitrogen isotopes, (2) revise existing estimates of sediment denitrification down by one-third through the use of spatially-explicit fluxes, and (3) provide independent evidence supporting the idea of a historically-balanced oceanic nitrogen cycle. These estimates are most sensitive to uncertainties in the global export production, the oxygen threshold for suboxic processes, and the efficiency of particle respiration under suboxic conditions. Ocean deoxygenation, an expected response to anthropogenic climate change, could increase denitrification by 14 Tg/year of nitrogen per 1 mmol/m3 of oxygen reduction if uniformly distributed, while leaving N2O production relatively unchanged.
Duteil, O, Daniele Bianchi, Eric D Galbraith, and Jorge L Sarmiento, et al., May 2012: Preformed and regenerated phosphate in ocean general circulation models: can right total concentrations be wrong?Biogeosciences, 9(5), DOI:10.5194/bg-9-1797-2012. Abstract
Phosphate distributions simulated by seven stateof-
the-art biogeochemical ocean circulation models are evaluated
against observations of global ocean nutrient distributions.
The biogeochemical models exhibit different structural
complexities, ranging from simple nutrient-restoring to
multi-nutrient NPZD type models. We evaluate the simulations
using the observed volume distribution of phosphate.
The errors in these simulated volume class distributions are
significantly larger when preformed phosphate (or regenerated
phosphate) rather than total phosphate is considered.
Our analysis reveals that models can achieve similarly good
fits to observed total phosphate distributions for a very different
partitioning into preformed and regenerated nutrient
components. This has implications for the strength and potential
climate sensitivity of the simulated biological carbon
pump. We suggest complementing the use of total nutrient
distributions for assessing model skill by an evaluation of the
respective preformed and regenerated nutrient components.
Kearney, Kelly A., Charles A Stock, Kerim Y Aydin, and Jorge L Sarmiento, July 2012: Coupling planktonic ecosystem and fisheries food web models for a pelagic ecosystem: Description and validation for the subarctic Pacific. Ecological Modelling, 237-238, DOI:10.1016/j.ecolmodel.2012.04.006. Abstract
We provide a modeling framework that fully couples a one-dimensional physical mixed layer model, a biogeochemical model, and an upper trophic level fisheries model. For validation purposes, the model has been parameterized for the pelagic Eastern Pacific Subarctic Gyre ecosystem. This paper presents a thorough description of the model itself, as well as an ensemble-based parameterization process that allows the model to incorporate the high level of uncertainty associated with many upper trophic level predator-prey processes. Through a series of model architecture experiments, we demonstrate that the use of a consistent functional response for all predator-prey interactions, as well as the use of density-dependent mortality rates for planktonic functional groups, are important factors in reproducing annual and seasonal observations. We present the results of a 50-year climatological simulation, which demonstrates that under contemporary physical forcing, the model is capable of reproducing long-term seasonal dynamics in primary production and biogeochemical cycling, while maintaining steady-state coexistence of upper trophic level functional groups at levels consistent with observations.
Kwon, Eun Young, M P Hain, D M Sigman, Eric D Galbraith, Jorge L Sarmiento, and J R Toggweiler, May 2012: North Atlantic ventilation of "southern-sourced" deep water in the glacial ocean. Paleoceanography, 27, PA2208, DOI:10.1029/2011PA002211. Abstract
One potential mechanism for lowering atmospheric CO2 during glacial times is an increase in the fraction of the global ocean ventilated by the North Atlantic, which produces deep water with a low concentration of unused nutrients and thus drives the ocean's biological pump to a high efficiency. However, the data indicate that during glacial times, a water mass low in 13C/12C and 14C/C occupied the deep Atlantic, apparently at the expense of North Atlantic Deep Water (NADW). This water is commonly referred to as "southern-sourced", because of its apparent entry into the Atlantic basin from the south, prompting the inference that it was ventilated at the Southern Ocean surface. Here, we propose that this deep Atlantic water mass actually included a large fraction of North Atlantic-venitlated water, the chemical characteristics of which were altered by recirculation in the deep Southern and Indo-Pacific Oceans. In an ocean model sensitivity experiment that reduces Antarctic Bottom Water formation and weakens its overturning circulation, we find that a much greater fraction of NADW is transported into the Southern Ocean without contacting the surface and is entrained and mixed into the southern-sourced deep water that spreads into the global abyssal ocean. Thus, North Atlantic ventilation takes over more of the ocean interior, lowering atmospheric CO2, and yet the abyssal Atlantic is filled from the south with old water low in 13C/12C and 14C/C, consistent with glacial data.
Cheung, William W., John P Dunne, Jorge L Sarmiento, and D J Pauly, July 2011: Integrating ecophysiology and plankton dynamics into projected maximum fisheries catch potential under climate change in the Northeast Atlantic. ICES Journal of Marine Science, 68(6), DOI:10.1093/icesjms/fsr012. Abstract
Previous global analyses projected shifts in species distributions and maximum fisheries catch potential across ocean basins by 2050 under the Special Report on Emission Scenarios (SRES) A1B. However, these studies did not account for the effects of changes in ocean biogeochemistry and phytoplankton community structure that affect fish and invertebrate distribution and productivity. This paper uses a dynamic bioclimatic envelope model that incorporates these factors to project distribution and maximum catch potential of 120 species of exploited demersal fish and invertebrates in the Northeast Atlantic. Using projections from the US National Oceanic and Atmospheric Administration's (NOAA) Geophysical Fluid Dynamics Laboratory Earth System Model (ESM2.1) under the SRES A1B, we project an average rate of distribution-centroid shift of 52 km decade−1 northwards and 5.1 m decade−1 deeper from 2005 to 2050. Ocean acidification and reduction in oxygen content reduce growth performance, increase the rate of range shift, and lower the estimated catch potentials (10-year average of 2050 relative to 2005) by 20–30% relative to simulations without considering these factors. Consideration of phytoplankton community structure may further reduce projected catch potentials by ∼10%. These results highlight the sensitivity of marine ecosystems to biogeochemical changes and the need to incorporate likely hypotheses of their biological and ecological effects in assessing climate change impacts.
We estimate water mass transformation rates resulting from surface buoyancy fluxes and interior diapycnal fluxes in the region south of 30°S in the ECCO model based state estimation and three free-running coupled climate models. The meridional transport of deep and intermediate waters across 30°S agrees well between models and observationally based estimates in the Atlantic Ocean, but not in the Indian and Pacific where the model based estimates are much smaller. Associated with this, in the models about half the southward flowing deep water is converted into lighter waters and half to denser bottom waters, whereas the observationally-based estimates convert most of the inflowing deep water to bottom waters. In the models, both Antarctic Intermediate Water (AAIW) and Antarctic Bottom Water (AABW) are formed primarily via an interior diapycnal transformation rather than being transformed at the surface via heat or freshwater fluxes. Given the small vertical diffusivity specified in the models in this region, we conclude that other processes such as cabbeling and thermobaricity must be playing an important role in water mass transformation. Finally, in the models, the largest contribution of the surface buoyancy fluxes in the Southern Ocean is to convert Upper Circumpolar Deep Water (UCDW) and Antarctic Intermediate Water (AAIW) into lighter Sub-Antarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW).
Downes, S M., A S Budnick, Jorge L Sarmiento, and Riccardo Farneti, June 2011: Impacts of wind stress on the Antarctic Circumpolar Current fronts and associated subduction. Geophysical Research Letters, 38, L11605, DOI:10.1029/2011GL047668. Abstract
Recent studies suggest that the overturning circulation in the Antarctic
Circumpolar Current (ACC) region shows a weak sensitivity to overlying wind
stress changes, due to balancing of changes in the eddy-induced and Eulerian
mean transports. Using an eddy-permitting coupled climate model, we
analyze the response of the ACC transport, and associated water mass subduction
rates, in response to an idealized poleward shift and intensification
of the westerlies. As in previous studies, we find a small increase in the net
ACC transport, and a poleward shift in the mean position of the ACC flow.
However, the ACC is restructured, with the Subantarctic Front (SAF) and
Polar Front (PF) branches shifting poleward by between 0.9o and 2.5o of latitude,
resulting in a weaker ACC flow in both the SAF and PF zones. The
wind stress anomaly drives a stronger northward Ekman transport of cool
surface waters, deepening the winter mixed layer and causing a 12.7 Sv increase
in the subduction of Subantarctic Mode Water (SAMW) north of the
SAF zone and a 6.5 Sv increase in the subduction of Antarctic Intermediate
Water (AAIW) within the SAF and PF zones. Our results suggest that
changes in the wind stress restructure the Southern Ocean large-scale circulation,
including the flow of the ACC in its primary jets, and that this affects
the formation rates of SAMW and AAIW in this complex region.
The distribution of radiocarbon (14C) in the ocean and atmosphere has fluctuated on timescales ranging from seasons to millennia. It is thought that these fluctuations partly reflect variability in the climate system, offering a rich potential source of information to help understand mechanisms of past climate change. Here, a long simulation with a new, coupled model is used to explore the mechanisms that redistribute 14C within the Earth system on inter-annual to centennial timescales. The model, CM2Mc, is a lower-resolution version of the Geophysical Fluid Dynamics Laboratory's CM2M model, uses no flux adjustments, and incorporates a simple prognostic ocean biogeochemistry model including 14C. The atmospheric 14C and radiative boundary conditions are held constant, so that the oceanic distribution of 14C is only a function of internal climate variability. The simulation displays previously-described relationships between tropical sea surface 14C and the model-equivalents of the El Niño Southern Oscillation and Indonesian Throughflow. Sea surface 14C variability also arises from fluctuations in the circulations of the subarctic Pacific and Southern Ocean, including North Pacific decadal variability, and episodic ventilation events in the Weddell Sea that are reminiscent of the Weddell Polynya of 1974–1976. Interannual variability in the air-sea balance of 14C is dominated by exchange within the belt of intense Southern Westerly winds, rather than at the convective locations where the surface 14C is most variable. Despite significant interannual variability, the simulated impact on air-sea exchange is an order of magnitude smaller than the recorded atmospheric 14C variability of the past millennium. This result partly reflects the importance of variability in the production rate of 14C in determining atmospheric 14C, but may also reflect an underestimate of natural climate variability, particularly in the Southern Westerly winds.
Kwon, Eun Young, Jorge L Sarmiento, J R Toggweiler, and T DeVries, September 2011: The control of atmospheric pCO2 by ocean ventilation change: The effect of the oceanic storage of biogenic carbon. Global Biogeochemical Cycles, 25, GB3026, DOI:10.1029/2011GB004059. Abstract
A simple analytical framework is developed relating the atmospheric partial pressure of CO2 to the globally-averaged concentrations of respired carbon ( ) and dissolved carbonate ( ) in the ocean. Assuming that the inventory of carbon is conserved in the ocean-atmosphere system (i.e. no seawater-sediment interactions), the resulting formula of = −0.0053Δ + 0.0034Δ suggests that atmospheric pCO2 would decrease by 5.3% and increase by 3.4% when and increase by 10 μmol kg−1, respectively. Using this analytical framework along with a 3-D global ocean biogeochemistry model, we show that the response of atmospheric pCO2 to changes in ocean circulation is rather modest because ∼30% of the change in atmospheric pCO2 caused by the accumulation of respired carbon is countered by a concomitant accumulation of dissolved carbonate in deep waters. Among the suite of circulation models examined here, the largest reduction in atmospheric pCO2 of 44–88 ppm occurs in a model where reduced overturning rates of both southern and northern sourced deep waters result in a four-fold increase in the Southern Ocean deep water ventilation age. On the other hand, when the ventilation rate of the southern-sourced water decreases, but the overturning rate of North Atlantic Deep Water increases, the resulting decrease in atmospheric pCO2 is only 14–34 ppm. The large uncertainty ranges in atmospheric pCO2 arise from uncertainty in how surface productivity responds to circulation change. Although the uncertainty is large, this study suggests that a synchronously reduced rate for the deep water formation in both hemispheres could lead to the large glacial reduction in atmospheric pCO2 of 80–100 ppm.
Palter, J B., S Lozier, Jorge L Sarmiento, and R G Williams, November 2011: The supply of excess phosphate across the Gulf Stream and the maintenance of subtropical nitrogen fixation. Global Biogeochemical Cycles, 25, GB4007, DOI:10.1029/2010GB003955. Abstract
The subtropical North Atlantic is considered a hot-spot for biological nitrogen fixation, with estimated rates between 1 and 20 x 1011 mol nitrogen fixed annually [ Gruber and Sarmiento , 1997; Mahaffey et al, 2005]. However, the region's nutrient reservoir beneath the euphotic zone is so enriched in nitrate relative to phosphate that it is perplexing how fixation might be sustained there. Here, we investigate whether the physical transport of excess phosphate into the subtropical gyre is sufficient to sustain nitrogen fixation in the gyre. Specifically, we assess the Ekman advection and isopycnal mixing of excess phosphate to the subtropical North Atlantic, using detailed hydrographic and nutrient sections occupied across the Gulf Stream combined with satellite wind data. Ekman advection and along-isopycnal mixing provide a source of approximately 2 x 1010 mol yr-1 of excess phosphate in the northwestern subtropics, a physical mechanism that has the potential to support more than 3 x 1011 mol yr-1 of biological nitrogen fixation, after accounting for alternative sinks of excess phosphate. This excess phosphate supply across the gyre's northern boundary and high nitrogen fixation there offers a mechanism that can explain both the maintenance of subtropical North Atlantic nitrogen fixation in a phosphate-poor environment and help account for the weak gradients in the proxies of fixation observed along interior circulation pathways of the gyre.
Sarmiento, Jorge L., Anand Gnanadesikan, I Marinov, and Richard D Slater, April 2011: The role of marine biota in the CO2 balance of the ocean-atmosphere system In The Role of Marine Biota in the Functioning of the Biosphere, Spain, Fundació and Fundación BBVA, 71-105.
The study of climate impacts on Living Marine Resources (LMRs) has increased rapidly in recent years with the availability of climate model simulations contributed to the assessment reports of the Intergovernmental Panel on Climate Change (IPCC). Collaboration between climate and LMR scientists and shared understanding of critical challenges for such applications are essential for developing robust projections of climate impacts on LMRs. This paper assesses present approaches for generating projections of climate impacts on LMRs using IPCC-class climate models, recommends practices that should be followed for these applications, and identifies priority developments that could improve current projections. Understanding of the climate system and its representation within climate models has progressed to a point where many climate model outputs can now be used effectively to make LMR projections. However, uncertainty in climate model projections (particularly biases and inter-model spread at regional to local scales), coarse climate model resolution, and the uncertainty and potential complexity of the mechanisms underlying the response of LMRs to climate limit the robustness and precision of LMR projections. A variety of techniques including the analysis of multi-model ensembles, bias corrections, and statistical and dynamical downscaling can ameliorate some limitations, though the assumptions underlying these approaches and the sensitivity of results to their application must be assessed for each application. Developments in LMR science that could improve current projections of climate impacts on LMRs include improved understanding of the multi-scale mechanisms that link climate and LMRs and better representations of these mechanisms within more holistic LMR models. These developments require a strong baseline of field and laboratory observations including long time-series and measurements over the broad range of spatial and temporal scales over which LMRs and climate interact. Priority developments for IPCC-class climate models include improved model accuracy (particularly at regional and local scales), inter-annual to decadal-scale predictions, and the continued development of earth system models capable of simulating the evolution of both the physical climate system and biosphere. Efforts to address these issues should occur in parallel and be informed by the continued application of existing climate and LMR models.
Bianchi, Daniele, Jorge L Sarmiento, Anand Gnanadesikan, Robert M Key, P Schlosser, and R Newton, August 2010: Low helium flux from the mantle inferred from simulations of oceanic helium isotope data. Earth and Planetary Science Letters, 297(3-4), DOI:10.1016/j.epsl.2010.06.037. Abstract
The high 3He/4He isotopic ratio of oceanic helium relative to the atmosphere has long been recognized as the signature of mantle 3He outgassing from the Earth's interior. The outgassing flux of helium is frequently used to normalize estimates of chemical fluxes of elements from the solid Earth, and provides a strong constraint to models of mantle degassing. Here we use a suite of ocean general circulation models and helium isotope data obtained by the World Ocean Circulation Experiment to constrain the flux of helium from the mantle to the oceans. Our results suggest that the currently accepted flux is overestimated by a factor of 2. We show that a flux of 527 ± 102 mol year− 1 is required for ocean general circulation models that produce distributions of ocean ventilation tracers such as radiocarbon and chlorofluorocarbons that match observations. This new estimate calls for a reevaluation of the degassing fluxes of elements that are currently tied to the helium fluxes, including noble gases and carbon dioxide.
Cheung, William W., and Jorge L Sarmiento, et al., January 2010: Large-scale redistribution of maximum fisheries catch potential in the global ocean under climate change. Global Change Biology, 16(1), DOI:10.1111/j.1365-2486.2009.01995.x. Abstract
Previous projection of climate change impacts on global food supply focuses solely on production from terrestrial biomes, ignoring the large contribution of animal protein from marine capture fisheries. Here, we project changes in global catch potential for 1066 species of exploited marine fish and invertebrates from 2005 to 2055 under climate change scenarios. We show that climate change may lead to large-scale redistribution of global catch potential, with an average of 30–70% increase in high-latitude regions and a drop of up to 40% in the tropics. Moreover, maximum catch potential declines considerably in the southward margins of semienclosed seas while it increases in poleward tips of continental shelf margins. Such changes are most apparent in the Pacific Ocean. Among the 20 most important fishing Exclusive Economic Zone (EEZ) regions in terms of their total landings, EEZ regions with the highest increase in catch potential by 2055 include Norway, Greenland, the United States (Alaska) and Russia (Asia). On the contrary, EEZ regions with the biggest loss in maximum catch potential include Indonesia, the United States (excluding Alaska and Hawaii), Chile and China. Many highly impacted regions, particularly those in the tropics, are socioeconomically vulnerable to these changes. Thus, our results indicate the need to develop adaptation policy that could minimize climate change impacts through fisheries. The study also provides information that may be useful to evaluate fisheries management options under climate change.
Crevoisier, Cyril, C Sweeney, M Gloor, Jorge L Sarmiento, and P P Tans, October 2010: Regional US carbon sinks from three-dimensional atmospheric CO2 sampling. Proceedings of the National Academy of Sciences, 107(43), DOI:10.1073/pnas.0900062107. Abstract
Studies diverge substantially on the actual magnitude of the North American carbon budget. This is due to the lack of appropriate data and also stems from the difficulty to properly model all the details of the flux distribution and transport inside the region of interest. To sidestep these difficulties, we use here a simple budgeting approach to estimate land-atmosphere fluxes across North America by balancing the inflow and outflow of CO2 from the troposphere. We base our study on the unique sampling strategy of atmospheric CO2 vertical profiles over North America from the National Oceanic and Atmospheric Administration/Earth System Research Laboratory aircraft network, from which we infer the three-dimensional CO2 distribution over the continent. We find a moderate sink of 0.5 ± 0.4 PgC y-1 for the period 2004–2006 for the coterminous United States, in good agreement with the forest-inventory-based estimate of the first North American State of the Carbon Cycle Report, and averaged climate conditions. We find that the highest uptake occurs in the Midwest and in the Southeast. This partitioning agrees with independent estimates of crop uptake in the Midwest, which proves to be a significant part of the US atmospheric sink, and of secondary forest regrowth in the Southeast. Provided that vertical profile measurements are continued, our study offers an independent means to link regional carbon uptake to climate drivers.
Gloor, M, Jorge L Sarmiento, and Nicolas Gruber, August 2010: What can be learned about carbon cycle climate feedbacks from the CO2 airborne fraction?Atmospheric Chemistry and Physics, 10(16), DOI:10.5194/acp-10-7739-2010. Abstract
The ratio of CO2 accumulating in the atmosphere to the CO2 flux into the atmosphere due to human activity, the airborne fraction AF, is central to predict changes in earth's surface temperature due to greenhouse gas induced warming. This ratio has remained remarkably constant in the past five decades, but recent studies have reported an apparent increasing trend and interpreted it as an indication for a decrease in the efficiency of the combined sinks by the ocean and terrestrial biosphere. We investigate here whether this interpretation is correct by analyzing the processes that control long-term trends and decadal-scale variations in the AF. To this end, we use simplified linear models for describing the time evolution of an atmospheric CO2 perturbation. We find firstly that the spin-up time of the system for the AF to converge to a constant value is on the order of 200–300 years and differs depending on whether exponentially increasing fossil fuel emissions only or the sum of fossil fuel and land use emissions are used. We find secondly that the primary control on the decadal time-scale variations of the AF is variations in the relative growth rate of the total anthropogenic CO2 emissions. Changes in sink efficiencies tend to leave a smaller imprint. Therefore, before interpreting trends in the AF as an indication of weakening carbon sink efficiency, it is necessary to account for trends and variations in AF stemming from anthropogenic emissions and other extrinsic forcing events, such as volcanic eruptions. Using atmospheric CO2 data and emission estimates for the period 1959 through 2006, and our simple predictive models for the AF, we find that likely omissions in the reported emissions from land use change and extrinsic forcing events are sufficient to explain the observed long-term trend in AF. Therefore, claims for a decreasing long-term trend in the carbon sink efficiency over the last few decades are currently not supported by atmospheric CO2 data and anthropogenic emissions estimates.
Henson, Stephanie A., Jorge L Sarmiento, John P Dunne, Laurent Bopp, Ivan D Lima, Scott C Doney, Jasmin G John, and C Beaulieu, February 2010: Detection of anthropogenic climate change in satellite records of ocean chlorophyll and productivity. Biogeosciences, 7(2), DOI:10.5194/bg-7-621-2010. Abstract
Global climate change is predicted to alter the ocean's biological productivity. But how will we recognise the impacts of climate change on ocean productivity? The most comprehensive information available on its global distribution comes from satellite ocean colour data. Now that over ten years of satellite-derived chlorophyll and productivity data have accumulated, can we begin to detect and attribute climate change-driven trends in productivity? Here we compare recent trends in satellite ocean colour data to longer-term time series from three biogeochemical models (GFDL, IPSL and NCAR). We find that detection of climate change-driven trends in the satellite data is confounded by the relatively short time series and large interannual and decadal variability in productivity. Thus, recent observed changes in chlorophyll, primary production and the size of the oligotrophic gyres cannot be unequivocally attributed to the impact of global climate change. Instead, our analyses suggest that a time series of similar to 40 years length is needed to distinguish a global warming trend from natural variability. In some regions, notably equatorial regions, detection times are predicted to be shorter (similar to 20-30 years). Analysis of modelled chlorophyll and primary production from 2001-2100 suggests that, on average, the climate change-driven trend will not be unambiguously separable from decadal variability until similar to 2055. Because the magnitude of natural variability in chlorophyll and primary production is larger than, or similar to, the global warming trend, a consistent, decades-long data record must be established if the impact of climate change on ocean productivity is to be definitively detected.
Efforts to test and improve terrestrial biosphere models (TBMs) using a variety of data sources have become increasingly common. However, geographically extensive forest inventories have been under-exploited in previous model-data fusion efforts. Inventory observations of forest growth, mortality, and biomass integrate processes across a range of time scales, including slow time-scale processes such as species turnover, that are likely to have important effects on ecosystem responses to environmental variation. However, the large number (thousands) of inventory plots precludes detailed measurements at each location, so that uncertainty in climate, soil properties, and other environmental drivers may be large. Errors in driver variables, if ignored, introduce bias into model-data fusion. We estimated errors in climate and soil drivers at U.S. Forest Inventory and Analysis (FIA) plots, and we explored the effects of these errors on model-data fusion with the Geophysical Fluid Dynamics Laboratory LM3V dynamic global vegetation model. When driver errors were ignored or assumed small at FIA plots, responses of biomass production in LM3V to precipitation and soil available water capacity appeared steeper than the corresponding responses estimated from FIA data. These differences became non-significant if driver errors at FIA plots were assumed large. Ignoring driver errors when optimizing LM3V parameter values yielded estimates for fine-root allocation that were larger than biometric estimates, which is consistent with the expected direction of bias. To explore if complications posed by driver errors could be circumvented by relying on intensive study sites where driver errors are small, we performed a power analysis. To accurately quantify the response of biomass production to spatial variation in mean annual precipitation within the eastern U.S. would require at least 40 intensive study sites, which is larger than the number of sites typically available for individual biomes in existing plot networks. Driver errors may be accommodated by several existing model-data fusion approaches, including hierarchical Bayesian methods and ensemble filtering methods; however, these methods are computationally expensive. We propose a new approach, in which the TBM functional response is fit directly to the driver-error-corrected functional response estimated from data, rather than to the raw observations.
Palter, J B., Jorge L Sarmiento, Anand Gnanadesikan, J Simeon, and Richard D Slater, November 2010: Fueling export production: nutrient return pathways from the deep ocean and their dependence on the Meridional Overturning Circulation. Biogeosciences, 7(11), DOI:10.5194/bg-7-3549-2010. Abstract
In the Southern Ocean, mixing and upwelling in the presence of heat and freshwater surface fluxes transform subpycnocline water to lighter densities as part of the upward branch of the Meridional Overturning Circulation (MOC). One hypothesized impact of this transformation is the restoration of nutrients to the global pycnocline, without which biological productivity at low latitudes would be significantly reduced. Here we use a novel set of modeling experiments to explore the causes and consequences of the Southern Ocean nutrient return pathway. Specifically, we quantify the contribution to global productivity of nutrients that rise from the ocean interior in the Southern Ocean, the northern high latitudes, and by mixing across the low latitude pycnocline. In addition, we evaluate how the strength of the Southern Ocean winds and the parameterizations of subgridscale processes change the dominant nutrient return pathways in the ocean. Our results suggest that nutrients upwelled from the deep ocean in the Antarctic Circumpolar Current and subducted in Subantartic Mode Water support between 33 and 75% of global export production between 30° S and 30° N. The high end of this range results from an ocean model in which the MOC is driven primarily by wind-induced Southern Ocean upwelling, a configuration favored due to its fidelity to tracer data, while the low end results from an MOC driven by high diapycnal diffusivity in the pycnocline. In all models, nutrients exported in the SAMW layer are utilized and converted rapidly (in less than 40 years) to remineralized nutrients, explaining previous modeling results that showed little influence of the drawdown of SAMW surface nutrients on atmospheric carbon concentrations.
Sarmiento, Jorge L., M Gloor, Nicolas Gruber, C Beaulieu, A R Jacobson, Sara E Mikaloff-Fletcher, Stephen W Pacala, and Keith B Rodgers, August 2010: Trends and regional distributions of land and ocean carbon sinks. Biogeosciences, 7(8), DOI:10.5194/bg-7-2351-2010. Abstract
We show here an updated estimate of the net land carbon sink (NLS) as a function of time from 1960 to 2007 calculated from the difference between fossil fuel emissions, the observed atmospheric growth rate, and the ocean uptake obtained by recent ocean model simulations forced with reanalysis wind stress and heat and water fluxes. Except for interannual variability, the net land carbon sink appears to have been relatively constant at a mean value of −0.27 Pg C yr−1 between 1960 and 1988, at which time it increased abruptly by −0.88 (−0.77 to −1.04) Pg C yr−1 to a new relatively constant mean of −1.15 Pg C yr−1 between 1989 and 2003/7 (the sign convention is negative out of the atmosphere). This result is detectable at the 99% level using a t-test. The land use source (LU) is relatively constant over this entire time interval. While the LU estimate is highly uncertain, this does imply that most of the change in the net land carbon sink must be due to an abrupt increase in the land sink, LS = NLS – LU, in response to some as yet unknown combination of biogeochemical and climate forcing. A regional synthesis and assessment of the land carbon sources and sinks over the post 1988/1989 period reveals broad agreement that the Northern Hemisphere land is a major sink of atmospheric CO2, but there remain major discrepancies with regard to the sign and magnitude of the net flux to and from tropical land.
While nutrient depletion scenarios have long shown that the high-latitude High Nutrient Low Chlorophyll (HNLC) regions are the most effective for sequestering atmospheric carbon dioxide, recent simulations with prognostic biogeochemical models have suggested that only a fraction of the potential drawdown can be realized. We use a global ocean biogeochemical general circulation model developed at GFDL and Princeton to examine this and related issues. We fertilize two patches in the North and Equatorial Pacific, and two additional patches in the Southern Ocean HNLC region north of the biogeochemical divide and in the Ross Sea south of the biogeochemical divide. We evaluate the simulations using observations from both artificial and natural iron fertilization experiments at nearby locations. We obtain by far the greatest response to iron fertilization at the Ross Sea site, where sea ice prevents escape of sequestered CO2 during the wintertime, and the CO2 removed from the surface ocean by the biological pump is carried into the deep ocean by the circulation. As a consequence, CO2 remains sequestered on century time-scales and the efficiency of fertilization remains almost constant no matter how frequently iron is applied as long as it is confined to the growing season. The second most efficient site is in the Southern Ocean. The North Pacific site has lower initial nutrients and thus a lower efficiency. Fertilization of the Equatorial Pacific leads to an expansion of the suboxic zone and a striking increase in denitrification that causes a sharp reduction in overall surface biological export production and CO2 uptake. The impacts on the oxygen distribution and surface biological export are less prominent at other sites, but nevertheless still a source of concern. The century time scale retention of iron in this model greatly increases the long-term biological response to iron addition as compared with simulations in which the added iron is rapidly scavenged from the ocean.
We present a new methodology for database-driven ecosystem model generation and apply the methodology to the world's 66 currently defined Large Marine Ecosystems. The method relies on a large number of spatial and temporal databases, including FishBase, SeaLifeBase, as well as several other databases developed notably as part of the Sea Around Us project. The models are formulated using the freely available Ecopath with Ecosim (EwE) modeling approach and software. We tune the models by fitting to available time series data, but recognize that the models represent only a first-generation of database-driven ecosystem models. We use the models to obtain a first estimate of fish biomass in the world's LMEs. The biggest hurdles at present to further model development and validation are insufficient time series trend information, and data on spatial fishing effort.
Gruber, Nicolas, and Jorge L Sarmiento, et al., February 2009: Oceanic sources, sinks, and transport of atmospheric CO2. Global Biogeochemical Cycles, 23, GB1005, DOI:10.1029/2008GB003349. Abstract
We synthesize estimates of the
contemporary net air-sea CO2 flux on the basis of an inversion of
interior ocean carbon observations using a suite of 10 ocean general
circulation models (Mikaloff Fletcher et al., 2006, 2007) and compare them
to estimates based on a new climatology of the air-sea difference of the
partial pressure of CO2 (pCO2) (Takahashi et
al., 2008). These two independent flux estimates reveal a consistent
description of the regional distribution of annual mean sources and sinks of
atmospheric CO2 for the decade of the 1990s and the early 2000s
with differences at the regional level of generally less than 0.1 Pg C a−1.
This distribution is characterized by outgassing in the tropics, uptake in
midlatitudes, and comparatively small fluxes in thehigh latitudes. Both
estimates point toward a small (∼ −0.3 Pg C a−1) contemporary CO2
sink in the Southern Ocean (south of 44°S), a result of the near
cancellation between a substantial outgassing of natural CO2 and
a strong uptake of anthropogenic CO2. A notable exception in the
generally good agreement between the two estimates exists within the
Southern Ocean: the ocean inversion suggests a relatively uniform uptake,
while the pCO2-based estimate suggests strong uptake in
the region between 58°S and 44°S, and a source in the region south of 58°S.
Globally and for a nominal period between 1995 and 2000, the contemporary
net air-sea flux of CO2 is estimated to be −1.7 ± 0.4 Pg C a−1
(inversion) and −1.4 ± 0.7 Pg C a−1 (pCO2-climatology),
respectively, consisting of an outgassing flux of river-derived carbon of
∼+0.5 Pg C a−1, and an uptake flux of anthropogenic carbon of
−2.2 ± 0.3 Pg C a−1 (inversion) and −1.9 ± 0.7 Pg C a−1
(pCO2-climatology). The two flux estimates also imply a
consistent description of the contemporary meridional transport of carbon
with southward ocean transport throughout most of the Atlantic basin, and
strong equatorward convergence in the Indo-Pacific basins. Both transport
estimates suggest a small hemispheric asymmetry with a southward transport
of between −0.2 and −0.3 Pg C a−1 across the equator. While the
convergence of these two independent estimates is encouraging and suggests
that it is now possible to provide relatively tight constraints for the net
air-sea CO2 fluxes at the regional basis, both studies are
limited by their lack of consideration of long-term changes in the ocean
carbon cycle, such as the recent possible stalling in the expected growth of
the Southern Ocean carbon sink.
The interannual to decadal variability in the timing and magnitude of the North Atlantic phytoplankton bloom is examined using a combination of satellite data and output from an ocean biogeochemistry general circulation model. The timing of the bloom as estimated from satellite chlorophyll data is used as a novel metric for validating the model's skill. Maps of bloom timing reveal that the subtropical bloom begins in winter and progresses northward starting in May in subpolar regions. A transition zone, which experiences substantial interannual variability in bloom timing, separates the two regions. Time series of the modeled decadal (1959–2004) variability in bloom timing show no long‐term trend toward earlier or delayed blooms in any of the three regions considered here. However, the timing of the subpolar bloom does show distinct decadal‐scale periodicity, which is found to be correlated with the North Atlantic Oscillation (NAO) index. The mechanism underpinning the relationship is identified as anomalous wind‐driven mixing conditions associated with the NAO. In positive NAO phases, stronger westerly winds result in deeper mixed layers, delaying the start of the subpolar spring bloom by 2–3 weeks. The subpolar region also expands during positive phases, pushing the transition zone further south in the central North Atlantic. The magnitude of the bloom is found to be only weakly dependent on bloom timing, but is more strongly correlated with mixed layer depth. The extensive interannual variability in the timing of the bloom, particularly in the transition region, is expected to strongly impact the availability of food to higher trophic levels.
Kwon, Eun Young, François W Primeau, and Jorge L Sarmiento, September 2009: The impact of remineralization depth on the air–sea carbon balance. Nature Geoscience, 2(9), DOI:10.1038/ngeo612. Abstract
As particulate organic carbon rains down from the surface ocean it is respired back to carbon dioxide and released into the ocean's interior. The depth at which this sinking carbon is converted back to carbon dioxide—known as the remineralization depth—depends on the balance between particle sinking speeds and their rate of decay. A host of climate-sensitive factors can affect this balance, including temperature1, oxygen concentration2, stratification, community composition3, 4 and the mineral content of the sinking particles5. Here we use a three-dimensional global ocean biogeochemistry model to show that a modest change in remineralization depth can have a substantial impact on atmospheric carbon dioxide concentrations. For example, when the depth at which 63% of sinking carbon is respired increases by 24 m globally, atmospheric carbon dioxide concentrations fall by 10–27 ppm. This reduction in atmospheric carbon dioxide concentration results from the redistribution of remineralized carbon from intermediate waters to bottom waters. As a consequence of the reduced concentration of respired carbon in upper ocean waters, atmospheric carbon dioxide is preferentially stored in newly formed North Atlantic Deep Water. We suggest that atmospheric carbon dioxide concentrations are highly sensitive to the potential changes in remineralization depth that may be caused by climate change.
Rodgers, Keith B., Robert M Key, Anand Gnanadesikan, Jorge L Sarmiento, John P Dunne, and A R Jacobson, et al., September 2009: Using altimetry to help explain patchy changes in hydrographic carbon measurements. Journal of Geophysical Research, C09013, DOI:10.1029/2008JC005183. Abstract
Here we use observations and ocean models to identify mechanisms driving large seasonal to interannual variations in dissolved inorganic carbon (DIC) and dissolved oxygen (O2) in the upper ocean. We begin with observations linking variations in upper ocean DIC and O2 inventories with changes in the physical state of the ocean. Models are subsequently used to address the extent to which the relationships derived from short-timescale (6 months to 2 years) repeat measurements are representative of variations over larger spatial and temporal scales. The main new result is that convergence and divergence (column stretching) attributed to baroclinic Rossby waves can make a first-order contribution to DIC and O2 variability in the upper ocean. This results in a close correspondence between natural variations in DIC and O2 column inventory variations and sea surface height (SSH) variations over much of the ocean. Oceanic Rossby wave activity is an intrinsic part of the natural variability in the climate system and is elevated even in the absence of significant interannual variability in climate mode indices. The close correspondence between SSH and both DIC and O2 column inventories for many regions suggests that SSH changes (inferred from satellite altimetry) may prove useful in reducing uncertainty in separating natural and anthropogenic DIC signals (using measurements from Climate Variability and Predictability's CO2/Repeat Hydrography program).
Correction: 10.1029/2009JC005835
This paper examines the sensitivity of atmospheric pCO2 to changes in ocean biology that result in drawdown of nutrients at the ocean surface. We show that the global inventory of preformed nutrients is the key determinant of atmospheric pCO2 and the oceanic carbon storage due to the soft-tissue pump (OCSsoft ). We develop a new theory showing that under conditions of perfect equilibrium between atmosphere and ocean, atmospheric pCO2 can be written as a sum of exponential functions of OCS soft . The theory also demonstrates how the sensitivity of atmospheric pCO2to changes in the soft-tissue pump depends on the preformed nutrient inventory and on surface buffer chemistry. We validate our theory against simulations of nutrient depletion in a suite of realistic general circulation models (GCMs). The decrease in atmospheric pCO2 following surface nutrient depletion depends on the oceanic circulation in the models. Increasing deep ocean ventilation by increasing vertical mixing or Southern Ocean winds increases the atmospheric pCO2 sensitivity to surface nutrient forcing. Conversely, stratifying the Southern Ocean decreases the atmospheric CO2 sensitivity to surface nutrient depletion. Surface CO2 disequilibrium due to the slow gas exchange with the atmosphere acts to make atmospheric pCO2 more sensitive to nutrient depletion in high-ventilation models and less sensitive to nutrient depletion in low-ventilation models. Our findings have potentially important implications for both past and future climates.
Marinov, I, Anand Gnanadesikan, Jorge L Sarmiento, J R Toggweiler, M J Follows, and B K Mignone, July 2008: Impact of oceanic circulation on biological carbon storage in the ocean and atmospheric pCO2. Global Biogeochemical Cycles, 22, GB3007, DOI:10.1029/2007GB002958. Abstract
We use both theory and ocean
biogeochemistry models to examine the role of the soft-tissue biological
pump in controlling atmospheric CO2. We demonstrate that
atmospheric CO2 can be simply related to the amount of inorganic
carbon stored in the ocean by the soft-tissue pump, which we term (OCSsoft). OCSsoftis linearly
related to the inventory of remineralized nutrient, which in turn is just
the total nutrient inventory minus the preformed nutrient inventory. In a
system where total nutrient is conserved, atmospheric CO2 can
thus be simply related to the global inventory of preformed nutrient.
Previous model simulations have explored how changes in the surface
concentration of nutrients in deepwater formation regions change the global
preformed nutrient inventory. We show that changes in physical forcing such
as winds, vertical mixing, and lateral mixing can shift the balance of
deepwater formation between the North Atlantic (where preformed nutrients
are low) and the Southern Ocean (where they are high). Such changes in
physical forcing can thus drive large changes in atmospheric CO2,
even with minimal changes in surface nutrient concentration. If Southern
Ocean deepwater formation strengthens, the preformed nutrient inventory and
thus atmospheric CO2 increase. An important consequence of these
new insights is that the relationship between surface nutrient
concentrations, biological export production, and atmospheric CO2
is more complex than previously predicted. Contrary to conventional wisdom,
we show that OCSsoftcan increase and atmospheric
CO2 decrease, while surface nutrients show minimal change and
export production decreases.
The silicic acid leakage hypothesis (SALH) attempts to explain part of the large and regular atmospheric CO2 changes over the last glacial-interglacial cycles. It calls for a reduction in the carbonate pump through a growth in diatoms at the expense of coccolithophorids in low-latitude surface waters, driven by a “leakage” of high-Si:N waters from the Southern Ocean. Recent studies that present low opal accumulation rates from the glacial eastern equatorial Pacific have challenged SALH. In a corollary to SALH, we argue that the key to SALH is the dominance of diatoms over coccolithophorids, and this does not depend on the magnitude of diatom production per se. In support of our claim, we show in a numerical model that atmospheric CO2 can be lowered with even a reduced absolute flux of silicic acid leakage, provided that Si:N in the leakage is elevated and that the excess Si can be used by diatoms to shift the floral composition in their favor.
Mignone, B K., Robert H Socolow, Jorge L Sarmiento, and M Oppenheimer, 2008: Atmospheric stabilization and the timing of carbon mitigation. Climatic Change, 88(3-4), DOI:10.1007/s10584-007-9391-8. Abstract
Stabilization of atmospheric CO2 concentrations below a pre-industrial doubling (~550 ppm) is a commonly cited target in climate policy assessment. When the rate at
which future emissions can fall is assumed to be fixed, the peak atmospheric concentration – or the stabilization “frontier” – is an increasing and convex function of the length of postponement. Here we find that a decline in emissions of 1% year−1 beginning today would place the frontier near 475 ppm and that when mitigation is postponed, options disappear (on average) at the rate of ~9 ppm year−1, meaning that delays of more than a decade will likely preclude stabilization below a doubling. When constraints on the future decline rate of emissions are relaxed, a particular atmospheric target can be realized in many ways, with scenarios that allow longer postponement of emissions reductions requiring greater increases in the intensity of future mitigation. However, the marginal rate of substitution between future mitigation and present delay becomes prohibitively large when the balance is shifted too far toward the future, meaning that some amount of postponement cannot be fully offset by simply increasing the intensity of future mitigation. Consequently, these
results suggest that a practical transition path to a given stabilization target in the most commonly cited range can allow, at most, one or two decades of delay.
Submarine groundwater discharge is defined as any flow of water at continental margins from the seabed to the coastal ocean, regardless of fluid composition or driving force1. The flux of submarine groundwater discharge has been hypothesized to be a pathway for enriching coastal waters in nutrients, carbon and metals. Here, we estimate the submarine groundwater flux from the inventory of 228Ra in the upper Atlantic Ocean, obtained by interpolating measurements at over 150 stations. Only 46% of the loss in 228Ra from radioactive decay is replenished by input from dust, rivers and coastal sediments. We infer that the remainder must come from submarine groundwater discharge. Using estimates of 228Ra concentrations in submarine groundwater discharge, we arrive at a total flux from submarine groundwater discharge of 2–4x1013 m3 yr-1, between 80 and 160% of the amount of freshwater entering the Atlantic Ocean from rivers. Submarine groundwater discharge is not a freshwater flux, but a flux of terrestrial and sea water that has penetrated permeable coastal sediments. Our assessment of the volume of submarine groundwater discharge confirms that this flux represents an important vehicle for the delivery of nutrients, carbon and metal to the ocean.
Rodgers, Keith B., Jorge L Sarmiento, Olivier Aumont, Cyril Crevoisier, C de Boyer Montégut, and N Metzl, June 2008: A wintertime uptake window for anthropogenic CO2 in the North Pacific. Global Biogeochemical Cycles, 22, GB2020, DOI:10.1029/2006GB002920. Abstract
An ocean model has been forced with NCEP reanalysis fluxes over 1948–2003 to evaluate the pathways and timescales associated with the uptake of anthropogenic CO2 over the North Pacific. The model reveals that there are two principal regions of uptake, the first in the region bounded by 35–45°N and 140–180°E, and the second along a band between 10–20°N and between 120°W and 180°E. For both of these regions, the dominant timescale of variability in uptake is seasonal, with maximum uptake occurring during winter and uptake being close to zero or slightly negative during summer when integrated over the basin. A decadal trend toward increased uptake of anthropogenic CO2 consists largely of modulations of the uptake maximum in winter. For detection of anthropogenic changes, this implies that in situ measurements will need to resolve the seasonal cycle in order to capture decadal trends in ΔpCO2. As uptake of anthropogenic CO2 occurs preferentially during winter, observationally based estimates which do not resolve the full seasonal cycle may result in underestimates of the rate of uptake of anthropogenic CO2. There is also a sizable circulation-driven decadal trend in the seasonal cycle of sea surface ΔpCO2 for the North Pacific, with maximum changes found near the boundary separating the subtropical and subpolar gyres in western and central regions of the basin. These changes are due to a trend in the large-scale circulation of the gyres, which itself is driven by a trend in the wind stress over the basin scale. This trend in the three-dimensional circulation is more important than the local trend in mixed layer depth (MLD) in contributing to the decadal trend in ΔpCO2.
Nitrogen fixation is crucial for maintaining biological productivity in the oceans, because it replaces the biologically available nitrogen that is lost through denitrification. But, owing to its temporal and spatial variability, the global distribution of marine nitrogen fixation is difficult to determine from direct shipboard measurements. This uncertainty limits our understanding of the factors that influence nitrogen fixation, which may include iron, nitrogen-to-phosphorus ratios, and physical conditions such as temperature. Here we determine nitrogen fixation rates in the world's oceans through their impact on nitrate and phosphate concentrations in surface waters, using an ocean circulation model. Our results indicate that nitrogen fixation rates are highest in the Pacific Ocean, where water column denitrification rates are high but the rate of atmospheric iron deposition is low. We conclude that oceanic nitrogen fixation is closely tied to the generation of nitrogen-deficient waters in denitrification zones, supporting the view that nitrogen fixation stabilizes the oceanic inventory of fixed nitrogen over time.
Dunne, John P., Jorge L Sarmiento, and Anand Gnanadesikan, December 2007: A synthesis of global particle export from the surface ocean and cycling through the ocean interior and on the seafloor. Global Biogeochemical Cycles, 21, GB4006, DOI:10.1029/2006GB002907. Abstract
We present a new synthesis of the oceanic
cycles of organic carbon, silicon, and calcium carbonate. Our calculations
are based on a series of algorithms starting with satellite-based primary
production and continuing with conversion of primary production to sinking
particle flux, penetration of particle flux to the deep sea, and
accumulation in sediments. Regional and global budgets from this synthesis
highlight the potential importance of shelves and near-shelf regions for
carbon burial. While a high degree of uncertainty remains, this analysis
suggests that shelves, less than 50 m water depths accounting for 2% of the
total ocean area, may account for 48% of the global flux of organic carbon
to the seafloor. Our estimates of organic carbon and nitrogen flux are in
generally good agreement with previous work while our estimates for CaCO3
and SiO2 fluxes are lower than recent work. Interannual
variability in particle export fluxes is found to be relatively small
compared to intra-annual variability over large domains with the single
exception of the dominating role of El Niño-Southern Oscillation variability
in the central tropical Pacific. Comparison with available sediment-based
syntheses of benthic remineralization and burial support the recent theory
of mineral protection of organic carbon flux through the deep ocean,
pointing to lithogenic material as an important carrier phase of organic
carbon to the deep seafloor. This work suggests that models which exclude
the role of lithogenic material would underestimate the penetration of POC
to the deep seafloor by approximately 16–51% globally, and by a much larger
fraction in areas with low productivity. Interestingly, atmospheric dust can
only account for 31% of the total lithogenic flux and 42% of the
lithogenically associated POC flux, implying that a majority of this
material is riverine or directly erosional in origin.
Jacobson, A R., Sara E Mikaloff-Fletcher, Nicolas Gruber, Jorge L Sarmiento, and M Gloor, 2007: A joint atmosphere-ocean inversion for surface fluxes of carbon dioxide: 1. Methods and global-scale fluxes. Global Biogeochemical Cycles, 21, GB1019, DOI:10.1029/2005GB002556. Abstract
We
have constructed an inverse estimate of surface fluxes of carbon dioxide
using both atmospheric and oceanic observational constraints. This global
estimate is spatially resolved into 11 land regions and 11 ocean regions,
and is calculated as a temporal mean for the period 1992–1996. The method
interprets in situ observations of carbon dioxide concentration in the ocean
and atmosphere with transport estimates from global circulation models.
Uncertainty in the modeled circulation is explicitly considered in this
inversion by using a suite of 16 atmospheric and 10 oceanic transport
simulations. The inversion analysis, coupled with estimates of river carbon
delivery, indicates that the open ocean had a net carbon uptake from the
atmosphere during the period 1992–96 of 1.7 PgC yr -1, consisting
of an uptake of 2.1 PgC yr-1 of anthropogenic carbon and a
natural outgassing of about 0.5 PgC yr-1 of carbon fixed on land
and transported through rivers to the open ocean. The formal uncertainty on
this oceanic uptake, despite a comprehensive effort to quantify sources of
error due to modeling biases, uncertain riverine carbon load, and
biogeochemical assumptions, is driven down to 0.2 PgC yr-1 by the
large number and relatively even spatial distribution of oceanic
observations used. Other sources of error, for which quantifiable estimates
are not currently available, such as unresolved transport and large region
inversion bias, may increase this uncertainty.
Jacobson, A R., Sara E Mikaloff-Fletcher, Nicolas Gruber, Jorge L Sarmiento, and M Gloor, 2007: A joint atmosphere-ocean inversion for surface fluxes of carbon dioxide: 2. Regional results. Global Biogeochemical Cycles, 21, GB1020, DOI:10.1029/2006GB002703. Abstract
We
report here the results from a coupled ocean-atmosphere inversion, in which
atmospheric CO2 gradients and transport simulations are combined
with observations of ocean interior carbon concentrations and ocean
transport simulations to provide a jointly constrained estimate of air-sea
and air-land carbon fluxes. While atmospheric data have little impact on
regional air-sea flux estimates, the inclusion of ocean data drives a
substantial change in terrestrial flux estimates. Our results indicate that
the tropical and southern land regions together are a large source of
carbon, with a 77% probability that their aggregate source size exceeds 1
PgC yr-1. This value is of similar magnitude to estimates of
fluxes in the tropics due to land-use change alone, making the existence of
a large tropical CO2 fertilization sink unlikely. This
terrestrial result is strongly driven by oceanic inversion results that
differ from flux estimates based on pCO2
climatologies, including a relatively small Southern Ocean sink (south of
44°S) and a relatively large sink in the southern temperate latitudes
(44°S–18°S). These conclusions are based on a formal error analysis of the
results, which includes uncertainties due to observational error transport
and other modeling errors, and biogeochemical assumptions. A suite of
sensitivity tests shows that these results are generally robust, but they
remain subject to potential sources of unquantified error stemming from the
use of large inversion regions and transport biases common to the suite of
available transport models.
Mikaloff-Fletcher, Sara E., Nicolas Gruber, A R Jacobson, M Gloor, Scott C Doney, Stephanie Dutkiewicz, M Gerber, M J Follows, Fortunat Joos, Keith Lindsay, D Menemenlis, A Mouchet, , and Jorge L Sarmiento, 2007: Inverse estimates of the oceanic sources and sinks of natural CO2 and the implied oceanic carbon transport. Global Biogeochemical Cycles, 21(GB1010), DOI:10.1029/2006GB002751. Abstract
We use an inverse method to estimate
the global-scale pattern of the air-sea flux of natural CO2,
i.e., the component of the CO2 flux due to the natural carbon
cycle that already existed in preindustrial times, on the basis of ocean
interior observations of dissolved inorganic carbon (DIC) and other
tracers, from which we estimate ΔCgasex, i.e.,
the component of the observed DIC that is due to the gas exchange of
natural CO2. We employ a suite of 10 different Ocean General
Circulation Models (OGCMs) to quantify the error arising from uncertainties
in the modeled transport required to link the interior ocean observations to
the surface fluxes. The results from the contributing OGCMs are weighted
using a model skill score based on a comparison of each model's simulated
natural radiocarbon with observations. We find a pattern of air-sea flux of
natural CO2 characterized by outgassing in the Southern Ocean
between 44°S and 59°S, vigorous uptake at midlatitudes of both hemispheres,
and strong outgassing in the tropics. In the Northern Hemisphere and the
tropics, the inverse estimates generally agree closely with the natural CO2
flux results from forward simulations of coupled OGCM-biogeochemistry models
undertaken as part of the second phase of the Ocean Carbon Model
Intercomparison Project (OCMIP-2). The OCMIP-2 simulations find far less
air-sea exchange than the inversion south of 20°S, but more recent forward
OGCM studies are in better agreement with the inverse estimates in the
Southern Hemisphere. The strong source and sink pattern south of 20°S was
not apparent in an earlier inversion study, because the choice of region
boundaries led to a partial cancellation of the sources and sinks. We show
that the inversely estimated flux pattern is clearly traceable to gradients
in the observed ΔCgasex, and that it is
relatively insensitive to the choice of OGCM or potential biases in ΔCgasex. Our inverse estimates imply a southward
interhemispheric transport of 0.31 ± 0.02 Pg C yr−1, most of
which occurs in the Atlantic. This is considerably smaller than the 1 Pg C
yr−1 of Northern Hemisphere uptake that has been inferred from
atmospheric CO2 observations during the 1980s and 1990s, which
supports the hypothesis of a Northern Hemisphere terrestrial sink.
Najjar, R G., X Jin, F Louanchi, Olivier Aumont, K Caldeira, Scott C Doney, J-C Dutay, M J Follows, Nicolas Gruber, Keith Lindsay, E Maier-Reimer, R Matear, K Matsumoto, Patrick Monfray, A Mouchet, James C Orr, G-K Plattner, Jorge L Sarmiento, R Schlitzer, Richard D Slater, M-F Weirig, Y Yamanaka, and Andrew Yool, 2007: Impact of circulation on export production, dissolved organic matter, and dissolved oxygen in the ocean: Results from Phase II of the Ocean Carbon-cycle Model Intercomparison Project (OCMIP-2). Global Biogeochemical Cycles, 21, GB3007, DOI:10.1029/2006GB002857. Abstract
Results are presented of export production, dissolved organic matter (DOM)
and dissolved oxygen simulated by 12 global ocean models participating in
the second phase of the Ocean Carbon-cycle Model Intercomparison Project. A
common, simple biogeochemical model is utilized in different
coarse-resolution ocean circulation models. The model mean (±1s)
downward flux of organic matter across 75 m depth is 17 ± 6 Pg C yr-1.
Model means of globally averaged particle export, the fraction of total
export in dissolved form, surface semilabile dissolved organic carbon (DOC),
and seasonal net outgassing (SNO) of oxygen are in good agreement with
observation-based estimates, but particle export and surface DOC are too
high in the tropics. There is a high sensitivity of the results to
circulation, as evidenced by (1) the correlation of surface DOC and export
with circulation metrics, including chlorofluorocarbon inventory and
deep-ocean radiocarbon, (2) very large intermodel differences in Southern
Ocean export, and (3) greater export production, fraction of export as DOM,
and SNO in models with explicit mixed layer physics. However, deep-ocean
oxygen, which varies widely among the models, is poorly correlated with
other model indices. Cross-model means of several biogeochemical metrics
show better agreement with observation-based estimates when restricted to
those models that best simulate deep-ocean radiocarbon. Overall, the results
emphasize the importance of physical processes in marine biogeochemical
modeling and suggest that the development of circulation models can be
accelerated by evaluating them with marine biogeochemical metrics.
Sarmiento, Jorge L., J Simeon, Anand Gnanadesikan, Nicolas Gruber, Robert M Key, and R Schlitzer, March 2007: Deep ocean biogeochemistry of silicic acid and nitrate. Global Biogeochemical Cycles, 21, GB1S90, DOI:10.1029/2006GB002720. Abstract
Observations of silicic acid and nitrate along the lower branch of the global conveyor belt circulation show that silicic acid accumulation by diatom opal dissolution occurs at 6.4 times the rate of nitrate addition by organic matter remineralization. The export of opal and organic matter from the surface ocean occurs at a Si:N mole ratio that is much smaller than this almost everywhere (cf. Sarmiento et al., 2004). The preferential increase of silicic acid over nitrate as the deep circulation progresses from the North Atlantic to the North Pacific is generally interpreted as requiring deep dissolution of opal together with shallow remineralization of organic matter (Broecker, 1991). However, Sarmiento et al. (2004) showed that the primary reason for the low silicic acid concentration of the upper ocean is that the waters feeding the main thermocline from the surface Southern Ocean are depleted in silicic acid relative to nitrate. By implication, the same Southern Ocean processes that deplete the silicic acid in the surface Southern Ocean must also be responsible for the enhanced silicic acid concentration of the deep ocean. We use observations and results from an updated version of the adjoint model of Schlitzer (2000) to confirm that this is indeed the case.
Sweeney, C, M Gloor, A R Jacobson, Robert M Key, Galen McKinley, Jorge L Sarmiento, and R Wanninkhof, 2007: Constraining global air-sea gas exchange for CO2 with recent bomb 14C measurements. Global Biogeochemical Cycles, 21, GB2015, DOI:10.1029/2006GB002784. Abstract
The 14CO2 released
into the stratosphere during bomb testing in the early 1960s provides a
global constraint on air-sea gas exchange of soluble atmospheric gases like
CO2. Using the most complete database of dissolved inorganic
radiocarbon, DI14C, available to date and a suite of ocean
general circulation models in an inverse mode we recalculate the ocean
inventory of bomb-produced DI14C in the global ocean and confirm
that there is a 25% decrease from previous estimates using older DI14C
data sets. Additionally, we find a 33% lower globally averaged gas transfer
velocity for CO2 compared to previous estimates (Wanninkhof,
1992) using the NCEP/NCAR Reanalysis 1 1954–2000 where the global mean winds
are 6.9 m s−1. Unlike some earlier ocean radiocarbon studies, the
implied gas transfer velocity finally closes the gap between small-scale
deliberate tracer studies and global-scale estimates. Additionally, the
total inventory of bomb-produced radiocarbon in the ocean is now in
agreement with global budgets based on radiocarbon measurements made in the
stratosphere and troposphere. Using the implied relationship between wind
speed and gas transfer velocity ks= 0.27
u102
(Sc/660)−0.5
and standard partial pressure difference climatology of CO2 we
obtain an net air-sea flux estimate of 1.3 ± 0.5 PgCyr−1 for
1995. After accounting for the carbon transferred from rivers to the deep
ocean, our estimate of oceanic uptake (1.8 ± 0.5 PgCyr−1)
compares well with estimates based on ocean inventories, ocean transport
inversions using ocean concentration data, and model simulations.
Behrenfeld, M J., R T O'Malley, D A Siegel, C R McClain, Jorge L Sarmiento, G C Feldman, J Milligan, P G Falkowski, R M Letelier, and E S Boss, 2006: Climate-driven trends in contemporary ocean productivity. Nature, 444(7120), DOI:10.1038/nature05317. Abstract
Contributing roughly half of the biosphere's net primary production (NPP)1, 2, photosynthesis by oceanic phytoplankton is a vital link in the cycling of carbon between living and inorganic stocks. Each day, more than a hundred million tons of carbon in the form of CO2 are fixed into organic material by these ubiquitous, microscopic plants of the upper ocean, and each day a similar amount of organic carbon is transferred into marine ecosystems by sinking and grazing. The distribution of phytoplankton biomass and NPP is defined by the availability of light and nutrients (nitrogen, phosphate, iron). These growth-limiting factors are in turn regulated by physical processes of ocean circulation, mixed-layer dynamics, upwelling, atmospheric dust deposition, and the solar cycle. Satellite measurements of ocean colour provide a means of quantifying ocean productivity on a global scale and linking its variability to environmental factors. Here we describe global ocean NPP changes detected from space over the past decade. The period is dominated by an initial increase in NPP of 1,930 teragrams of carbon a year (Tg C yr-1), followed by a prolonged decrease averaging 190 Tg C yr-1. These trends are driven by changes occurring in the expansive stratified low-latitude oceans and are tightly coupled to coincident climate variability. This link between the physical environment and ocean biology functions through changes in upper-ocean temperature and stratification, which influence the availability of nutrients for phytoplankton growth. The observed reductions in ocean productivity during the recent post-1999 warming period provide insight on how future climate change can alter marine food webs
Crevoisier, Cyril, M Gloor, E Gloaquen, Larry W Horowitz, Jorge L Sarmiento, C Sweeney, and P P Tans, 2006: A direct carbon budgeting approach to infer carbon sources and sinks. Design and synthetic application to complement the NACP observation network. Tellus B, 58B(5), DOI:10.1111/j.1600-0889.2006.00214.x. Abstract
In order to exploit the upcoming regular measurements of vertical carbon dioxide (CO2 profiles over North America implemented in the framework of the North American Carbon Program (NACP), we design a direct carbon budgeting approach to infer carbon sources and sinks over the continent using model simulations. Direct budgeting puts a control volume on top of North America, balances air mass in- and outflows into the volume and solves for the surface fluxes. The flows are derived from the observations through a geostatistical interpolation technique called Kriging combined with transport fields from weather analysis. The use of CO2 vertical profiles simulated by the atmospheric transport model MOZART-2 at the planned 19 stations of the NACP network has given an estimation of the error of 0.39 GtC yr-1 within the model world. Reducing this error may be achieved through a better estimation of mass fluxes associated with convective processes affecting North America. Complementary stations in the north-west and the north-east are also needed to resolve the variability of CO2 in these regions. For instance, the addition of a single station near 52°N; 110°W is shown to decrease the estimation error to 0.34 GtC yr-1.
Jin, X, Nicolas Gruber, John P Dunne, Jorge L Sarmiento, and R A Armstrong, June 2006: Diagnosing the contribution of phytoplankton functional groups to the production and export of particulate organic carbon, CaCO3, and opal from global nutrient and alkalinity distributions. Global Biogeochemical Cycles, 20, GB2015, DOI:10.1029/2005GB002532. Abstract
We diagnose the contribution of four main phytoplankton functional groups to the production and export of particulate organic carbon (POC), CaCO3, and opal by combining in a restoring approach global oceanic observations of nitrate, silicic acid, and alkalinity with a simple size-dependent ecological/biogeochemical model. In order to determine the robustness of our results, we employ three different variants of the ocean general circulation model (OGCM) required to transport and mix the nutrients and alkalinity into the upper ocean. In our standard model, the global export of CaCO3 is diagnosed as 1.1 PgC yr−1 (range of sensitivity cases 0.8 to 1.2 PgC yr−1) and that of opal as 180 Tmol Si yr−1 (range 160 to 180 Tmol Si yr−1). CaCO3 export is found to have three maxima at approximately 40¡ÆS, the equator, and around 40¡ÆN. In contrast, the opal export is dominated by the Southern Ocean with a single maximum at around 60¡ÆS. The molar export ratio of inorganic to organic carbon is diagnosed in our standard model to be about 0.09 (range 0.07 to 0.10) and found to be remarkably uniform spatially. The molar export ratio of opal to organic nitrogen varies substantially from values around 2 to 3 in the Southern Ocean south of 45¡ÆS to values below 0.5 throughout most of the rest of the ocean, except for the North Pacific. Irrespective of which OGCM is used, large phytoplankton dominate the export of POC, with diatoms alone accounting for 40% of this export, while the contribution of coccolithophorids is only about 10%. Small phytoplankton dominate net primary production (NPP) with a fraction of ¡70%. Diatoms and coccolithophorids account for about 15% and less than 2% of NPP, respectively. These diagnosed contributions of the main phytoplankton functional groups to NPP are also robust across all OGCMs investigated. Correlation and regression analyses reveal that the variations in the relative contributions of diatoms and coccolithophorids to NPP can be predicted reasonably well on the basis of a few key parameters.
Modelling studies have demonstrated that the nutrient and carbon cycles in the Southern Ocean play a central role in setting the air–sea balance of CO2 and global biological production1, 2, 3, 4, 5, 6, 7, 8. Box model studies1, 2, 3, 4 first pointed out that an increase in nutrient utilization in the high latitudes results in a strong decrease in the atmospheric carbon dioxide partial pressure (pCO2). This early research led to two important ideas: high latitude regions are more important in determining atmospheric pCO2 than low latitudes, despite their much smaller area, and nutrient utilization and atmospheric pCO2 are tightly linked. Subsequent general circulation model simulations show that the Southern Ocean is the most important high latitude region in controlling pre-industrial atmospheric CO2 because it serves as a lid to a larger volume of the deep ocean5, 6. Other studies point out the crucial role of the Southern Ocean in the uptake and storage of anthropogenic carbon dioxide7 and in controlling global biological production8. Here we probe the system to determine whether certain regions of the Southern Ocean are more critical than others for air–sea CO2 balance and the biological export production, by increasing surface nutrient drawdown in an ocean general circulation model. We demonstrate that atmospheric CO2 and global biological export production are controlled by different regions of the Southern Ocean. The air–sea balance of carbon dioxide is controlled mainly by the biological pump and circulation in the Antarctic deep-water formation region, whereas global export production is controlled mainly by the biological pump and circulation in the Subantarctic intermediate and mode water formation region. The existence of this biogeochemical divide separating the Antarctic from the Subantarctic suggests that it may be possible for climate change or human intervention to modify one of these without greatly altering the other.
Mignone, B K., Anand Gnanadesikan, Jorge L Sarmiento, and Richard D Slater, January 2006: Central role of Southern Hemisphere winds and eddies in modulating the oceanic uptake of anthropogenic carbon. Geophysical Research Letters, 33, L01604, DOI:10.1029/2005GL024464. Abstract
Although the world ocean is known to be a major sink of anthropogenic carbon dioxide, the exact processes governing the magnitude and regional distribution of carbon uptake remain poorly understood. Here we show that Southern Hemisphere winds, by altering the Ekman volume transport out of the Southern Ocean, strongly control the regional distribution of anthropogenic uptake in an ocean general circulation model, while winds and isopycnal thickness mixing together, by altering the volume of light, actively-ventilated ocean water, exert strong control over the absolute magnitude of anthropogenic uptake. These results are provocative in suggesting that climate-mediated changes in pycnocline volume may ultimately control changes in future carbon uptake.
Mikaloff-Fletcher, Sara E., Nicolas Gruber, A R Jacobson, Scott C Doney, Stephanie Dutkiewicz, M Gerber, M J Follows, Fortunat Joos, Keith Lindsay, D Menemenlis, A Mouchet, , and Jorge L Sarmiento, 2006: Inverse estimates of anthropogenic CO2 uptake, transport, and storage by the ocean. Global Biogeochemical Cycles, 20, GB2002, DOI:10.1029/2006GB002751. Abstract
Regional air-sea fluxes of anthropogenic CO2 are estimated using a Green's function inversion method that combines data-based estimates of anthropogenic CO2 in the ocean with information about ocean transport and mixing from a suite of Ocean General Circulation Models (OGCMs). In order to quantify the uncertainty associated with the estimated fluxes owing to modeled transport and errors in the data, we employ 10 OGCMs and three scenarios representing biases in the data-based anthropogenic CO2 estimates. On the basis of the prescribed anthropogenic CO2 storage, we find a global uptake of 2.2 ± 0.25 Pg C yr−1, scaled to 1995. This error estimate represents the standard deviation of the models weighted by a CFC-based model skill score, which reduces the error range and emphasizes those models that have been shown to reproduce observed tracer concentrations most accurately. The greatest anthropogenic CO2 uptake occurs in the Southern Ocean and in the tropics. The flux estimates imply vigorous northward transport in the Southern Hemisphere, northward cross-equatorial transport, and equatorward transport at high northern latitudes. Compared with forward simulations, we find substantially more uptake in the Southern Ocean, less uptake in the Pacific Ocean, and less global uptake. The large-scale spatial pattern of the estimated flux is generally insensitive to possible biases in the data and the models employed. However, the global uptake scales approximately linearly with changes in the global anthropogenic CO2 inventory. Considerable uncertainties remain in some regions, particularly the Southern Ocean.
Patra, Prabir K., Jasmin G John, Jorge L Sarmiento, and Songmiao Fan, et al., March 2006: Sensitivity of inverse estimation of annual mean CO2 sources and sinks to ocean-only sites versus all-sites observational networks. Geophysical Research Letters, 33, L05814, DOI:10.1029/2005GL025403. Abstract
Inverse estimation of carbon dioxide (CO2) sources and sinks uses atmospheric CO2 observations, mostly made near the Earth's surface. However, transport models used in such studies lack perfect representation of atmospheric dynamics and thus often fail to produce unbiased forward simulations. The error is generally larger for observations over the land than those over the remote/marine locations. The range of this error is estimated by using multiple transport models (16 are used here). We have estimated the remaining differences in CO2 fluxes due to the use of ocean-only versus all-sites (i.e., over ocean and land) observations of CO2 in a time-independent inverse modeling framework. The fluxes estimated using the ocean-only networks are more robust compared to those obtained using all-sites networks. This makes the global, hemispheric, and regional flux determination less dependent on the selection of transport model and observation network.
Sarmiento, Jorge L., and Nicolas Gruber, 2006: Ocean Biogeochemical Dynamics, Princeton, NJ: Princeton University Press, 526pp.
We present new empirical and mechanistic models for predicting the export of organic carbon out of the surface ocean by sinking particles. To calibrate these models, we have compiled a synthesis of field observations related to ecosystem size structure, primary production and particle export from around the globe. The empirical model captures 61% of the observed variance in the ratio of particle export to primary production (the pe ratio) using sea-surface temperature and chlorophyll concentrations (or primary productivity) as predictor variables. To describe the mechanisms responsible for pe-ratio variability, we present size-based formulations of phytoplankton grazing and sinking particle export, combining them into an alternative, mechanistic model. The formulation of grazing dynamics, using simple power laws as closure terms for small and large phytoplankton, reproduces 74% of the observed variability in phytoplankton community composition wherein large phytoplankton augment small ones as production increases. The formulation for sinking particle export partitions a temperature-dependent fraction of small and large phytoplankton grazing into sinking detritus. The mechanistic model also captures 61% of the observed variance in pe ratio, with large phytoplankton in high biomass and relatively cold regions leading to more efficient export. In this model, variability in primary productivity results in a biomass-modulated switch between small and large phytoplankton pathways.
Orr, James C., V J Fabry, Olivier Aumont, Laurent Bopp, Scott C Doney, Richard A Feely, Anand Gnanadesikan, Nicolas Gruber, A Ishida, Fortunat Joos, Robert M Key, Keith Lindsay, E Maier-Reimer, R Matear, Patrick Monfray, A Mouchet, R G Najjar, G-K Plattner, Keith B Rodgers, C L Sabine, Jorge L Sarmiento, R Schlitzer, Richard D Slater, I J Totterdell, M-F Weirig, Y Yamanaka, and Andrew Yool, September 2005: Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature, 437(7059), DOI:10.1038/nature04095. Abstract
Today's surface ocean is saturated with respect to calcium carbonate, but increasing atmospheric carbon dioxide concentrations are reducing ocean pH and carbonate ion concentrations, and thus the level of calcium carbonate saturation. Experimental evidence suggests that if these trends continue, key marine organisms such as corals and some plankton will have difficulty maintaining their external calcium carbonate skeletons. Here we use 13 models of the ocean-carbon cycle to assess calcium carbonate saturation under the IS92a 'business-as-usual' scenario for future emissions of anthropogenic carbon dioxide. In our projections, Southern Ocean surface waters will begin to become undersaturated with respect to aragonite, a metastable form of calcium carbonate, by the year 2050. By 2100, this undersaturation could extend throughout the entire Southern Ocean and into the subarctic Pacific Ocean. When live pteropods were exposed to our predicted level of undersaturation during a two-day shipboard experiment, their aragonite shells showed notable dissolution. Our findings indicate that conditions detrimental to high-latitude ecosystems could develop within decades, not centuries as suggested previously.
A suite of standard ocean hydrographic and circulation metrics are applied to the equilibrium physical solutions from 13 global carbon models participating in phase 2 of the Ocean Carbon-cycle Model Intercomparison Project (OCMIP-2). Model-data comparisons are presented for sea surface temperature and salinity, seasonal mixed layer depth, meridional heat and freshwater transport, 3-D hydrographic fields, and meridional overturning. Considerable variation exists among the OCMIP-2 simulations, with some of the solutions falling noticeably outside available observational constraints. For some cases, model-model and model-data differences can be related to variations in surface forcing, subgrid-scale parameterizations, and model architecture. These errors in the physical metrics point to significant problems in the underlying model representations of ocean transport and dynamics, problems that directly affect the OCMIP predicted ocean tracer and carbon cycle variables (e.g., air-sea CO2 flux, chlorofluorocarbon and anthropogenic CO2 uptake, and export production). A substantial fraction of the large model-model ranges in OCMIP-2 biogeochemical fields (±25–40%) represents the propagation of known errors in model physics. Therefore the model-model spread likely overstates the uncertainty in our current understanding of the ocean carbon system, particularly for transport-dominated fields such as the historical uptake of anthropogenic CO2. A full error assessment, however, would need to account for additional sources of uncertainty such as more complex biological-chemical-physical interactions, biases arising from poorly resolved or neglected physical processes, and climate change.
Gnanadesikan, Anand, John P Dunne, Robert M Key, K Matsumoto, Jorge L Sarmiento, Richard D Slater, and P S Swathi, December 2004: Oceanic ventilation and biogeochemical cycling: Understanding the physical mechanisms that produce realistic distributions of tracers and productivity. Global Biogeochemical Cycles, 18(4), GB4010, DOI:10.1029/2003GB002097. Abstract
Differing models of the ocean circulation support different rates of ventilation, which in turn produce different distributions of radiocarbon, oxygen, and export production. We examine these fields within a suite of general circulation models run to examine the sensitivity of the circulation to the parameterization of subgridscale mixing and surface forcing. We find that different models can explain relatively high fractions of the spatial variance in some fields such as radiocarbon, and that newer estimates of the rate of biological cycling are in better agreement with the models than previously published estimates. We consider how different models achieve such agreement and show that they can accomplish this in different ways. For example, models with high vertical diffusion move young surface waters into the Southern Ocean, while models with high winds move more young North Atlantic water into this region. The dependence on parameter values is not simple. Changes in the vertical diffusion coefficient, for example, can produce major changes in advective fluxes. In the coarse-resolution models studied here, lateral diffusion plays a major role in the tracer budget of the deep ocean, a somewhat worrisome fact as it is poorly constrained both observationally and theoretically.
Matsumoto, K, Jorge L Sarmiento, Robert M Key, Olivier Aumont, J L Bullister, K Caldeira, J-M Campin, Scott C Doney, H Drange, J-C Dutay, M J Follows, Y Gao, Anand Gnanadesikan, Nicolas Gruber, A Ishida, Fortunat Joos, Keith Lindsay, E Maier-Reimer, J Marshall, R Matear, Patrick Monfray, A Mouchet, R G Najjar, G-K Plattner, R Schlitzer, Richard D Slater, P S Swathi, I J Totterdell, M-F Weirig, Y Yamanaka, Andrew Yool, and James C Orr, April 2004: Evaluation of ocean carbon cycle models with data-based metrics. Geophysical Research Letters, 31, L07303, DOI:10.1029/2003GL018970. Abstract
New radiocarbon and chlorofluorocarbon-11 data from the World Ocean Circulation Experiment are used to assess a suite of 19 ocean carbon cycle models. We use the distributions and inventories of these tracers as quantitative metrics of model skill and find that only about a quarter of the suite is consistent with the new data-based metrics. This should serve as a warning bell to the larger community that not all is well with current generation of ocean carbon cycle models. At the same time, this highlights the danger in simply using the available models to represent the state-of-the-art modeling without considering the credibility of each model.
A number of large-scale sequestration strategies have been considered to help mitigate rising levels of atmospheric carbon dioxide (CO2). Here, we use an ocean general circulation model (OGCM) to evaluate the efficiency of one such strategy currently receiving much attention, the direct injection of liquid CO2 into selected regions of the abyssal ocean. We find that currents typically transport the injected plumes quite far before they are able to return to the surface and release CO2 through air–sea gas exchange. When injected at sufficient depth (well within or below the main thermocline), most of the injected CO2 outgasses in high latitudes (mainly in the Southern Ocean) where vertical exchange is most favored. Virtually all OGCMs that have performed similar simulations confirm these global patterns, but regional differences are significant, leading efficiency estimates to vary widely among models even when identical protocols are followed. In this paper, we make a first attempt at reconciling some of these differences by performing a sensitivity analysis in one OGCM, the Princeton Modular Ocean Model. Using techniques we have developed to maintain both the modeled density structure and the absolute magnitude of the overturning circulation while varying important mixing parameters, we estimate the sensitivity of sequestration efficiency to the magnitude of vertical exchange within the low-latitude pycnocline. Combining these model results with available tracer data permits us to narrow the range of model behavior, which in turn places important constraints on sequestration efficiency.
Sarmiento, Jorge L., Nicolas Gruber, M Brzezinski, and John P Dunne, January 2004: High-latitude controls of thermocline nutrients and low latitude biological productivity. Nature, 427, 56-60. Abstract PDF
The ocean's biological pump strips nutrients out of the surface waters and exports them into the thermocline and deep waters. If there were no return path of nutrients from deep waters, the biological pump would eventually deplete the surface waters and thermocline of nutrients; surface biological productivity would plummet. Here we make use of the combined distributions of silicic acid and nitrate to trace the main nutrient return path from deep waters by upwelling in the Southern Ocean and subsequent entrainment into subantarctic mode water. We show that the subantarctic mode water, which spreads throughout the entire Southern Hemisphere and North Atlantic Ocean, is the main source of nutrients for the thermocline. We also find that an additional return path exists in the northwest corner of the Pacific Ocean, where enhanced vertical mixing, perhaps driven by tides, brings abyssal nutrients to the surface and supplies them to the thermocline of the North Pacific. Our analysis has important implications for our understanding of large-scale controls on the nature and magnitude of low-latitude biological productivity and its sensitivity to climate change.
Sarmiento, Jorge L., Richard D Slater, R T Barber, Laurent Bopp, Scott C Doney, A C Hirst, J Kieypas, R Matear, U Mikolajewicz, Patrick Monfray, V Soldatov, S A Spall, and Ronald J Stouffer, September 2004: Response of ocean ecosystems to climate warming. Global Biogeochemical Cycles, 18, GB3003, DOI:10.1029/2003GB002134. Abstract
We examine six different coupled climate model simulations to determine the ocean biological response to climate warming between the beginning of the industrial revolution and 2050. We use vertical velocity, maximum winter mixed layer depth, and sea ice cover to define six biomes. Climate warming leads to a contraction of the highly productive marginal sea ice biome by 42% in the Northern Hemisphere and 17% in the Southern Hemisphere, and leads to an expansion of the low productivity permanently stratified subtropical gyre biome by 4.0% in the Northern Hemisphere and 9.4% in the Southern Hemisphere. In between these, the subpolar gyre biome expands by 16% in the Northern Hemisphere and 7% in the Southern Hemisphere, and the seasonally stratified subtropical gyre contracts by 11% in both hemispheres. The low-latitude (mostly coastal) upwelling biome area changes only modestly. Vertical stratification increases, which would be expected to decrease nutrient supply everywhere, but increase the growing season length in high latitudes. We use satellite ocean color and climatological observations to develop an empirical model for predicting chlorophyll from the physical properties of the global warming simulations. Four features stand out in the response to global warming: (1) a drop in chlorophyll in the North Pacific due primarily to retreat of the marginal sea ice biome, (2) a tendency toward an increase in chlorophyll in the North Atlantic due to a complex combination of factors, (3) an increase in chlorophyll in the Southern Ocean due primarily to the retreat of and changes at the northern boundary of the marginal sea ice zone, and (4) a tendency toward a decrease in chlorophyll adjacent to the Antarctic continent due primarily to freshening within the marginal sea ice zone. We use three different primary production algorithms to estimate the response of primary production to climate warming based on our estimated chlorophyll concentrations. The three algorithms give a global increase in primary production of 0.7% at the low end to 8.1% at the high end, with very large regional differences. The main cause of both the response to warming and the variation between algorithms is the temperature sensitivity of the primary production algorithms. We also show results for the period between the industrial revolution and 2050 and 2090.
Sarmiento, Jorge L., John P Dunne, and R A Armstrong, 2004: Do We Now Understand The Ocean’s Biological Pump?U.S. JGOFS News, 12, 1-5.
Gao, Y, Songmiao Fan, and Jorge L Sarmiento, April 2003: Aeolian iron input to the ocean through precipitation scavenging: A modeling perspective and its implication for natural iron fertilization in the ocean. Journal of Geophysical Research, 108(D7), 4221, DOI:10.1029/2002JD002420. Abstract
Aeolian dust input may be a critical source of dissolved iron for phytoplankton growth in some oceanic regions. We used an atmospheric general circulation model (GCM) to simulate dust transport and removal by dry and wet deposition. Model results show extremely low dust concentrations over the equatorial Pacific and Southern Ocean. We find that wet deposition through precipitation scavenging accounts for ~40% of the total deposition over the coastal oceans and ~60% over the open ocean. Our estimates suggest that the annual input of dissolved Fe by precipitation scavenging ranges from 0.5 to 4 × 1012 g yr-1, which is 4-30% of the total aeolian Fe fluxes. Dissolved Fe input through dry deposition is significantly lower than that by wet deposition, accounting for only 0.6-2.4 % of the total Fe deposition. Our upper limit estimate on the fraction of dissolved Fe in the total atmospheric deposition is thus more than three times higher than the value of 10% currently considered as an upper limit for dissolved Fe in Aeolian fluxes. As iron input through precipitation may promote episodic phytoplankton growth in the ocean, measurements of dissolved iron in rainwater over the oceans are needed for the study of oceanic biogeochemical cycles.
Gloor, M, Nicolas Gruber, Jorge L Sarmiento, C L Sabine, Richard A Feely, and C Rödenbeck, 2003: A first estimate of present and preindustrial air-sea (CO2 flux patterns based on ocean interior carbon measurements and models. Geophysical Research Letters, 30(1), 1010, DOI:10.1029/2002GL015594. Abstract
The exchange of CO2 across the air-sea interface is a main determinant of the distribution of atmospheric CO2 from which major conclusions about the carbon cycle are drawn, yet our knowledge of atmosphere-ocean fluxes still has major gaps. A new analysis based on recent ocean dissolved inorganic carbon data and on models permits us to separately estimate the preindiustrial and present air-sea CO2 flux distributions without requiring knowledge of the gas exchange coefficient. We find a smaller carbon sink at mid to high latitudes of the sourhtern hemisphere than previous data based estimates and a shift of ocean uptake to lower latitude regions compared to estimates and simulations. The total uptake of anthropogenic CO2 for 1990 is 1.8 (±0.4) Pg C yr-1 . Our ocean based results support the interpretation of the latitudinal distribution of atmospheric CO2 data as evidence for a large northern hemisphere land carbon sink.
Increasing oceanic productivity by fertilizing nutrient-rich regions with iron has been proposed as a mechanism to offset anthropogenic emissions of carbon dioxide. Earlier studies examined the impact of large-scale fertilization of vast reaches of the ocean for long periods of time. We use an ocean general circulation model to consider more realistic scenarios involving fertilizing small regions (a few hundred kilometers on a side) for limited periods of time (of order 1 month). A century after such a fertilization event, the reduction of atmospheric carbon dioxide is between 2% and 44% of the initial pulse of organic carbon export to the abyssal ocean. The fraction depends on how rapidly the surface nutrient and carbon fields recover from the fertilization event. The modeled recovery is very sensitive to the representation of biological productivity and remineralization. Direct verification of the uptake would be nearly impossible since changes in the air-sea flux due to fertilization would be much smaller than those resulting from natural spatial variability. Because of the sensitivity of the uptake to the long-term fate of the iron and organic matter, indirect verification by measurement of the organic matter flux would require high vertical resolution and long-term monitoring. Finally, the downward displacement of the nutrient profile resulting from an iron-induced productivity spurt may paradoxically lead to a long-term reduction in biological productivity. In the worst-case scenario, removing 1 ton of carbon from the atmosphere for a century is associated with a 30-ton reduction in biological export of carbon.
Gurney, K R., A Lauer, A S Denning, P Rayner, D F Baker, Philippe Bousquet, Lori Bruhwiler, Yu-Han Chen, Philippe Ciais, Songmiao Fan, I Y Fung, M Gloor, M Heimann, K Higuchi, Jasmin G John, Eva Kowalczyk, T Maki, Shamil Maksyutov, P Peylin, Michael J Prather, B Pak, Jorge L Sarmiento, S Taguchi, T Takahashi, and C-W Yuen, 2003: TransCom 3 CO2 inversion intercomparison: 1. Annual mean control results and sensitivity to transport and prior flux information. Tellus B, 55B(2), 555-579. Abstract PDF
Spatial and temporal variations of atmospheric CO2 concentration contain information about surface sources and sinks, which can be quantitatively interpreted through tracer transport inversion. Previous CO2 inversion calculations obtained differing results due to different data, methods and transport models used. To isolate the sources of uncertainty, we have conducted a set of annual mean inversion experiments in which 17 different transport models or model variants were used to calculate regional carbon sources and sinks from the same data with a standardized method. Simulated transport is a significant source of uncertainty in these calculations, particularly in the response to prescribed "background" fluxes due to fossil fuel combustion, a balanced terrestrial biosphere, and air-sea gas exchange. Individual model-estimated fluxes are often a direct reflection of their response to these background fluxes. Models that generate strong surface maxima near background exchange locations tend to require larger uptake near those locations. Models with weak surface maxima tend to have less uptake in those same regions but may infer small sources downwind. In some cases, individual model flux estimates cannot be analyzed through simple relationships to background flux responses but are likely due to local transport differences or particular responses at individual CO2 observing locations. The response to the background biosphere exchange generates the greatest variation in the estimated fluxes, particularly over land in the Northern Hemisphere. More observational data in the tropical regions may help in both lowering the uncertain tropical land flux uncertainties and constraining the northern land estimates because of compensation between these two broad regions in the inversion. More optimistically, examination of the model-mean retrieved fluxes indicates a general insensitivity to the prior fluxes and the prior flux uncertainties. Less uptake in the Southern Ocean than implied by oceanographic observations, and an evenly distributed northern land sink, remain in spite of changes in this aspect of the inversion setup.
Matsumoto, K, Anand Gnanadesikan, Nicolas Gruber, Robert M Key, and Jorge L Sarmiento, 2003: Inconsistent model uptake of anthropogenic tracers in the Southern Ocean. Geochimica et Cosmochimica Acta, 67(18), Suppl 1, A278. PDF
McNeil, B I., R Matear, Robert M Key, J L Bullister, and Jorge L Sarmiento, 2003: Anthropogenic CO2 uptake by the ocean based on the Global Chlorofluorocarbon Data Set. Science, 299(5604), 235-239. Abstract PDF
We estimated the oceanic inventory of anthropogenic carbon dioxide (CO2) from 1980 to 1999 using a technique based on the global chorofluorocarbon data set. Our analysis suggests that the ocean stored 14.8 petagrams of anthropogenic carbon from mid-1980 to mid-1989 and 17.9 petagrams of carbon from mid-1990 to mid-1999, indicating an oceanwide net uptake of 1.6 and 2.0 ± 0.4 petagrams of carbon per year, respectively. Our results provide an upper limit on the solubility-driven anthropogenic CO2 flux into the ocean, and they suggest that most ocean general circulation models are overestimating oceanic anthropogenic CO2 uptake over the past two decades.
Toggweiler, J R., Anand Gnanadesikan, S Carson, R Murnane, and Jorge L Sarmiento, March 2003: Representation of the carbon cycle in box models and GCMs: 1. Solubility pump. Global Biogeochemical Cycles, 17(1), 1026, DOI:10.1029/2001GB001401. Abstract
Bacastow [1996], Broecker et al. [1999], and Archer et al.[2000] have called attention recently to the fact that box models and general circulation models (GCMs) represent the thermal partitioning of CO2 between the warm surface ocean and cold deep ocean in different ways. They attribute these differences to mixing and circulation effects in GCMs that are not resolved in box models. The message that emerges from these studies is that box models have overstated the importance of the ocean's polar regions in the carbon cycle. A reduced role for the polar regions has major implications for the mechanisms put forth to explain glacial - interglacial changes in atmospheric CO2. In parts 1 and 2 of this paper, a new analysis of the ocean's carbon pumps is carried out to examine these findings. This paper, part 1, shows that unresolved mixing and circulation effects in box models are not the main reason for box model-GCM differences. The main factor is very different kinds of restrictions on gas exchange in polar areas. Polar outcrops in GCMs are much smaller than in box models, and they are assumed to be ice covered in an unrealistic way. This finding does not support a reduced role for the ocean's polar regions in the cycling of organic carbon, the subject taken up in part 2.
Box models of the ocean/atmosphere CO2 system rely on mechanisms at polar outcrops to alter the strength of the ocean's organic carbon pump. GCM-based carbon system models are reportedly less sensitive to the same processes. Here we separate the carbon pumps in a three-box model and the GCM-based Princeton Ocean Biogeochemistry Model to show how the organic pumps operate in the two kinds of models. The organic pumps are found to be quite different in two respects. Deep water in the three-box model is relatively well equilibrated with respect to the pCO2 of the atmosphere while deep water in the GCM tends to be poorly equilibrated. This makes the organic pump inherently stronger in the GCM than in the three-box model. The second difference has to do with the role of polar nutrient utilization. The organic pump in the GCM is shown to have natural upper and lower limits that are set by the initial PO4 concentrations in the deep water formed in the North Atlantic and Southern Ocean. The strength of the organic pump can swing between these limits in response to changes in deep-water formation that alter the mix of northern and southern deep water. Thus, unlike the situation in the three-box model, the organic pump in the GCM can become weaker or stronger without changes in polar nutrient utilization.
Watson, A J., James C Orr, Anand Gnanadesikan, Robert M Key, Jorge L Sarmiento, and Richard D Slater, 2003: Carbon dioxide fluxes in the global ocean In Ocean Biogeochemistry: A Synthesis of the Joint Global Ocean Flux Study (JGOFS), Berlin, Germany, Springer-Verlag, 123-143.
Brzezinski, M, C J Pride, V M Franck, D M Sigman, Jorge L Sarmiento, K Matsumoto, Nicolas Gruber, G H Rau, and K H Coale, 2002: A switch from Si(OH)4 to NO3- depletion in the glacial Southern Ocean. Geophysical Research Letters, 29(12), DOI:10.1029/2001GL014349. Abstract
Phytoplankton in the Antarctic deplete silicic acid (Si(OH)4) to a far greater extent than they do nitrate (NO3-). This pattern can be reversed by the addition of iron which dramatically lowers diatom Si(OH)4:NO3- uptake ratios. Higher iron supply during glacial times would thus drive the Antarctic towards NO3- depletion with excess Si(OH)4 remaining in surface waters. New 30Si and 15N records from Antarctic sediments confirm diminished Si(OH)4 use and enhanced NO3- depletion during the last three glaciations. The present low-Si(OH)4 water is transported northward to at least the subtropics. We postulate that the glacial high-Si(OH)4 water similarly may have been transported to the subtropics and beyond. This input of Si(OH)4 may have caused diatoms to displace coccolithophores at low latitudes, weakening the carbonate pump and increasing the depth of organic matter remineralization. These effects may have lowered glacial atmospheric pCO2 by as much as 60 ppm.
Doney, Scott C., A Kleypas, Jorge L Sarmiento, and P G Falkowski, 2002: The US JGOFS Synthesis and modeling project - an introduction. Deep-Sea Research, Part II, 49(1-3), 1-20. Abstract PDF
The field data collected as part of the international Joint Global Ocean Flux Study (JGOFS) provide an unprecedented view of marine biogeochemistry and the ocean carbon cycle. Following the completion of a series of regional process studies, a global CO2 survey, and a decade of sampling at two open-ocean time-series, US JGOFS initiated in 1997 a final research phase, the Synthesis and Modeling Project (SMP). The objective of the US JGOFS SMP is to "synthesize knowledge gained from the US JGOFS and related studies into a set of models that reflect our current understanding of the oceanic carbon cycle". Here we present an overview of the SMP and highlight the early scientific results from the project.
Dutay, J-C, J L Bullister, Scott C Doney, James C Orr, R G Najjar, Jorge L Sarmiento, and Richard D Slater, et al., 2002: Evaluation of ocean model ventilation with CFC-11: comparison of 13 global ocean models. Ocean Modelling, 4(2), 89-120. Abstract PDF
We compared the 13 models participating in the Ocean Carbon Model Intercomparison Project (OCMIP) with regards to their skill in matching observed distributions of CFC-11. This analysis characterizes the abilities of these models to ventilate the ocean on timescales relevant for anthropogenic CO2 uptake. We found a large range in the modeled global inventory (±30%), mainly due to differences in ventilation from the high latitudes. In the Southern Ocean, models differ particularly in the longitudinal distribution of the CFC uptake in the intermediate water, whereas the latitudinal distribution is mainly controlled by the subgrid-scale parameterization. Models with isopycnal diffusion and eddy-induced velocity parameterization produce more realistic intermediate water ventilation. Deep and bottom water ventilation also varies substantially between the models. Models coupled to a sea-ice model systematically provide more realistic AABW formation source region; however these same models also largely overestimate AABW ventilation if no specific parameterization of brine rejection during sea-ice formation is included. In the North Pacific Ocean, all models exhibit a systematic large underestimation of the CFC uptake in the thermocline of the subtropical gyre, while no systematic difference toward the observations is found in the subpolar gyre. In the North Atlantic Ocean, the CFC uptake is globally underestimated in subsurface. In the deep ocean, all but the adjoint model failed to produce the two recently ventilated branches observed in the North Atlantic Deep Water (NADW). Furthermore, simulated transport in the Deep Western Boundary Current (DWBC) is too sluggish in all but the isopycnal model, where it is too rapid.
Gnanadesikan, Anand, Richard D Slater, Nicolas Gruber, and Jorge L Sarmiento, 2002: Oceanic vertical exchange and new production: a comparison between models and observations. Deep-Sea Research, Part II, 49(1-3), 363-401. Abstract PDF
This paper explores the relationship between large-scale vertical exchange and the cycling of biologically active nutrients within the ocean. It considers how the parameterization of vertical and lateral mixing effects estimates of new production (defined as the net uptake of phosphate). A baseline case is run with low vertical mixing in the pycnocline and a relatively low lateral diffusion coefficient. The magnitude of the diapycnal diffusion coefficient is then increased within the pycnocline, within the pycnocline of the Southern Ocean, and in the top 50 m, while the lateral diffusion coefficient is increased throughout the ocean. It is shown that it is possible to change lateral and vertical diffusion coefficients so as to preserve the structure of the pycnocline while changing the pathways of vertical exchange and hence the cycling of nutrients. Comparisons between the different models reveal that new production is very sensitive to the level of vertical mixing within the pycnocline, but only weakly sensitive to the level of lateral and upper ocean diffusion. The results are compared with two estimates of new production based on ocean color and the annual cycle of nutrients. On a global scale, the observational estimates are most consistent with the circulation produced with a low diffusion coefficient within the pycnocline, resulting in a new production of around 10 GtC yr -1. On a regional level, however, large differences appear between observational and model based estimates. In the tropics, the models yield systematically higher levels of new production than the observational estimates. Evidence from the Eastern Equatorial Pacific suggests that this is due to both biases in the data used to generate the observational estimates and problems with the models. In the North Atlantic, the observational estimates vary more than the models, due in part to the methodology by which the nutrient-based climatology is constructed. In the North Pacific, the modelled values of new production are all much lower than the observational estimates, probably as a result of the failure to form intermediate water with the right properties. The results demonstrate the potential usefulness of new production for evaluating circulation models.
Gruber, Nicolas, and Jorge L Sarmiento, 2002: Large-scale biogeochemical-physical interactions in elemental cycles In The Sea, Volume 12, edited by A. R. Robinson, J. J. McCarthy, and B. J. Rothschild, New York, NY, John Wiley & Sons, Inc, 337-399.
Information about regional carbon sources and sinks can be derived from variations in observed atmospheric CO2 concentrations via inverse modelling with atmospheric tracer transport models. A consensus has not yet been reached regarding the size and distribution of regional carbon fluxes obtained using this approach, partly owing to the use of several different atmospheric transport models 1-9. Here we report estimates of surface-atmosphere CO2 fluxes from an intercomparison of atmospheric CO2 inversion models (the TransCom 3 project), which includes 16 transport models and model variants. We find an uptake of CO2 in the southern extratropical ocean less than that estimated from ocean measurements, a result that is not sensitive in transport models or methodological approaches. We also find a northern land carbon sink that is distributed relatively evenly among the continents of the Northern Hemisphere, but these results show some sensitivity to transport differences among models, especially in how they respond to seasonal terrestrial exchange of CO2. Overall, carbon fluxes integrated over latitudinal zones are strongly constrained by observations in the middle to high latitudes. Further significant constraints to our understanding of regional carbon fluxes will therefore require improvements in transport models and expansion of the CO2 observation network with the tropics.
Iglesias-Rodriguez, M D., R A Armstrong, Richard A Feely, Raleigh Hood, A Kleypas, J D Milliman, C L Sabine, and Jorge L Sarmiento, 2002: Progress made in study of ocean's calcium carbonate budget. EOS, 83(34), 365, 374-375.
Matsumoto, K, Jorge L Sarmiento, and M Brzezinski, 2002: Silicic acid leakage from the Southern Ocean: a possible explanation for glacial atmospheric pCO2. Global Biogeochemical Cycles, 16(3), DOI:10.1029/2001GB001442. Abstract
Using a simple box model, we investigate the effects of a reduced Si:N uptake ratio by Antarctic phytoplankton on the marine silica cycle and atmospheric pCO2. Recent incubation experiments demonstrate such a phenomenon in diatoms when iron is added [Hutchins and Bruland, 1998; Takeda, 1998; Franck et al., 2000]. The Southern Ocean may have supported diatoms with reduced Si:N uptake ratios compared to today during the dustier glacial times [Petit et al., 1999]. A similar reduction in the uptake ratio may be realized with an increased production of nondiatom phytoplankton such as Phaeocystis. Our model shows that reduced Si:N export ratios in the Southern Ocean create excess silicic acid, which may then be leaked out to lower latitudes. Any significant consumption of the excess silicic acid by diatoms that leads to an enhancement in their growth at the expense of coccolithophorids diminishes CaCO3 production and therefore diminishes the carbonate pump. In our box model the combination of a reduced carbonate pump and an open system carbonate compensation draw down steady state atmospheric CO2 from the interglacial 277 to 230–242 ppm, depending on where the excess silicic acid is consumed. By comparison, the atmospheric pCO2 sensitivity of general circulation models to carbonate pump forcing is ~3.5–fold greater, which, combined with carbonate compensation, can account for peak glacial atmospheric pCO2. We discuss the importance of the initial rain ratio of CaCO3 to organic carbon on atmospheric pCO2 and relevant sedimentary records that support and constrain this "silicic acid leakage" scenario.
Peylin, P, D F Baker, Jorge L Sarmiento, Philippe Ciais, and Philippe Bousquet, 2002: Influence of transport uncertainty on annual mean and seasonal inversions of atmospheric CO2 data. Journal of Geophysical Research, 107(D19), 4385, DOI:10.1029/2001JD000857. Abstract
Inversion methods are often used to estimate surface CO2 fluxes from atmospheric CO2 concentration measurements, given an atmospheric transport model to relate the two. The published estimates disagree strongly on the location of the main sources and sinks, however. Are these differences due to the different time spans considered, or are they artifacts of the method and data used? Here we assess the uncertainty in such estimates due to the choice of time discretization of the measurements and fluxes, the spatial resolution of the fluxes, and the transport model. A suite of 27 Bayesian least squares inversions has been run, given by varying the number of flux regions solved for (7, 12, and 17), the time discretization (annual/annual, annual/monthly, and monthly/monthly for the fluxes/data), and the transport model (TM2, TM3, and GCTM), while holding all other inversion details constant. The estimated fluxes from this ensemble of inversions for the land + ocean sum are stable over large zonal bands, but the spread in the results increases when considering the longitudinal flux distribution inside these bands. On average for 1990–1994 the inversions place a large CO2 uptake north of 30°N (3.2 ± 0.3 GtC yr-1), mostly over the land regions, with more in Eurasia than North America. The ocean fluxes are generally smaller than given by Takahashi et al. [1999], especially south of 15°S and in the global total, where they are less than half as large. A small uptake is found for the tropical land regions, suggesting that growth more than compensates for deforestation there. The results for the different transport models are consistent with their known mixing properties; the longitudinal pattern of their land biosphere rectifier, in particular, strongly influences the regional partitioning of the flux in the north. While differences between the transport models contribute significantly to the spread of the results, an equivalent or even larger spread is due to the time discretization method used: Solving for annual mean fluxes with monthly mean measurements tended to give spurious land/ocean flux partition in the north. We suggest then that this time discretization method be avoided. Overall, the uncertainty quoted for the estimated fluxes should include not only the random error calculated by the inversion equations but also all the systematic errors in the problem, such as those addressed in this study.
We use an ocean biogeochemical-transport box model of the top 100 m of the water column to estimate the CaCO3 to organic carbon export ratio from observations of the vertical gradients of potential alkalinity and nitrate. We find a global average molar export ratio of 0.06 ± 0.03. This is substantially smaller than earlier estimates of 0.25 on which a majority of ocean biogeochemical models had based their parameterization of CaCO3 production. Contrary to the pattern of coccolithophore blooms determined from satellite observations, which show high latitude predominance, we find maximum export ratios in the equatorial region and generally smaller ratios in the subtropical and subpolar gyres. Our results suggest a dominant contribution to global calcification by low-latitude nonbloom forming coccolithophores or other organisms such as foraminifera and pteropods.
Organic carbon buried in sediments as coal, natural gas, and oil over literally hundreds of millions of years is being consumed as a result of human activities and returned to the atmosphere as carbon dioxide (CO2) on a time scale of a few centuries. The energy harvested from this transformation of fossil fuels supplies us with electricity, heat, transportation, and industrial power. The clearing of forests for agricultural lands and the harvesting of wood, both of which remove carbon-bearing vegetation, have also added CO2 to the atmosphere, in amounts equivalent to more than half of the fossil fuel source. The CO2 added to the atmosphere because of man's activities, and the way it is currently distributed within the land, air, and sea, is depicted in the carbon cycle diagram shown in figure 1.
Deutsch, Curtis A., Nicolas Gruber, Robert M Key, Jorge L Sarmiento, and A Ganachaud, 2001: Denitrification and N2 fixation in the Pacific Ocean. Global Biogeochemical Cycles, 15(2), 483-506. Abstract PDF
We establish the fixed nitrogen budget of the Pacific Ocean based on nutrient fields from the recently completed World Ocean Circulation Experiment (WOCE). The budget includes denitrification in the water column and sediments, nitrogen fixation, atmospheric and riverine inputs, and nitrogen divergence due to the large-scale circulation. A water column denitrification rate of 48 ± 5 Tg N yr-1 is calculated for the Eastern Tropical Pacific using N* [Gruber and Sarmiento, 1997] and water mass age tracers. On the basis of rates in the literature, we estimate sedimentary denitrification to remove an additional 15 ± 3 Tg N yr-1 . We then calculate the total nitrogen divergence due to the large scale circulation through the basin, composed of flows through a zonal transect at 32°S, and through the Indonesian and Bering straits. Adding atmospheric deposition and riverine fluxes results in a net divergence of nitrogen from the basin of -4 ± 12 Tg N yr-1 . Pacific nitrogen fixation can be extracted as a residual component of the total budget, assuming steady state. We find that nitrogen fixation would have to contribute 59 ± 14 Tg N yr-1 in order to balance the Pacific nitrogen budget. This result is consistent with the tentative global extrapolations of Gruber and Sarmiento [1997], based on nitrogen fixation rates estimated for the North Atlantic. Our estimated mean areal fixation rate is within the range of direct and geochemical rate estimates from a single location near Hawaii [Karl et al., 1997]. Pacific nitrogen fixation occurs primarily in the western part of the subtropical gyres where elevated N* signals are found. These regions are also supplied with significant amounts of iron via atmospheric dust deposition, lending qualitative support to the hypothesis that nitrogen fixation is regulated in part by iron supply.
Gloor, M, Nicolas Gruber, T M C Hughes, and Jorge L Sarmiento, 2001: Estimating net air-sea fluxes from ocean bulk data: Methodology and application to the heat cycle. Global Biogeochemical Cycles, 15(4), 767-782. Abstract PDF
A novel method to estimate annual mean heat, water, and gas exchange fluxes between the ocean and the atmosphere is proposed that is complementary to the traditional approach based on air-sea gradients and bulk exchange parameterization. The new approach exploits the information on surface exchange fluxes contained in the distribution of temperature, salinity, and dissolved gases in the ocean interior. We use an Ocean General Circulation Model to determine how the distribution in the ocean interior is linked to surface fluxes. We then determine with least squares the surface fluxes that are most compatible with the observations. To establish and test the method, we apply it to ocean temperature data to estimate heat fluxes across the air-sea interface for which a number of climatological estimates exists. We also test the sensitivity of the inversion results to data coverage, differences in ocean transport, variations in the surface flux pattern and a range of spatial resolutions. We find, on the basis of the World Ocean Circulation Experiment (WOCE) data network augmented with selected high-quality pre-WOCE data, that we are able to constrain heat exchange fluxes for 10 - 15 regions of the ocean, whereby these fluxes nearly balance globally without enforcing a conservation constraint. Our results agree well with heat flux estimates on the basis of bulk exchange parameterization, which generally require constraints to ensure a global net heat flux of zero. We also find that the heat transports implied by our inversely estimated fluxes are in good agreement with a large range of heat transport estimates based on hydrographic data. Increasing the number of regions beyond the 10 - 15 regions considered here is severely limited because of modeling errors. The inverse method is fairly robust to the modeling of the spatial patterns of the surface fluxes; however, it is quite sensitive to the modeling of ocean transport. The most striking difference between our estimates and the heat flux climatologies is a large heat loss of 0.64 PW to the atmosphere from the Southern Ocean and a large heat gain by the subpolar South Atlantic of 0.56 PW. These results are consistent with the large gain of carbon dioxide called for by Takahashi et al. [1999] in his recent analysis of the air-sea flux of carbon dioxide but inconsistent with the large loss of oxygen and carbon dioxide such as those of Stephens et al. [1998].
Gruber, Nicolas, M Gloor, Songmiao Fan, and Jorge L Sarmiento, 2001: Air-sea flux of oxygen estimated from bulk data: Implications for the marine and atmospheric oxygen cycles. Global Biogeochemical Cycles, 15(4), 783-803. Abstract PDF
We estimate the annual net air-sea fluxes of oxygen for 13 regions on the basis of a steady state inverse modeling technique that is independent of air-sea gas exchange parameterizations. The inverted data consist of the observed oceanic oxygen concentration after a correction has been applied to account for biological cycling. We find that the tropical oceans (13°S-13°N) emit ~212 Tmol O2 yr -1 , which is compensated by uptake of 148 Tmol yr-1 in the Northern Hemisphere (>13°N) and by uptake of 65 Tmol yr-1 in the Southern Hemisphere (<13°S). These results imply that the dominant feature of oxygen transport in the combined ocean-atmosphere system is the existence of a closed circulation cell in each hemisphere. These two cells consist of O2 uptake by the ocean in the middle and high latitudes of both hemispheres and transport in the ocean toward the tropics, where O2 is lost to the atmosphere and transported in the atmosphere back toward the poles. We find an asymmetry in the two cells involving O2 uptake in the temperate regions of the Northern Hemisphere versus loss of O2 in the temperate regions of the Southern Hemisphere. There is an additional asymmetry between the Atlantic basin, which has a net southward transport at all latitudes north of 36°S, in agreement with independent transport estimates, versus the Indian and Pacific Oceans, which have a strong equatorward transport everywhere. We find that these inverse estimates are relatively insensitive to details in the inversion scheme but are sensitive to biases in the ocean general circulation model that provides the linkage between surface fluxes and ocean interior concentrations. Forward simulations of O2 in an atmospheric tracer transport model using our inversely estimated oxygen fluxes as a boundary condition agree reasonably well with observations of atmospheric potential oxygen (APO O2 + CO2 ). Our results indicate that the north-south asymmetry in the strength of the two hemispheric cells coupled with a strong asymmetry in fossil fuel emissions can explain much of the observed interhemispheric gradient in APO. Therefore it might not be necessary to invoke the existence of a large southward interhemispheric transport of O2 in the ocean, such as proposed by Stephens et al. [1998]. However, we find that uncertainties in the modeled APO distribution stemming from seasonal atmospheric rectification effects and the limited APO data coverage prevent the currently available APO data from providing strong constraints on the magnitude of interhemispheric transport.
Orr, James C., E Maier-Reimer, U Mikolajewicz, Patrick Monfray, Jorge L Sarmiento, J R Toggweiler, N K Taylor, J Palmer, Nicolas Gruber, C L Sabine, C Le Quéré, Robert M Key, and J Boutin, 2001: Estimates of anthropogenic carbon uptake from four three-dimensional global ocean models. Global Biogeochemical Cycles, 15(1), 43-60. Abstract PDF
We have compared simulations of anthropogenic CO2 in the four three-dimensional ocean models that participated in the first phase of the Ocean Carbon-Cycle Model Intercomparison Project (OCMIP), as a means to identify their major differences. Simulated global uptake agrees to within ± 19%, giving a range of 1.85±0.35 Pg C yr -1 for the 1980-1989 average. Regionally, the Southern Ocean dominates the present-day air-sea flux of anthropogenic CO2 in all models, with one third to one half of the global uptake occurring south of 30°S. The highest simulated total uptake in the Southern Ocean was 70% larger than the lowest. Comparison with recent data-based estimates of anthropogenic CO2 suggest that most of the models substantially overestimate storage in the Southern Ocean; elsewhere they generally underestimate storage by less than 20%. Globally, the OCMIP models appear to bracket the real ocean's present uptake, based on comparison of regional data-based estimates of anthropogenic CO2 and bomb 14C. Column inventories of bomb 14C have become more similar to those for anthropogenic CO2 with the time that has elapsed between the Geochemical Ocean Sections Study (1970s) and World Ocean Circulation Experiment (1990s) global sampling campaigns. Our ability to evaluate simulated anthropogenic CO2 would improve if systematic errors associated with the date-based estimates could be provided regionally.
For the period 1980–89, we estimate a carbon sink in the coterminous United States between 0.30 and 0.58 petagrams of carbon per year (petagrams of carbon = 1015 grams of carbon). The net carbon flux from the atmosphere to the land was higher, 0.37 to 0.71 petagrams of carbon per year, because a net flux of 0.07 to 0.13 petagrams of carbon per year was exported by rivers and commerce and returned to the atmosphere elsewhere. These land-based estimates are larger than those from previous studies (0.08 to 0.35 petagrams of carbon per year) because of the inclusion of additional processes and revised estimates of some component fluxes. Although component estimates are uncertain, about one-half of the total is outside the forest sector. We also estimated the sink using atmospheric models and the atmospheric concentration of carbon dioxide (the tracer-transport inversion method). The range of results from the atmosphere-based inversions contains the land-based estimates. Atmosphere- and land-based estimates are thus consistent, within the large ranges of uncertainty for both methods. Atmosphere-based results for 1980–89 are similar to those for 1985–89 and 1990–94, indicating a relatively stable U.S. sink throughout the period.
Gloor, M, Songmiao Fan, Stephen W Pacala, and Jorge L Sarmiento, 2000: Optimal sampling of the atmosphere for purpose of inverse modeling: A model study. Global Biogeochemical Cycles, 14(1), 407-428. Abstract PDF
The 66 stations of the GLOBALVIEW-CO2 sampling network (GLOBALVIEW-CO2: Cooperative Atmospheric Data Integration Project - Carbon Dioxide, (1997)) are located primarily remotely from continents where signals of fossil fuel consumption and biospheric exchange are diluted. It is thus not surprising that inversion studies are able to estimate terrestrial sources and sinks only to a very limited extent. The poor constraint on terrestrial fluxes propagates to the oceans and strongly limits estimates of oceanic fluxes as well, at least if no use is made of other information such as isotopic ratios. We analyze here the resolving power of the GLOBALVIEW-CO2 network, compare the efficiency of different measurement strategies, and determine optimal extensions to the present network. We find the following: (1) GLOBALVIEW-CO2 is well suited to characterize the meridional distribution of sources and sinks but is poorly suited to separate terrestrial from oceanic sinks at the same latitude. The most poorly constrained regions are South America, Africa, and southern hemispheric oceans. (2) To improve the network, observing stations need to be positioned on the continents near to the largest biospheric signals despite the large diurnal and seasonal fluctuations associated with biological activity and the dynamics of the PBL. The mixing in the atmosphere is too strong to allow positioning of stations remote from large fluxes. Our optimization results prove to be fairly insensitive to the details of model transport and the inversion model with the addition of ~ 10 optimally positioned stations. (3) The best measurement strategy among surface observations, N-S airplane transects, and vertical profiles proves to be vertical profiles. (4) Approximately 12 optimally positioned vertical profiles or 30 surface stations in addition to GLOBALVIEW-CO2 would reduce estimate uncertainties caused by insufficient data coverage from ~ 1 Pg C yr -1 per region to ~ 0.2 Pg C yr -1 per region.
Murnane, R, and Jorge L Sarmiento, 2000: Roles of biology and gas exchange in determining the 13C distribution in the ocean and the preindustrial gradient in atmospheric 13C. Global Biogeochemical Cycles, 14(1), 389-405. Abstract PDF
We examine the processes responsible for the distribution of 13C in a global ocean model. The dominant sources of gradients are biological processes and the temperature effect on isotopic fractionation. However, in a model without biology developed to examine the temperature effect of isotropic fractionation in isolation, we find an almost uniform 13C distribution. Extremely slow 13C air-sea equilibrium does not permit the surface ocean to come into equilibrium with the atmosphere and 13C in the ocean thus becomes well mixed. However biological effects, which are interior to the ocean, are strongly expressed and minimally effected by air-sea exchange. Biological fractionation thus dominates the oceanic 13C distribution. An important feature of the model is an extremely large northward transport of isotopic anomaly. The transfer from the ocean to the Northern Hemisphere atmosphere of 120 Pg C 0/00 is equivalent in magnitude to the signal that would be generated by a net terrestrial biospheric uptake of 5 Pg C yr -1 from the Northern Hemisphere atmosphere, or an 1-2 0/00 disequilibrium between terrestrial respiration and photosynthesis. Improved ocean model simulations and observational analysis are required to test for the possible existence of such a large oceanic transport of isotopic anomaly,
Sarmiento, Jorge L., Patrick Monfray, E Maier-Reimer, Olivier Aumont, R Murnane, and James C Orr, 2000: Sea-air CO2 fluxes and carbon transport: A comparison of three ocean general circulation models. Global Biogeochemical Cycles, 14(4), 1267-1281. Abstract PDF
Many estimates of the atmospheric carbon budget suggest that most of the sink for CO2 produced by fossil fuel burning and cement production must be in the Northern Hemisphere. Keeling et al. [1989] hypothesized that this asymmetry could be explained instead by a northward preindustrial transport of ~1 Pg C y-1 in the atmosphere balanced by an equal and opposite southward transport in the ocean. We explore this hypothesis by examining the processes that determine the magnitude of the preindustrial interhemispheric flux of carbon in three ocean carbon models. This study is part of the first stage of the Ocean Carbon Model Intercomparison Project organized by International Geosphere Biosphere Programme Global Analysis, Interpretation, and Modelling Task Force. We find that the combination of interhemispheric heat transport (with its associated carbon transport), a finite gas exchange, and the biological pump, yield a carbon flux of only -0.12 to +0.04 Pg C y-1 across the equator (positive to the north). An important reason for the low carbon transport is the decoupling of the carbon flux from the interhemispheric heat transport due to the long sea-air equilibration time for surface CO2. A possible additional influence on the interhemispheric exchange is oceanic transport of carbon from rivers.
Suntharalingam, P, and Jorge L Sarmiento, 2000: Factors governing the oceanic nitrous oxide distribution: Simulations with an ocean general circulation model. Global Biogeochemical Cycles, 14(1), 429-454. Abstract PDF
A global model of the oceanic nitrous oxide distribution is developed to evaluate current understanding of the processes governing nitrous oxide formation and distribution in the open ocean. N2O is treated as a nonconserved tracer in a global ocean general circulation model subject to biological sources in the oceanic interior and gas exchange at the ocean surface. A simple scalar parameterization linking N2O production to oxygen consumption (and based on observed correlations between excess N2O and apparent oxygen utilization) is suceessful in reproducing the large-scale features of the observed distribution, namely, high surface supersaturations in regions of upwelling and biological productivity, and values close to equilibrium in the oligotrophic subtropical gyres. The majority of the oceanic N2O source is produced in the upper water column (over 75% above 600 m) and effluxes directly to the atmosphere in the latitude band of formation. The observed structure at depth is not as well reproduced by this model, which displays excessive N2O production in the deep ocean. An alternative source of parameterization, which accounts for processes which result in a depth variation in the relationship between N2O production and oxygen consumption, yields an improved representation of the deep distribution. The surface distribution and sea-air flux are, however, determined primarily by the upper ocean source and, therefore, are relatively insensitive to changes in the nature of deep oceanic N2O production.
Suntharalingam, P, Jorge L Sarmiento, and J R Toggweiler, 2000: Global significance of nitrous-oxide production and transport from oceanic low-oxygen zones: A modeling study. Global Biogeochemical Cycles, 14(4), 1353-1370. Abstract PDF
Recent studies of marine nitrous oxide have focused attention on the suboxic and low-oxygen zones associated with ocean basin eastern boundaries. It has been suggested that complex N2O cycling mechanisms in these regions may provide a net source to the oceanic interior and a significant portion of the ocean-atmosphere flux. In this study we evaluate the global significance of N2O formation in these regions. N2O is treated as a nonconserved tracer in an ocean general circulation model: a simple source function is developed which models N2O production as a function of organic matter remineralization and local oxygen concentration. Model results are evaluated against both surface and deep observational data sets. The oceanic oxygen minimum zones are predominantly found in the upperwater column of tropical latitudes and overlain by regions of strong upwelling in the surface ocean. Simulations of increased N2O production under low-oxygen conditions indicate that the majority of the N2O thus formed escapes directly to the atmosphere and is not subject to significant meridional transport. Results indicate that while enhanced N2O production in these regions cannot be held accountable for the majority of the sea-air flux and interior distribution, it may, however, have significance for the local distribution and provide as much as 25-50% of the global oceanic source.
Fan, Songmiao, T Blaine, and Jorge L Sarmiento, 1999: Terrestrial carbon sink in the Northern Hemisphere estimated from the atmospheric CO2 difference between Mauna Loa and the South Pole since 1959. Tellus B, 51B(5), 863-870. Abstract PDF
The difference between Mauna Loa and South Pole atmospheric CO2 concentrations from 1959 to the present scales linearly with CO2 emissions from fossil fuel burning and cement production (together called fossil CO2). An extrapolation to zero fossil CO2 emission has been used to suggest that the atmospheric CO2 concentration at Mauna Loa was 0.8 ppm less than that at the South Pole before the industrial revolution, associated with a northward atmospheric transport of about 1 Gt C yr-1 (Keeling et al., 1989a). Mass conservation requires an equal southward transport in the ocean. However, our ocean general circulation and biogeochemistry model predicts a much smaller pre-industrial carbon transport. Here, we present a new analysis of the Mauna Loa and South Pole CO2 data, using a general circulation model and a 2-box model of the atmosphere. It is suggested that the present CO2 difference between Mauna Loa and the South Pole is caused by, in addition to fossil CO2 sources and sinks, a pre-industrial interhemispheric flux of 0.5-0.7 Gt C yr-1 , and a terrestrial sink of 0.8-1.2 Gt C yr-1 in the mid-latitude Northern Hemisphere, balanced by a tropical deforestation source that has been operating continuously in the period from 1959 to the present.
Fan, Songmiao, Jorge L Sarmiento, M Gloor, and Stephen W Pacala, 1999: On the use of regularization techniques in the inverse modeling of atmospheric carbon dioxide. Journal of Geophysical Research, 104(D17), 21,503-21,512. Abstract PDF
The global distribution of carbon sources and sinks is estimated from atmospheric CO2 measurements using an inverse method based on the Geophysical Fluid Dynamics Laboratory SKYHI atmospheric general circulation model. Applying the inverse model without any regularization yields unrealistically large CO2 fluxes in the tropical regions. We examine the use of three regularization techniques that are commonly used to stabilize inversions: truncated singular value decomposition, imposition of a priori flux estimates, and use of a quadratic inequality constraint. The regularization techniques can all be made to minimize the unrealistic fluxes in the tropical regions. This brings inversion estimated CO2 fluxes for oceanic regions in the tropics and in the Southern Hemisphere into better agreement with independent estimates of the air-sea exchange. However, one cannot assume that stabilized inversions give accurate estimates, as regularization merely holds the fluxes to a priori estimates or simply reduces them in magnitude in regions that are not resolvable by observations. By contrast, estimates of flux and uncertainty for the temperate North Atlantic, temperate North Pacific, and boreal and temperate North American regions are far less sensitive to the regularization parameters, consistent with the fact that these regions are better constrained by the present observations.
Gloor, M, Songmiao Fan, Stephen W Pacala, Jorge L Sarmiento, and M Ramonet, 1999: A model-based evaluation of inversions of atmospheric transport, using annual mean mixing ratios, as a tool to monitor fluxes of nonreactive trace substances like CO2 on a continental scale. Journal of Geophysical Research, 104(D12), 14,245-14,260. Abstract PDF
The inversion of atmospheric transport of CO2 may potentially be a means for monitoring compliance with emission treaties in the future. There are two types of errors though, which may cause errors in inversions: (1) amplification of high-frequency data variability given the information loss in the atmosphere by mixing and (2) systematic errors in the CO2flux estimates caused by various approximations used to formulate the inversions. In this study we use simulations with atmospheric transport models and a time independent inverse scheme to estimate these errors as a function of network size and the number of flux regions solved for. Our main results are as follows: (1) When solving for 10-20 source regions, the average uncertainty of flux estimates caused by amplification of high-frequency data variability alone decreases strongly with increasing number of stations for up to ~150 randomly positions stations and then levels off (for 150 stations of the order of ±0.2 Pg C yr-1). As a rule of thumb, about 10 observing stations are needed per region to be estimated. (2) Of all the sources of systematic errors, modeling error is the largest. Our estimates of SF6 emissions from five continental regions simulated with 12 different AGCMs differ by up to a factor of 2. The number of observations needed to overcome the information loss due to atmospheric mixing is hence small enough to permit monitoring of fluxes with inversions on a continental scale in principle. Nevertheless errors in transport modeling are still too large for inversions to be a quantitatively reliable option for flux monitoring.
Murnane, R, Jorge L Sarmiento, and C Le Quéré, 1999: Spatial distribution of air-sea CO2 fluxes and the interhemispheric transport of carbon by the oceans. Global Biogeochemical Cycles, 13(2), 287-305. Abstract PDF
The dominant processes controlling the magnitude and spatial distribution of the preindustrial air-sea flux of CO2 are atmosphere-ocean heat exchange and the biological pump, coupled with the direct influence of ocean circulation resulting from the slow time-scale of air-sea CO2 gas exchange equilibration. The influence of the biological pump is greatest in surface outcrops of deep water, where the excess deep ocean carbon resulting from net remineralization can escape to the atmosphere. In a steady state other regions compensate for this loss by taking up CO2 to give a global net air-sea CO2 flux of zero. The predominant outcrop region is the Southern Ocean, where the loss to the atmosphere of biological pump CO2 is large enough to cancel the gain of CO2 due to cooling. The influence of the biological pump on uptake of anthropogenic CO2 is small: a model including biology takes up 4.9% less than a model without it. Our model does not predict the large southward interhemispheric transport of CO2 that has been suggested by atmospheric carbon transport constraints.
Sabine, C L., Robert M Key, K M Johnson, F J Millero, A Poisson, Jorge L Sarmiento, D W R Wallace, and C D Winn, 1999: Anthropogenic CO2 inventory of the Indian Ocean. Global Biogeochemical Cycles, 13(1), 179-198. Abstract PDF
This study presents basin-wide anthropogenic CO2 inventory estimates for the Indian Ocean based on measurements from the World Ocean Circulation Experiment/Joint Global Ocean Flux Study global survey. These estimates employed slightly modified d C* and time series techniques originally proposed by Gruber et al. [1996] and Wallace [1995], respectively. Together, the two methods yield the total oceanic anthropogenic CO2 and the carbon increase over the past 2 decades. The highest concentrations and the deepest penetrations of anthropogenic carbon are associated with the Subtropical Convergence at around 30° to 40°S. With both techniques, the lowest anthropogenic CO2 column inventories are observed south of 50°S. The total anthropogenic CO2 inventory north of 35°S was 13.6 ± 2 Pg C in 1995. The inventory increase since GEOSECS (Geochemical Ocean Sections Program) was 4.1 ± 1 Pg C for the same area. Approximately 6.7 ± 1 Pg C are stored in the Indian sector of the Southern Ocean, giving a total Indian Ocean inventory of 20.3 ± 3 Pg C for 1995. These estimates are compared to anthropogenic CO2 inventories estimated by the Princeton ocean biogeochemistry model. The model predicts an Indian Ocean sink north of 35°S that is only 0.61-0.68 times the results presented here; while the Southern Ocean sink is nearly 2.6 times higher than the measurement-based estimate. These results clearly identify areas in the models that need further examination and provide a good baseline for future studies of the anthropogenic inventory.
Sarmiento, Jorge L., and T M C Hughes, 1999: Anthropogenic CO2 uptake in a warming ocean. Tellus B, 51B(2), 560-561. PDF
Fan, Songmiao, M Gloor, Jerry D Mahlman, Stephen W Pacala, Jorge L Sarmiento, T Takahashi, and P P Tans, 1998: A large terrestrial carbon sink in North America implied by atmospheric and oceanic carbon dioxide data and models. Science, 282(5388), 442-446. Abstract PDF
Atmospheric carbon dioxide increased at a rate of 2.8 petagrams of carbon per year (Pg C year-1) during 1988 to 1992 (1 Pg = 1015 grams). Given estimates of fossil carbon dioxide emissions, and net oceanic uptake, this implies a global terrestrial uptake of 1.0 to 2.2 Pg C year-1. The spatial distribution of the terrestrial carbon dioxide uptake is estimated by means of the observed spatial patterns of the greatly increased atmospheric carbon dioxide data set available from 1988 onward, together with two atmospheric transport models, two estimates of the sea-air flux, and an estimate of the spatial distribution of fossil carbon dioxide emissions. North America is the best constrained continent, with a mean uptake of 1.7 ± 0.5 Pg C year-1, mostly south of 51 degrees north. Eurasia-North Africa is relatively weakly constrained, with a mean uptake of 0.1 ± 0.6 Pg C year-1. The rest of the world's land surface is poorly constrained, with a mean source of 0.2 ± 0.9 Pg C year-1.
A 1995 report of the Intergovernmental Panel on Climate Change provides a set of illustrative anthropogenic CO2 emission models leading to stabilization of atmospheric CO2 concentrations ranging from 350 to 1,000 p.p.m. (refs 1 - 4). Ocean carbon-cycle models used in calculating these scenarios assume that oceanic circulation and biology remain unchanged through time. Here we examine the importance of this assumption by using a coupled atmosphere-ocean model of global warming for the period 1765 to 2065. We find a large potential modification to the ocean carbon sink in a vast region of the Southern Ocean where increased rainfall leads to surface freshening and increased stratification. The increased stratification reduces the downward flux of carbon and the loss of heat to the atmosphere, both of which decrease the oceanic uptake of anthropogenic CO2 relative to a constant-climate control scenario. Changes in the formation, transport and cycling of biological material may counteract the reduced uptake, but the response of the biological community to the climate change is difficult to predict on present understanding. Our simulation suggests that such physical and biological changes might already be occurring, and that they could substantially affect the ocean carbon sink over the next few decades.
Gruber, Nicolas, and Jorge L Sarmiento, 1997: Global patterns of marine nitrogen fixation and denitrification. Global Biogeochemical Cycles, 11(2), 235-266. Abstract PDF
A new quasi-conservative tracer N*, defined as a linear combination of nitrate and phosphate, is proposed to investigate the distribution of nitrogen fixation and denitrification in the world oceans. Spatial patterns of N* are determined in the difference ocean basins using data from the Geochemical Ocean Sections Study (GEOSECS) cruises (1972-1978) and from eight additional cruises in the Atlantic Ocean. N* is low (< -3 µmol kg-1) in the Arabian Sea and in the eastern tropical North and South Pacific. This distribution is consistent with direct observations of water column denitrification in these oxygen minimum zones. Low N*concentrations in the Bering Sea and near the continental shelves of the east and west coasts of North America also indicate a sink of N* due to benthic denitrification. High concentrations of N* (>2.0 µmol kg--1) indicative of prevailing nitrogen fixation are found in the thermocline of the tropical and subtropical North Atlantic and in the Mediterranean. This suggests that on a global scale these basins are acting as sources of fixed nitrogen, while the Indian Ocean and parts of the Pacific Ocean are acting as sinks. Nitrogen fixation is estimated in the North Atlantic Ocean (10° N - 50° N) using the N* distribution along isopycnal surfaces and information about the water age. We calculate a fixation rate of 28 Tg N yr-1 which is about 3 times larger than the most recent global estimate. Our result is in line, however, with some recent suggestions that pelagic nitrogen fixation may be seriously underestimated. The implied flux of 0.072 mol N m-2 yr-1 is sufficient to meet all the nitrogen requirement of the estimated net community production in the mixed layer during summer at the Bermuda Atlantic Time-series Study (BATS) site in the northwestern Sargasso Sea. Extrapolation of our North Atlantic estimate to the global ocean suggests that the present-day budget of nitrogen in the ocean may be in approximate balance.
Gruber, Nicolas, Jorge L Sarmiento, and T F Stocker, 1996: An improved method for detecting anthropogenic CO2 in the oceans. Global Biogeochemical Cycles, 10(4), 809-837. Abstract PDF
An improved method has been developed for the separation of the anthropogenic CO2 from the large natural background variability of dissolved inorganic carbon (C) in the ocean. This technique employs a new quasi-conservative carbon tracer Delta C*, which reflects the uptake of anthropogenic CO2 and the air-sea disequilibrium when a water parcel loses contact with the atmosphere. The air-sea disequilibrium component can be discriminated from the anthropogenic signal using either information about the water age or the distribution of Delta C* in regions not affected by the anthropogenic transient. This technique has been applied to data from the North Atlantic sampled during the Transient Tracers in the Ocean North Atlantic (TTO NAS) and Tropical Atlantic study (TTO TAS) cruises in 1981-1983. The highest anthropogenic CO2 concentrations and specific inventories (inventory per square meter) are found in the subtropical convergence zone. In the North Atlantic, anthropogenic CO2 has already invaded deeply into the interior of the ocean, north of 50°N it has even reached the bottom. Only waters below 3000 m and south of 30°N are not yet affected. We estimate an anthropogenic CO2 inventory of 20 plus or minus 4 Gt C in the North Atlantic between 10°N and 80°N. The 2.5-dimensional ocean circulation model of Stocker et al. [1994] and the three-dimensional ocean general circulation biogeochemistry model of Sarmiento et al. [1995] predict anthropogenic CO2 inventories of 18.7 Gt C and 18.4 Gt C, respectively, in good agreement with the observed inventory. Important differences exist on a more regional scale, associated with known deficiencies of the models.
Joos, Fortunat, M S Bruno, R Fink, U Siegenthaler, T F Stocker, C Le Quéré, and Jorge L Sarmiento, 1996: An efficient and accurate representation of complex oceanic and biospheric models of anthropogenic carbon uptake. Tellus B, 48B, 397-417. Abstract PDF
Establishing th link between atmospheric CO2 concentration and anthropogenic carbon emissions requires the development of complex carbon cycle models of the primary sinks, the ocean and terrestrial biosphere. Once such models have been developed, the potential exists to use pulse response functions to characterize their behaviour. However, the application of response functions based on a pulse increase in atmospheric CO2 to characterize oceanic uptake, the conventional technique, does not yield a very accurate result due to nonlinearities in the aquatic carbon chemistry. Here, we propose the useof an ocean mixed-layer pulse response function that characterizes the surface to deep ocean mixing in combination with a separate equation describing air-sea exchange. The use of a mixed-layer pulse response function avoids the problem arising from the nonlinearities of the carbon chemistry and gives therefore more accurate results. The response function is also valid for tracers other than carbon. We found that tracer uptake of the HILDA and Box-Diffusion model can be represented exactly by the new method. For the Princeton 3-D model, we find that the agreement between the complete model and its pulse substitute is better than 4% for the cumulative uptake of anthropogenic carbon for the period 1765 to 2300 applying the IPCC stabilization scenarios S450 and S750 and better than 2% for the simulated inventory and surface concentration of bomb-produced radiocarbon. By contrast, the use of atmospheric response functions gives deviations up to 73% for the cumulative CO2 uptake as calculated with the Princeton 3-D model. We introduce the use of a decay response function for calculating the potential carbon storage on land as a substitute for terrestrial biosphere models that describe the overturning of assimilated carbon. This, in combination with an equation describing the net primary productivity permits us to exactly characterize simple biosphere models. As the time scales of biospheric overturning are one key aspect to determine the amount of anthropogenic carbon which might be sequestered by the biosphere, we suggest that decay response functions should be used as a simple and standardized measure to compare different models and to improve understanding of their behaviour. We provide analytical formulations for mixed-layer and terrestrial biosphere decay pulse response functions which permit us to easily build a substitute for the "Bern" carbon cycle model (HILDA). Furthermore, mixed-layer response functions for the Box-Diffusion, a 2-D model, and the Princeton 3-D model are given.
Sarmiento, Jorge L., and C Le Quéré, 1996: Oceanic carbon dioxide uptake in a model of century-scale global warming. Science, 274(5291), 1346-1350. Abstract PDF
In a model of ocean-atmosphere interaction that excluded biological proceses, the oceanic uptake of atmospheric carbon dioxide (CO2) was substantially reduced in scenarios involving global warming relative to control scenarios. The primary reason for the reduced uptake was the weakening or collapse of the ocean thermohaline circulation. Such a large reduction in this ocean uptake would have a major inpact on the future growth rate of atmospheric CO2. Model simulations that include a simple representation of biological processes show a potentially large offsetting effect resulting from the downward flux of biogenic carbon. However, the magnitude of the offset is difficult to quantify with present knowledge.
Anderson, L A., and Jorge L Sarmiento, 1995: Global ocean phosphate and oxygen simulations. Global Biogeochemical Cycles, 9(4), 621-636. Abstract PDF
We examine the role of dissolved organic matter (DOM) and the stoichiometric ratios of organic matter remineralization in determining the magnitude and distribution of remineralization of organic matter in the oceanic water column, and the impact of this remineralization on tracer distributions. Our aim is to improve the parameterization of relevant processes in ocean general circulation models by bringing the models into closer agreement with new observational constraints that suggest substantial differences from previous work (lower DOM levels and higher -O2/P ratios). We used phosphate and apparent oxygen utilization (AOU) to analyze the effect of the remineralization profile on water column tracer distributions. The primary impact of DOM cycling is to modify the distribution of remineralization over that obtained from a model with particle cycling only. Changing the oxygen-to-phosphorous stoichiometric ratio modifies the magnitude of oxygen utilization, but preserves its basic distribution. We find that even a small amount of DOM is sufficient to prevent the problem of nutrient trapping. Improved phosphate and AOU simulations are obtained when the amount of DOM is reduced and the deep ocean -O2/P ratio is increased in accord with recent observations of these properties.
Joos, Fortunat, and Jorge L Sarmiento, 1995: Der anstieg des atmospharischen kohlendioxids. Physikalische Blatter, 51(5), 405-411.
Sarmiento, Jorge L., C Le Quéré, and Stephen W Pacala, 1995: Limiting future atmospheric carbon dioxide. Global Biogeochemical Cycles, 9(1), 121-137. Abstract PDF
We estimate anthropogenic carbon emissions required to stabilize future atmospheric CO at various levels ranging from 350 ppm to 750 ppm. Over the next three centuries, uptake by the ocean and terrestrial biosphere would permit emissions to be 3 to 6 times greater than the total atmospheric increase, with each of them contributing approximately equal amounts. Owing to the nonlinear dependence of oceanic and terrestrial biospheric uptake on CO concentration, the uptake by these two sinks decreases substantially at higher atmospheric CO levels. The uptake also decreases with increased atmospheric CO growth rate. All the stabilization scenarios require a substantial future reduction in emissions.
Shaffer, G, and Jorge L Sarmiento, 1995: Biogeochemical cycling in the global ocean. 1. A new, analytical model with continuous vertical resolution and high-latitude dynamics. Journal of Geophysical Research, 100(C2), 2659-2672. Abstract PDF
A new, simple analytical model of ocean chemistry is presented which includes continuous vertical resolution, high-latitude dynamics, air-sea exchange and sea ice cover. In this high-latitude exchange/interior diffusion-advection (HILDA) model, ocean physics are represented by four parameters: k and w, an eddy diffusion coefficient and a deep upwelling velocity in the stratified interior; q, a rate of lateral exchange between the interior and a well-mixed, deep polar ocean; and u, an exchange velocity between surface and deep layers in the polar ocean. First, estimates are made of ice-free and ice-covered areas at high latitudes, surface temperatures, and air-sea exchange velocities from available data. Then values of the physical parameters are estimated from simultaneous, least mean square fits of model solutions for temperature (T) and "abiotic" carbon 14 to interior profiles of T and carbon 14 and surface layer carbon 14 values all derived from available data. Best fit values for k, w, q, and u are 3.2x10-5 m2 s-1, 2.0x10-8 m s -1, 7.5x10-11 s-1 and 1.9x10-6 m s-1 respectively. These results are interpreted in terms of modes of ocean circulation and mixing and compared with results from other simpler and more complex models. In parts 2 and 3 of this series, these values for k, w, q and u are taken as inputs for studying phosphorus, oxygen, and carbon cycling in the global ocean with the HILDA model.
Suntharalingam, P, and Jorge L Sarmiento, 1995: Modeling global air-sea N2O fluxes - A sensitivity analysis of the gas-exchange formulation In Air-Water Gas Transfer, Selected papers from the Third International Symposium on Air-Water Gas Transfer, July 24-27, 1995,, AEON Verlag & Studio, 843-853. Abstract
Two models of the global oceanic N2O flux distribution are presented, and the sensitivity of these models to the gas-exchange formulation is examined. The N2O models discussed are a multi-variate regression method based on surface delta p N2O measurements from Weiss et al. [1992], and a model of the oceanic N2O cycle embedded in the ocean general circulation model (OGCM) of Toggweiler et al. [1989]. The gas-exchange parameterizations considered are from Wanninkhof [1992] and Liss and Merlivat [1986]. Results indicate that the formulation of Wanninkhof [1992] may be better suited for modeling oceanic N2O fluxes using a data-based model or an OGCM.
Anderson, L A., and Jorge L Sarmiento, 1994: Redfield ratios of remineralization determined by nutrient data analysis. Global Biogeochemical Cycles, 8(1), 65-80. Abstract PDF
A nonlinear inverse method is applied to nutrient data upon approximately 20 neutral surfaces in each of the South Atlantic, Indian, and Pacific basins, between 400 and 4000 m depth. By accounting for the gradients in nutrients due to the mixing of "preformed" concentrations of the major water masses, the nutrient changes due to biological activity are examined, and the time-mean, basin-wide Redfield ratios calculated. It is found that the P/N/Corg-O2 ratios of nutrient regeneration between 400 and 4000 m (corrected for the effect of denitification) are approximately constant with depth and basin, at a value of 1/16 ± 1/117 ± 14/170 ± 10. These ratios agree with those of fresh organic matter, suggesting that the flux of organic material to the deep ocean may be dominated by fast-sinking matter produced by sporadic, high-productivity events. Sedimentary denitrification reduces the N/P utilization ratio to 12 ± 2 between 1000 and 3000 m. In the Indian and Pacific basins the Corg/Cinorg regeneration ratio decreases from approximately 7 ± 3 at 400 m to 3 ± 1 at 1000 m and to 1 ± 0.5 at 4000 m, suggesting a significant amount of calcium carbonate dissolution above the calcite lysoclines in the Indian and Pacific Oceans.
Murnane, R, J K Cochran, and Jorge L Sarmiento, 1994: Estimates of particle- and thorium-cycling rates in the northwest Atlantic Ocean. Journal of Geophysical Research, 99(C2), 3373-3392. Abstract PDF
We provide least squares estimates of particle-cycling rate constants and their errors at 13 depths in the Northwest Atlantic Ocean using a compilation of published results and conservation equations for thorium and particle-cycling. The predicted rates of particle aggregation and disaggregation vary through the water column. The means and standard deviations, based on lognormal probability distributions, for the lowest and highest rates of aggregation (B2) and disaggregation (B-2) in the water column are 8 ± 27 y-1 < B2 <18 ± 90 y-1, and 580 ± 2000 y-1< B-2<3 X103 ± 104 y-1. Median values for these rates are 2.1 y-1< B2< 3.2 y-1, and 149 y-1< B-2 < 156 y-1. Predicted rate constants for thorium adsorption (k1 = 5.0 ± 1.0x 104 m3 kg y-1) and desorption (k-1 = 3.1 ± 1.5 y-1) are consistent with previous estimates. Least squares estimates of the sum of the time dependence and transport terms from the particle and thorium conservation equations are on the same order as other terms in the conservation equations. Forcing this sum to equal zero would change the predicted rates. Better estimates of the time dependence of thorium activities and particle concentrations and of the concentration and flux of particulate organic matter would help to constrain estimates of B2 and B-2.
Fasham, M J., Jorge L Sarmiento, Richard D Slater, H W Ducklow, and R G Williams, 1993: Ecosystem behavior at Bermuda Station "S" and Ocean Weather Station "India": A general circulation model and observational analysis. Global Biogeochemical Cycles, 7(2), 379-415. Abstract
A model of biological production in the euphotic zone of the North Atlantic has been developed by coupling a seven-compartment nitrogen-based ecosystem model with a three-dimensional seasonal general circulation model. The predicted seasonal cycles of phytoplankton, zooplankton, bacteria, nitrate, ammonium, primary production, and particle flux have been compared to data from Bermuda Station "S" and Ocean Weather Station "India". Bearing in mind the simplicity of the model and the paucity of data, the results are encouraging. However, deficiencies in the physical model lead to winter nitrate values at Bermuda being overestimated, and at both positions the predicted magnitude of the spring phytoplankton bloom was too high. Simulations were carried out with different detrital sinking rates and it was found that a sinking rate of 10 m d-1 gave the best agreement with observations. The model was used to investigate the factors affecting the population growth of phytoplankton and it was found that the model supported the generally held theory that the spring bloom is initiated by the cessation of physical mixing. After the bloom, phytoplankton are controlled by zooplankton grazing. At Ocean Weather Station "India" the model reproduced the observed high summer nitrate levels and suggested that these high values are caused by a combination of high vertical nitrate transport, ammonium inhibition of nitrate uptake, and zooplankton grazing control. The model demonstrated the critical importance of zooplankton in understanding ecosystem dynamics and highlights the need for more observational data on the seasonal cycles of zooplankton biomass and growth rates.
Murphy, E, and Jorge L Sarmiento, et al., 1993: Global extrapolation In Towards a Model of Ocean Biogeochemical Processes, NATO Series I, Vol. 10, Berlin, Germany, Springer-Verlag, 21-46.
Sarmiento, Jorge L., Richard D Slater, M J Fasham, H W Ducklow, J R Toggweiler, and G T Evans, 1993: A seasonal three-dimensional ecosystem model of nitrogen cycling in the North Atlantic euphotic zone. Global Biogeochemical Cycles, 7(2), 417-450. Abstract
A seven-component upper ocean ecosystem model of nitrogen cycling calibrated with observations at Bermuda Station "S" has been coupled to a three-dimensional seasonal general circulation model (GCM) of the North Atlantic Ocean. The aim of this project is to improve our understanding of the role of upper ocean biological processes in controlling surface chemical distributions, and to develop approaches for assimilating large data sets relevant to this problem. A comparison of model predicted chlorophyll with satellite coastal zone color scanner observations shows that the ecosystem model is capable of responding realistically to a variety of physical forcing environments. Most of the discrepancies identified are due to problems with the GCM model. The new production predicted by the model is equivalent to 2 to 2.8 mol m-2 yr-1 of carbon uptake, or 8 to 12 GtC/yr on a global scale. The southern half of the subtropical gyre is the only major region of the model with almost complete surface nitrate removal (nitrate<0.1 mmol m-3). Despite this, almost the entire model is nitrate limited in the sense that any addition of nitrate supply would go predominately into photosynthesis. The only exceptions are some coastal upwelling regions and the high latitudes during winter, where nitrate goes as high as ~10 mmol m-3 .
Siegenthaler, U, and Jorge L Sarmiento, 1993: Atmospheric carbon dioxide and the ocean. Nature, 365(6442), 119-125. Abstract PDF
The ocean is a significant sink for anthropogenic carbon dioxide, taking up about a third of the emissions arising from fossil-fuel use and tropical deforestation. Increases in the atmospheric carbon dioxide concentration account for most of the remaining emissions, but there still appears to be a 'missing sink' which may be located in the terrestrial biosphere. Atmospheric CO2 measurements made from ice cores recovered ar Siple and Amundsen-Scott Stations were used in the preparation of this review.
Slater, Richard D., Jorge L Sarmiento, and M J Fasham, 1993: Some parametric and structural simulations with a three-dimensional ecosystem model of nitrogen cycling in the North Atlantic euphotic zone In Towards a Model of Ocean Biogeochemical Processes, NATO Series I, Vol. 10, Berlin, Germany, Springer-Verlag, 261-294.
Najjar, R G., Jorge L Sarmiento, and J R Toggweiler, 1992: Downward transport and fate of organic matter in the ocean: Simulations with a general circulation model. Global Biogeochemical Cycles, 6(1), 45-76. Abstract
A phosphorous-based model of nutrient cycling has been developed and used in conjunction with a general circulation model to evaluate the roles of the dissolved and sinking particulate phases in the downward transport of organic matter in the ocean. If sinking particles dominate the downward transport and remineralize in accord with observations made primarily with sediment traps, we find in equatorial upwelling regions that particle fluxes and thermocline nutrient concentrations are higher than observed. These enhanced fluxes and concentrations are a result of what we term "nutrient trapping," a positive feedback whereby high upwelling produces high new production that results in remineralization and enhanced nutrient concentrations in the upwelling water, which further increases new production. Nutrient trapping in shallow upwelling zones can be eliminated by increasing the particle flux length scale, which suggests that if sinking particles dominate the downward transport of organic matter then the flux length scale is longer than observed. Even with a longer particle flux length scale, we find that nutrients are trapped in some deep convective regions of the southern ocean, where new production is predicted to be much higher than the observed primary production. In simulations where the downward transport of organic matter takes place primarily in a dissolved phase, nutrient trapping is completely eliminated, in both upwelling and convective regions. The models with dissolved organic matter also agree fairly well with nutrient transports in the north Atlantic Ocean calculated from observed nutrient and hydrographic data (Rintoul and Wunsch, 1991). Our results therefore support the dissolved organic nitrogen and carbon measurements made with the high-temperature combustion technique of Suzuki, et al. (1985) and Sugimura and Suzuki (1988) and suggest that there exists an as-yet undiscovered pool of dissolved organic phosphorous in the ocean. We also use the various models to make an estimate of global new production of 2.9 to 3.6 mol C/m2/yr (12 to 15 Gt C/yr).
Sarmiento, Jorge L., 1992: Biogeochemical ocean models In Climate System Modeling, Cambridge University Press, 519-551.
Sarmiento, Jorge L., James C Orr, and U Siegenthaler, 1992: A perturbation simulation of CO2 uptake in an ocean general circulation model. Journal of Geophysical Research, 97(C3), 3621-3645. Abstract
The uptake of anthropogenic CO2 by the ocean is simulated using a perturbation approach in a three-dimensional global general circulation model. Atmospheric pCO2 is prescribed for the period 1750-1990 using the combined Siple ice core and Mauna Loa records. For the period 1980 to 1989, the average flux of CO2 into the ocean is 1.9 GtC/yr. However, the bomb radiocarbon simulation of Toggweiler, et al. (1989b) shows that the surface to deep ocean exchange in this model is too sluggish. Hence the CO2 uptake calculated by the model is probably below the actual value. The observed atmospheric increase in 1980 to 1989 is 3.2 GtC/yr., for a combined atmosphere-ocean total of 5.1 GtC/yr. This is comparable to the estimated fossil CO2 production of 5.4 GtC/yr., implying that other sources and sinks (such as from deforestation, enhanced growth of land biota, and changes in the ocean carbon cycle) must be approximately in balance. The sensitivity of the uptake to the gas exchange rate is small: a 100% increase in gas exchange rate gives only a 9.2% increase in cumulative oceanic uptake. Details of the penetration into different oceanic regions are discussed.
Sarmiento, Jorge L., and E T Sundquist, 1992: Revised budget for the oceanic uptake of anthropogenic carbon dioxide. Nature, 356(6370), 589-593. Abstract PDF
Tracer-calibrated models of the total uptake of anthropogenic CO2 by the world's oceans give estimates of about 2 gigatones carbon per year, significantly larger than a recent estimate of 0.3-0.8 Gt C yr-1 for the synoptic air-to-sea CO2 influx. Although both estimates require that the global CO2 budget must be balanced by a large unknown terrestrial sink, the latter estimate implies a much larger terrestrial sink, and challenges the ocean model calculations on which previous CO2 budgets were based. The discrepancy is due in part to the net flux of carbon to the ocean by rivers and rain, which must be added to the synoptic air-to-sea CO2flux to obtain the total oceanic uptake of anthropogenic CO2. Here we estimate the magnitude of this correction and of several other recently proposed adjustments to the synoptic air-sea CO2 exchange. These combined adjustments minimize the apparent inconsistency, and restore estimates of the terrestrial sink to values implied by the modelled oceanic uptake.
Joos, Fortunat, Jorge L Sarmiento, and U Siegenthaler, 1991: Estimates of the effect of Southern Ocean iron fertilization on atmospheric CO2 concentrations. Nature, 349(6312), 772-774. Abstract PDF
It has been suggested that fertilizing the ocean with iron might offset the continuing increase in atmospheric CO2 by enhancing the biological uptake of carbon, thereby decreasing the surface-ocean partial pressure of CO2 and drawing down CO2 from the atmosphere. Using a box model, we present estimates of the maximum possible effect of iron fertilization, assuming that iron is continuously added to the phosphate-rich waters of the Southern Ocean, which corresponds to 16% of the world ocean surface. We find that after 100 years of fertilization, the atmospheric CO2 concentration would be 59 p.p.m. below what it would have been with no fertilization, assuming no anthropogenic CO2 emissions, and 90-107 p.p.m. less when anthropogenic emissions are included in the calculation. Such a large uptake of CO2 is unllikely to be achieved in practice, owing to a variety of constraints that require further study; the effect of iron fertilization on the ecology of the Southern Ocean also remains to be evaluated. Thus, the most effective and reliable strategy for reducing future increases in atmospheric CO2 continues to be control of anthropogenic emissions.
Joos, Fortunat, U Siegenthaler, and Jorge L Sarmiento, 1991: Possible effects of iron fertilization in the southern ocean on atmospheric CO2 concentration. Global Biogeochemical Cycles, 5(2), 135-150. Abstract PDF
Recently, it was proposed (Baum, 1990 and Martin, et al., 1990a, 1990b) that the southern ocean should be fertilized with iron to stimulate biological productivity, thus enhancing the flux of organic carbon from surface to depth, thereby lowering the concentration of inorganic carbon in surface water and in turn the atmospheric CO2 concentration. We explore the possible impact of a hypothetical iron fertilization on atmospheric CO2 levels during the next century using a high-latitude exchange/interior diffusion advection model. Assuming as an upper-limit scenario that it is possible to stimulate the uptake of the abundant nutrients in the southern ocean, the maximum atmospheric CO2 depletion is 58 ppm after 50 years and 107 ppm after 100 years. This scenario requires completely effective Fe fertilization to be carried out over 16% of the world ocean area. Sensitivity studies and comparison with other models suggest that the errors in these limits due to uncertainties in the transport parameters, which are determined by calibrating the model with radiocarbon and validated with CFC-11 measurements, range from -29% to +17%. If iron-stimulated biological productivity is halted during the six winter months, the additional oceanic CO2 uptake is reduced by 18%. Possible changes in surface water alkalinity alter the result of iron fertilization by less than +9% to -28%. Burial of the iron-induced particle flux as opposed to remineralization in the deep ocean has virtually no influence on the atmospheric response for the considered time scale of 100 years. If iron fertilization were terminated, CO2 would escape from the ocean and soon cancel the effect of the fertilization. The factors which determine the atmospheric CO2 reduction most strongly are the area of fertilization, the extent to which biology utilizes the abundant nutrients, and the magnitude of future CO2 emissions. The possible effect of fertilizing the ocean with iron is small compared to the expected atmospheric CO2 increase over the next century, unless the increase is kept small by means of stringent measures to control CO2 emissions.
Nuttle, W K., J S Wroblewski, and Jorge L Sarmiento, 1991: Advances in modeling ocean primary production and its role in the global carbon cycle In Global Change and Relevant Space Observations, Oxford, UK, Pergamon Press, Inc., 67-76. Abstract
The oceans contain a large fraction of the carbon in the Earth's biosphere. Therefore, understanding the global carbon cycle, particularly the changes in atmospheric CO2 and their effects on climate, requires an accounting of CO2 exchanges between the atmosphere and the ocean. Primary production in the ocean, i.e., uptake and assimilation of CO2 by phytoplankton, plays an important role in this exchange. Ocean production is linked to nutrient cycles, mixing and circulation on a number of scales. Several university research groups are using Coastal Zone Color Scanner imagery to study ocean production and the links between physical and biological oceanographic processes and the carbon cycle. We review their recent accomplishments.
Sarmiento, Jorge L., 1991: Oceanic uptake of anthropogenic CO2: The major uncertainties. Global Biogeochemical Cycles, 5(4), 309-313. PDF
Sarmiento, Jorge L., 1991: Slowing the buildup of fossil CO2 in the atmosphere by iron fertilization: A comment. Global Biogeochemical Cycles, 5(1), 1-2. PDF
Sarmiento, Jorge L., and James C Orr, 1991: Three-dimensional simulations of the impact of the southern ocean nutrients depletion on atmospheric CO2 and ocean chemistry. Limnology and Oceanography, 36(8), 1928-1950. Abstract PDF
Surface nutrient concentrations in the Southern Ocean are an important indicator of the atmosphere-ocean chemical balance that played a key role in ice-age reduction of atmospheric pCO2 and would play a role in any Fe fertilization scenario for increasing oceanic uptake of anthropogenic CO2. The response of the ocean and atmosphere to a scenario of extreme depletion of Southern Ocean surface nutrients by an increase in the organic matter flux to the deep ocean is examined with a three-dimensional model of ocean circulation coupled to a one-box model of the atmosphere. After 100 years, the increase in the organic matter flux is 6-30 Gt C yr-1 -about twice the global new production determined by the same model for the present ocean. The removal of nutrients from surface waters of the Southern Ocean reduces the nutrient content of the near-surface and intermediate depth waters of the entire ocean, resulting in a 0.5-1.9 Gt C yr-1 reduction of low-latitude new production. The deep circumpolar waters, enriched in nutrients by regeneration of organic matter, spread into the deep and bottom waters of the remainder of the ocean, giving an overall downward shift of nutrients from surface and intermediate to circumpolar and deep waters. The oceanic total C distribution is also shifted downward, resulting in uptake of atmospheric CO2 of 46-85 ppm (98-181 Gt C) in the first 100 yr. The oxygen content shifts upward in the water column, approximately mirroring the downward shift of nutrients. Some of the oxygen shifted to the upper ocean escapes to the atmosphere. As a consequence, the global average oceanic content of oxygen, presently 168 µmol kg-1, is reduced by 6-20 µmol kg-1, with anoxia developing in the southwestern Indian Ocean.
Murnane, R, Jorge L Sarmiento, and M P Bacon, 1990: Thorium isotopes, particle cycling models, and inverse calculations of model rate constants. Journal of Geophysical Research, 95(C9), 16,195-16,206. Abstract PDF
Generalized models of thorium and particle cycling, data from Station P, and an inversion technique are used to obtain rate estimates of important biological and chemical transformations occurring in the water column. We first verify the inversion technique using an idealized data set generated by a finite difference model, and then apply the inversion technique to data from Station P. With the Station P data, predicted rate constants for adsorption and release of thorium between the dissolved and small particle phases are consistent with the results from other workers. The predicted rate constants for the interaction between small and large particles are smaller than previous estimates. The predicted concentration of large rapidly sinking particles is greater than the concentration of suspended non-sinking particles is 20 m d-1. This sinking rate is an order of magnitude smaller than the large particle sinking rate inferred from sediment trap mass fluxes at two levels in the water column. The reason we predict a high large particle concentration and slow settling velocity has not been uniquely determined. Possible modifications of the current model that could help to reconcile the differences between observations and model predictions include: 1) two classes of rapidly sinking particles or rate constants that change with depth, 2) direct interactions between the large particle and dissolved phases, and 3) incorporation of a continuous distribution of particle size and settling velocity.
Sarmiento, Jorge L., G Thiele, Robert M Key, and W S Moore, 1990: Oxygen and nitrate new production and remineralizaion in the North Atlantic subtropical gyre. Journal of Geophysical Research, 95(C10), 18,303-18,315. Abstract PDF
New estimates are obtained of oxygen utilization rates on isopycnal surfaces in the North Atlantic subtropical gyre thermocline based on tritium inventories (2.4-3.5 mol m-2yr-1) and 228Ra measurements (8.5 ± 0.8 mol m-2yr-1). Arguments are given for why the tritium inventory oxygen utilization rate estimate may be too low. The 228Ra results are combined with recent estimates of oxygen utlization within the thermocline (Jenkins, 1987) as well as estimates of oxygen production in the mixed layer (Spitzer and Jenkins, 1989; Musgrave et al., 1988), to suggest a tentative overall oxygen balance for the whole water column. The new production of oxygen in the surface ocean (~4.6 ± 1.6 mol m-2yr-1) appears to be lower than the estimated utilization within the thermocline (~8.5 ± 0.8 mol m-2yr-1), suggesting that there may be a net lateral import of organic matter into the thermocline equivalent to a new production of ~3.9 ± 1.8 mol m-2yr-1. The nitrogen balance is consistent with these results. An estimate for the total nitrogen remineralization rate in the thermocline is obtained from the oxygen utilization rate by using an -O2:N Redfield ratio of 9.1 ± 0.4 for remineralization (Minster and Boulahdid, 1987), giving a nitrogen remineralization rate of ~0.93 ± 0.10 mol m-2yr-1. Subtracting off the estimated lateral export of nitrate of ~0.51 ± 0.21 mol m-2yr-1, which is presumed to be balanced by a lateral import of dissolved organic nitrogen (Rintoul and Wunsch, 1990), gives a nitrate flux into the surface of ~0.42 ± 0.23 mol m-2yr-1, which is comparable to the estimate of 0.6 ± 0.2 mol m-2yr-1 obtained by Jenkins (1988) near Bermuda as well as the 100-m particulate nitrogen flux of 0.33 mol m-2yr-1 obtained Altabet (1989) near Bermuda.
Thiele, G, and Jorge L Sarmiento, 1990: Tracer dating and ocean ventilation. Journal of Geophysical Research, 95(C6), 9377-9391. Abstract PDF
The interpretation of transient tracer observations depends on difficult to obtain information on the evolution in time of the tracer boundary conditions and interior distributions. Recent studies have attempted to circumvent this problem by making use of a derived quantity, age, based on the simultaneous distribution of two complementary tracers, such as tritium and its daughter, helium 3. The age is defined with reference to the surface such that the boundary condition takes on a constant value of zero. We use a two-dimensional model to explore the circumstances under which such a combination of conservation equations for two complementary tracers can lead to a cancellation of the time derivative terms. An interesting aspect of this approach is that mixing can serve as a source or sink of tracer based age. We define an idealized "ventilation age tracer" that is conservative with respect to mixing, and we explore how its behavior compares with that of the tracer-based ages over a range of advective and diffusive parameters.
Sarmiento, Jorge L., M J Fasham, U Siegenthaler, R G Najjar, and J R Toggweiler, 1989: In Models of Chemical Cycling in the Oceans: Progress Report II, Ocean Tracers Laboratory Report #6, Princeton, NJ, Princeton University, 46 pp. Abstract
In a previous progress report (Toggweiler, et al., 1987) we argued that the most difficult obstacle that needed to be overcome in developing predictive, dynamical, 3-D models of geochemical cycling in the oceans was to develop approaches for simulating the role of biological processes. In this report we update our progress on developing an ecosystem-level description of upper ocean fluxes and on simulating the penetration of anthropogenic CO2 into the ocean. if the natural carbon cycle and ocean circulation are in steady state, one needs to know only the pre-anthropogenic surface total carbon and alkalinity to predict the uptake of fossil CO2 by the oceans. As a first simple approximation, we fix the surface alkalinity at a constant value of 2300 ueq kg-1, and fix the pre-anthropogenic surface total carbon to the value that gives the pre-anthropogenic pCO2 of 280 ppm everywhere. This neglects details of the natural cycles of CO2 due to temperature as well as biology that give rise to non-equilibrium pre-anthropogenic pCO2 levels over much of the ocean. Several fossil CO2 uptake experiments have been performed with this approach, both with 3-D ocean circulation models and with a new box model that incorporates features not included in previous box models. Another approach we are working on is a determination of the pre-anthropogenic surface total carbon and alkalinity based on the observed surface nutrient distributions and an assumed Redfield stochiometry. This approach gives us the concentration of pre-anthropogenic total carbon and alkalinity that we need for the steady state simulations of fossil CO2 uptake discussed above. It also provides a simple way of simulating the effects of biology and temperature in the euphotic region of the ocean, allowing us to put major emphasis on processes occurring below the euphotic zone. Our simulations of processes below the euphotic zone suggest an important role for substances not caught in sediment traps, such as dissolved organic matter. We have made considerable progress on the development of ecosystem models of the upper ocean and have performed a first experiment incorporating these models into a 3-D ocean circulation model of the North Atlantic. Such models are necessary for predicting how the atmospheric pCO2 will be affected should the ocean circulation and biology begin to change in response to a greenhouse climate.
Sarmiento, Jorge L., 1988: In A Chemical Tracer Strategy for WOCE: Report of a Workshop Held in Seattle, Washington, U.S. WOCE Planning Report Number 10, 181 pp.
We examine the hypothesis that global scale episodes of anoxia such as occurred in the Cretaceous are due to high productivity and/or stagnation of the circulation. Two modes of ocean circulation are considered: a thermohaline overturning cell, essentially vertical, which involves global scale upwelling into the surface followed by sinking in deep water formation regions; and an approximately horizontal cell which connects the abyss directly with deeply convecting waters in deep water formation regions. Modern analogs for these processes are formation of North Atlantic Deep Water and Antarctic Bottom Water, respectively. Over most of the oceans the surface new production is nutrient limited and thus directly proportional to the supply of nutrients by the vertical overturning cell. A reduction in oxygen can only be brought about by increased vertical overturning associated with increased production. In addition, the model shows that as the deep ocean becomes lower in oxygen, the sensitivity of the oxygen levels to the meridional circulation decreases such that it becomes difficult or impossible to achieve.
We examine the causes of anoxia in regions such as the Eastern Mediterranean, which have exchange over sills with adjacent basins. Box models show that the concentration of the limiting nutrient is the major determinant of deep oxygen levels. The most effective way of increasing nutrient concentrations to the point where anoxia occurs is to change the flow pattern across the sills ventilating the basins. With a sill exchange pattern such as that in the present Strait of Sicily, it is difficult to obtain anoxia in the Eastern Mediterranean without also driving the Western Mediterranean to low oxygen and high nutrient levels. Episodes of anoxia in the Eastern Mediterranean are associated with a freshening of surface waters. A reversal in flow directions, presumably resulting from the observed freshening, will inevitably lead to anoxia associated with increased sediment burial rates of the limiting nutrient and will leave the Western Mediterranean largely unaffected, in keeping with the observational evidence.
Sarmiento, Jorge L., J R Toggweiler, and R G Najjar, 1988: Ocean carbon-cycle dynamics and atmospheric pCO2. Philosophical Transactions of the Royal Society of London, A, 325, 3-21. Abstract
Mechanisms are identified whereby processes internal to the oceans can give rise to rapid changes in atmospheric pCO2. One such mechanism involves exchange between the atmosphere and deep ocean through the high-latitude outcrop regions of the deep waters. The effectiveness of communication between the atmosphere and deep ocean is determined by the rate of exchange between the surface and deep ocean against the rate of biological uptake of the excess carbon brought up from the abyss by this exchange. Changes in the relative magnitude of these two processes can lead to atmospheric pCO2 values ranging between 165 p.p.m. (by volume) and 425 p.p.m. compared with a pre-industrial value of 280 p.p.m. Another such mechanism involves the separation between regeneration of alkalinity and total carbon that occurs in the oceans because of the fact that organic carbon is regenerated primarily in the upper ocean whereas CaCO3 is dissolved primarily in the deep ocean. The extent of separation depends on the rate of CaCO3 formation at the surface against the rate of upward mixing of deep waters. This mechanism can lead to atmospheric values in excess of 20000 p.p.m., although values greater than 1100 p.p.m. are unlikely because calcareous organisms would have difficulty surviving in the undersaturated surface waters that develop at this point. A three-dimensional model that is being developed to further study these and other problems provides illustrations of them and also suggests the possibility that there is a long-lived form of non-sinking carbon playing a major role in carbon cycling.
Sarmiento, Jorge L., 1987: Tracers and modeling. Reviews of Geophysics, 25(6), 1417-1419. PDF
Kawase, M, and Jorge L Sarmiento, 1986: Circulation and nutrients in middepth Atlantic waters. Journal of Geophysical Research, 91(C8), 9749-9770. Abstract PDF
Isopycnal analyses of distributions of salinity, oxygen, apparent oxygen utilization, nitrate, and silica in the depth range 800-3000 m in th North and Tropical Atlantic Ocean are carried out using data from the Transient Tracers in the Oceans, Metero cruise 56 leg 5, Atlantis II cruise 109 legs 1 and 3, and the GEOSECS Atlantic Study. The results are summarized in isopycnal distribution maps and property-property plots. The layer between 800 and 1500 m is oxygen poor and nutrient rich and is poorly ventilated. Below this is a better aerated layer with the deep western boundary current, part of which separates along the equator. Regeneration of nutrients and consumption of oxygen persist into the deep ocean in regions off the west coast of Africa. The Mediterranean Outflow Water apparently is causing significant cross-isopycnal mixing through salt fingering. The resultant cross-isopycnal velocity may be large enough to cause significant vorticity stretching.
Moore, W S., Jorge L Sarmiento, and Robert M Key, 1986: Tracing the Amazon component of surface Atlantic water using 228Ra, salinity and silica. Journal of Geophysical Research, 91(C2), 2574-2580. Abstract PDF
High 228Ra/226Ra activity ratios characteristic of waters in the Amazon estuary provide a sensitive indicator of the presence of these waters in the Atlantic Ocean. A conservative mixing model utilizing the 228Ra/226activity ratio (AR) tied to absolute measurements in the estuary allows us to estimate that 20-34% of the surface water east of the Antilles during June, 5-9% from the same area during December, and 15-20% of the eastern Caribbean surface water during December are derived from the Amazon estuary. Differences in 228Ra input occur in response to variable stratification of water near the river mouth. During high discharge, intense vertical mixing enriches the water in the estuary in 228Ra. A large fraction of this water moves to the north and east of Antilles, where its relatively high 228Ra/226 AR distinguishes it over 1500 km from its source. During low discharge (northern hemisphere fall) a significant fraction of river water passes northwest of the zone of intense mixing into a vertically stratified region where 228Ra gain is lower. This water is transported by the Guiana Current along the coast of South America and into the Caribbean.
Olson, D B., G H Ostlund, and Jorge L Sarmiento, 1986: Western boundary undercurrent off the Bahamas. Journal of Physical Oceanography, 16(2), 233-240. Abstract PDF
Two tritium sections through the deep western boundary current east of the Bahamas, taken in late 1980 and early 1981, are presented. Tritium from the bomb tests in the late 1950s and early 1960s is used to identify recently formed deep waters in the sections. High concentrations are found in the North Atlantic deep water. Low tritium values occur in the Labrador Sea water found above the core of this deep water. This is consistent with the suggestion by Talley and McCartney that this water mass has not been ventilated at the temperatures observed in these sections since the mid-1950s. Tritium in the sections is correlated with maxima in potential vorticity. This is inconsistent with deep convection as a direct source for the water mass. The potential vorticity maxima may be associated with plume dynamics near the overflow regions or with the dynamics of the deep western boundary current. The sections are south of the section discussed by Jenkins and Rhines, where high tritium concentrations were found along the topography on the Blake-Bahama Outer Ridge between 3.5- and 4.5-km depth in late 1977. In the sections farther south, a similar maximum is found, but it is at a 0.6 degrees C warmer potential temperature and separated from the topography. Tritium is found at the temperature it appears in the Jenkins and Rhines section. In contrast to their concentrated feature, the tritium in the later sections is spread out into a layer that extends into the ocean interior to the limit of the sections in these temperature ranges. This, coupled with dynamic height fields, suggests that the boundary current feeds an offshore flow into the ocean interior east of the Bahamas. The change in the temperature where the tritium maximum is found implies variations in the formation and spread of North Atlantic deep water on fairly short time scales.
Sarmiento, Jorge L., 1986: Modeling oceanic transport of dissolved constituents In The Role of Air-Sea Exchange in Geochemical Cycling, Amsterdam, The Netherlands, Reidel Publishing Co, 65-82.
Sarmiento, Jorge L., 1986: On the north and tropical Atlantic heat balance. Journal of Geophysical Research, 91(C10), 11,677-11,689. Abstract
The heat balance deduced from a three-dimensional, seasonally driven primitive equation model of the North Atlantic is described and compared with observations. The greatest response to the seasonal forcing occurs in the region between ~5 degrees N and 10 degrees N, where the northward heat transport goes from a minimum of <0 in summer, when the North Equatorial Countercurrent dominates the surface flow and the Brazil Current is at a minimum, to a maximum of >1.4 x 1015 W in the winter, when the Brazil Current and northward Ekman transport are at a maximum. Elsewhere in the tropics and subtropics the range in transport is smaller. In the northern hemisphere (~12 degrees to 32 degrees N), there is a significant semiannual component due to Ekman transport. The large seasonal changes in heat storage in the tropics are caused primarily by transport divergence rather than surface heat flux. In the annual mean, the equatorial region has a large surface heat flux gain associated primarily with the conversion of ~6 x 106 m3 s-1 of northward geostrophic flow with theta < 20 degrees C in the southern hemisphere, to a surface Ekman flow with theta ~ 27 degrees C in the northern hemisphere. The seasonal variation in heat storage within the subtropical and subpolar gyres is due almost entirely to the surface heat flux. However, seasonal variations in Ekman transport do lead to a relatively large annual cycle in northward heat transport (e.g., 0.5 x 1015 W to 0.9 x 1015 W at ~35 degrees N), with maximum transport occurring during the summer when the southward Ekman transport is at a minimum. A comparison of the model results with heat storage estimates shows that the model and data agree very well in the tropics but that at higher latitudes the model underestimates the seasonal variations because of inadequate vertical penetration of heating during periods of warming.
Sarmiento, Jorge L., 1986: Three-dimensional ocean models for predicting the distribution of three-dimensional ocean models for predicting the distribution of CO2 between the ocean and atmosphere In Changing Carbon Cycle: A Global Analysis, Springer-Verlag, 279-294.
Sarmiento, Jorge L., and P E Biscaye, 1986: Radon 222 in the benthic boundary layer. Journal of Geophysical Research, 91(C1), 833-844. Abstract
A detailed survey of radon 222 and temperature profiles of the benthic boundary layer in the Hatteras Abyssal Plain shows a strong correlation between the structure of both. The apparent vertical diffusivities estimated from radon 222 are of the order of 50 cm2 s-1 in the mixed layer, and of the order of 1 cm-2 s-1 above it. One profile appears to have been taken in a frontal zone where isotherms dip sharply into the sediments. This is the only profile where there is significant penetration of radon above the mixed layer. Several other profiles suggest that the temperature may take longer than several radon half-lives to adjust to new mixing and advection regimes. In such cases, one often sees considerable structure in the radon profile within the region where the potential temperature is well mixed.
Sarmiento, Jorge L., and E Gwinn, 1986: Strontium 90 fallout prediction. Journal of Geophysical Research, 91(C6), 7631-7646.
Sarmiento, Jorge L., and J R Toggweiler, 1986: A preliminary model of the role of upper ocean chemical dynamics in determining oceanic oxygen and atmospheric carbon dioxide levels In Dynamic Processes in the Chemistry of the Upper Ocean, Plenum Press, 233-240. Abstract
A first version is presented of equations for a three-dimensional model of nutrient and carbon cycling in the oceans. An analytical solution of these equations has been obtained for a one-and-a-half-dimensional "pipe" model. This solution shows that atmospheric CO2 can be varied by changing the level of preformed nutrients. It is suggested that this mechanism may explain the lower pCO values of the last ice age.
Brewer, P G., Jorge L Sarmiento, and W M Smethie, Jr, 1985: Transient Tracers in the Ocean (TTO) Program: The North Atlantic Study, 1981; The Tropical Atlantic Study, 1983. Journal of Geophysical Research, 90(C4), 6903-6905. PDF
Kawase, M, and Jorge L Sarmiento, 1985: Nutrients in the Atlantic thermocline. Journal of Geophysical Research, 90(C5), 8961-8979. Abstract PDF
A set of maps are presented of nutrient distribution on isopycnal surfaces in the North and tropical Atlantic Ocean main thermocline. The data used in producing these maps are from the Transient Tracers in the Ocean (TTO) North Atlantic Study and Tropical Atlantic Study, an associated German study (Meteor 56/5), two cross-Atlantic sections from cruise 109 of the Atlantis II, and the GEOSECS program. The nutrient distributions reflect primarily the sources at the northern and southern outcrops of the isopycnal surfaces, the in situ regeneration due to decomposition of sinking organic materials, and the interior physical processes as inferred from thermocline models and the distribution of conservative properties such as salinity. However, silica also exhibits behavior that cannot be explained by in situ regeneration. A simple phenomenological model suggests that cross-isopycnal advection and mixing in the equatorial region may play an important role in the nutrient dynamics. These data should prove of great value in constraining models of physical as well as biogeochemical processes.
Key, Robert M., R F Stallard, W S Moore, and Jorge L Sarmiento, 1985: Distribution and flux of 226Ra and 228Ra in the Amazon River estuary. Journal of Geophysical Research, 90(C4), 6995-7004. Abstract PDF
Measurements of 226Ra and 228Ra in the Amazon River estuary show that desorption from river-borne suspended particulate matter in the estuary increases the riverine flux of both isotopes to the ocean by a factor of approximately 5 over the flux attributable to radium dissolved in the river water alone. The total Amazon flux supplies approximately 0.20% of the 226Ra and approximately 2.6% of the 228Ra standing crops in the near-surface Atlantic (0-200 m). Diffusive flux from estuarine and shelf sediments and desorption from resuspended sediments in the region of the estuary approximately double the estuarine 226Ra concentration and quadruple the estuarine 228Ra concentration above that caused by the dissolved and desorbed river components alone.
Moore, W S., Robert M Key, and Jorge L Sarmiento, 1985: Techniques for precise mapping of 226Ra and 228Ra in the Ocean. Journal of Geophysical Research, 90(C4), 6983-6994. Abstract
Improvements in the analyses of 226Ra and 228Ra in seawater made possible by better extraction and processing techniques reduce significantly the errors associated with these measurements. These improvements and the extensive sampling for Ra isotopes conducted on the TTO North Atlantic Study should enable us to use the distribution of 228Ra to study mixing processes on a 3-15 year time scale in both the upper and deep North Atlantic. The 228Ra profiles already analyzed show a closer resemblance to GEOSECS tritium data than to TTO tritium data in the upper ocean. This is because the transient tracer tritium was responding on a 10-year time scale during GEOSECS and a 20-year time scale during TTO. The steady state tracer 228Ra should always respond on a time scale of 8 years. Thus the 228Ra data obtained on TTO should provide a means to extend the features of the GEOSECS tritium field to the regions of the TTO study. The 226Ra data are of high enough quality to identify features associated with different water masses. Changes in the positions of the deep-water masses since the GEOSECS cruise are revealed by the 226Ra data
Toggweiler, J R., and Jorge L Sarmiento, 1985: Glacial to interglacial changes in atmospheric carbon dioxide: The critical role of ocean surface water in high latitudes In The Carbon Cycle and Atmospheric CO2: Natural Variations Archean to Present, Geophysical Monograph 32, Washington, DC, American Geophysical Union, 163-184. Abstract PDF
Recent measurements of the CO2 content of air bubbles trapped in glacial ice have shown that the partial pressure of atmospheric COsub>2 during the last ice age was baout 70 ppm lower than during the interglacial. Isotopic measurements on surface- and bottom-dwelling forams living during the ice age have shown that the 13C/12C gradient between the ocean's surface and bottom layers was 25% larger during the last ice age than at present. Broecker (1982) proposed that an increase in the phospate content of the deep sea could explain these observations. We follow up here on a proposal by Sarmiento and Toggweiler (1984) that glacial to interglacial changes in PCO2 are related to changes in the nutrient content of high-latitude surface water. We develop a four-box model of the ocean and atmosphere which includes low- and high-latitude surface boxes, an atmosphere, and a deep ocean. In simplest form the model equations show that the CO2 content of high-latitude surface water is directly connected to the huge reservoir of CO2 in deep water through the nutrient content of high-latitude surface water. The relationship between the CO2 content of low latitude surface water and the deep sea is more indirect and depends to a large extent on transport of CO2 through the atmosphere from high latitudes. We illustrate how the 14C content of the atmosphere and that of high-latitude surface water constrain model solutions for the present ocean and how ice age 13C observations constrain ice age parameters. We propose that the low ice age PCO2 can be produced by a reduction in local exchange between high-latitude surface water and deep water. The model requires that the current exchange rate of about 50 Sv be reduced to about 10 Sv. We review evidence in the geologic record for widespread changes in deep convection around Antarctica about 14,000 years ago which are synchronous with the change in atmospheric PCO2.
Sarmiento, Jorge L., and J R Toggweiler, 1984: New model for the role of the oceans in determining atmospheric PCO2. Nature, 308(5960), 621-624.
Sarmiento, Jorge L., 1983: A simulation of bomb tritium entry into the Atlantic Ocean. Journal of Physical Oceanography, 13(10), 1924-1939. Abstract PDF
Tritium is used in a model calibration study that is aimed at developing three-dimensional ocean circulation and mixing models for climate and geochemical simulations. The North Atlantic tritium distribution is modeled using a three-dimensional advective field predicted by a primitive equation ocean circulation model. The effect of wintertime convection is parameterized by homogenizing the tracer to the observed March mixed-layer depth. Mixing is parameterized by horizontal and vertical Fickian diffusivities of 5 x 10-6 cm2 s-1 and 0.5 cm2 s-1, respectively.
The spreading of tritium in the model is dominated by advection in the horizontal, and by wintertime convection and advection in the vertical. The horizontal and vertical mixing provided by the model have negligible effect. A comparison of the model tracer fields with observations shows that most of the basic patterns of the tritium field are reproduced. The model's mean vertical penetration of 543 m in 1972 is comparable to the 592 m penetration obtained from the data. The major discrepancy between model and data is an inadequate penetration into deeper portions of the northwestern subtropical gyre main thermocline. Some of the problems that may contribute to this are identified.
A tritium simulation with a smoothed input gives a penetration depth of only 395 m. The smoothing puts a high fraction of the tritium into low-latitude, low-penetration regions such as the equator. This suggests that great care needs to be exercised in using simplified models of tritium observations to predict the behavior of tracers with different input functions, like fossil fuel CO2.
Sarmiento, Jorge L., 1983: A tritium box model of the North Atlantic thermocline. Journal of Physical Oceanography, 13(7), 1269-1274. Abstract PDF
A box model of 1972 tritium observations on isopycnal surfaces in the main thermocline of the North Atlantic subtropical gyre is used to estimate the time scales and volume of exchange of the thermocline with respect to surface waters. The flux of water between the surface and the thermocline implied by this model (~ 40 x 106 m3 s-1) greatly exceeds the downward Ekman pumping (~8 x 106 m3 s-1). This suggests that mixing and convective overturning are the dominant mechanisms for exchange between surface waters and the interior geostrophic flow. The flux rate is approximately the same size as conventional estimates of the Sverdrup transport. This suggests that ventilation of the thermocline may occur by recirculation combined with a very efficient exchange across the poleward boundary of the gyre.
Three-dimensional solutions are obtained for the circulation of the North Atlantic using a robust diagnostic model. In contrast to previous diagnostic models the robust diagnostic model incorporates the conservation of the large-scale fields of heat and salinity as well as momentum. An approximate fit to observed fields of temperature and salinity is obtained by a closure condition. The method is robust in the sense that it does not have the extreme sensitivity to the density input fields of the classical diagnostic method. Equilibrium solutions are obtained by numerical integration of the time-dependent equations. Error estimates for the velocity field can be obtained indirectly from the numerical solutions. Temperature observations used as input have an effective resolution of 3 degrees x 3 degrees of latitude and longitude and a sampling error of plus or minus 0.15 degrees C. The equivalent vertically integrated velocity error is estimated to be plus or minus 0.5-1.0 cm/s depending on bottom topography. The suitability of the model for geochemical work is judged by comparison with heat and salinity balance estimates. Best results are obtained for the case in which the model has a minimum observational constraint below the surface.
Sarmiento, Jorge L., C G H Rooth, and W Broecker, 1982: Radium 228 as a tracer of basin wide processes in the abyssal ocean. Journal of Geophysical Research, 87(C12), 9694-9698. Abstract
simple model of isopycnal mixing in a circular basin is developed in order to examine the utility of the 5.75-year half-life tracer radium 228 for studying basin wide processes in the deep ocean. The model shows that it is possible to resolve diffusivities of approximately less than 8 x 107 cm2 s-1 in a basin of ~3000-km diameter with profiles measured near the center and edge of the basin. A least squares fit of the model to four abyssal profiles measured during GEOSECS in the North Atlantic Basin gives an isopycnal diffusivity of 6 x 107 cm2 s-1.
Sarmiento, Jorge L., C G H Rooth, and W Roether, 1982: The North Atlantic tritium distribution in 1972. Journal of Geophysical Research, 87(C10), 8047-8056. Abstract
The distribution of tritium in the North Atlantic in 1972 is compared with the distribution of salinity and the first-order potential vorticity as mapped on six constant potential density surfaces in the North Atlantic. The picture presented suggests an advective transport regime which is consistent with currently developing notions of the large-scale gyre structure. Lateral mixing along isopycnals appears to be important in the northwestern region of the subtropical gyre, and vertical (cross-isopycnal) mixing needs to be invoked only in near surface layers.
Sarmiento, Jorge L., and C G H Rooth, 1980: A comparison of vertical and isopycnal mixing models in the deep sea based on Radon 222 measurements. Journal of Geophysical Research, 85(C3), 1515-1518. Abstract
A two-dimensional model is developed for the one-dimensional depth-dependent distribution of a radioactive tracer with a bottom source (e.g., radon 222) in a weakly baroclinic fluid. The tracer transport away from the boundary is separated into two components with cardinal directions along and perpendicular to isopycnal surfaces in the fluid. Contrary to the case of a strictly one-dimensional mixing model, where properties such as heat and buoyancy have the same vertical eddy diffusivity as the tracer so that their vertical fluxes are uniquely related, this model yields a relation between vertical buoyancy and heat fluxes and the tracer flux which depends on the relative magnitude of the isopycnal and cross-isopycnal diffusivities. To close the problem, weassume that the interior vertical buoyancy flux is balanced by a horizontal cross-isopycnal Ekman drift at the bottom. As an example, a bottom radon 222 profile (Geosecs station 31) is analyzed. The buoyancy flux implied by the apparent vertical radon 222 diffusivity of A two-dimensional model is developed for the one-dimensional depth-dependent distribution of a radioactive tracer with a bottom source (e.g., radon 222) in a weakly baroclinic fluid. The tracer transport away from the boundary is separated into two components with cardinal directions along and perpendicular to isopycnal surfaces in the fluid. Contrary to the case of a strictly one-dimensional mixing model, where properties such as heat and buoyancy have the same vertical eddy diffusivity as the tracer so that their vertical fluxes are uniquely related, this model yields a relation betweenvertical buoyancy and heat fluxes and the tracer flux which depends on the relative magnitude of the isopycnal and cross-isopycnal diffusivities. To close the problem, we assume that the interior vertical buoyancy flux is balanced by a horizontal cross-isopycnal Ekman drift at the bottom. As an example, a bottom radon 222 profile (Geosecs station 31) is analyzed. The buoyancy flux implied by the apparent vertical radon 222 diffusivity of 46 cm2 s-1 is 2.1 x 10-6 cm2 s-3, requiring a bottom friction velocity (u*) of 0.94 cm s-1 to balance it in a one-dimensional model. In the two-dimensional isopycnal mixing model the buoyancy flux can take on any value between 0 and 2.1 x 10 -6 cm2 s -3 with corresponding values for u *, which was not measured, of 0-0.94 cm-1. For an assumed u* value of 0.1 cm s-1 the cross-isopycnal diffusivity is 0.5 cm2 s-1, implying a vertical buoyancy flux of 2.3 x 10-8 cm2 s-3, and the diffusivity parallel to the isopycnals is 3.8 x 106 cm2s-1.
Sarmiento, Jorge L., W Broecker, and P E Biscaye, 1978: Excess bottom Radon 222 distribution in deep ocean passages. Journal of Geophysical Research, 83(C10), 5068-5076. Abstract
Radon 222 and STD profiles were obtained as part of the Geosecs program in the Vema Channel in the southwest Atlantic Ocean and in the Samoan, Clarion, and Wake Island passages in the Pacific Ocean. The standing crop of excess radon 222 is higher in the passages than at other nearby locations. The most likely explanation for this is that there is a high flux of radon 222 from the floor of the passages. Since much of the floor is covered with manganese nodules and encrustations, the high flux of radon 222 may be attributable to the high concentrations of radium 226 in the outer few millimeters of such deposits. Laboratory measurements of radon 222 emissivity from manganese encrustations obtained in the Vema Channel support this hypothesis. The excess radon 222 in the Vema Channel and Wake Island Passage is found in substantial quantities at heights above bottom greatly exceeding the heights at which excess radon 222 is found in nonpassage areas. The horizontal diffusion of radon emanating from the walls of the passages is unlikely to be the cause of the observed concentrations because the ratio of wall surface area to water volume is very low. The profiles must therefore be a result of exceptionally high apparent vertical mixing in the passages. Further work is needed to determine the nature of this apparent vertical mixing. The excess radon 222 and STD data in all four passages have been fit with an empirical model in which it is assumed that the buoyancy flux is constant with distance above bottom. The fits are very good and yield apparent buoyancy fluxes that are between 1 and 3 orders of magnitude greater than those obtained at nearby stations outside the passages for three of the four passages.
