Sedigh Marvasti, S, Anand Gnanadesikan, A A Bidokhti, John P Dunne, and S Ghader, February 2016: Challenges in modelling spatiotemporally varying phytoplankton blooms in the Northwestern Arabian Sea and Gulf of Oman. Biogeosciences, 13(4), DOI:10.5194/bg-13-1049-2016. Abstract
We examine interannual variability of phytoplankton blooms in northwestern Arabian Sea and Gulf of Oman. Satellite data (SeaWIFS ocean color) shows two climatological blooms in this region, a wintertime bloom peaking in February and a summertime bloom peaking in September. A pronounced anti-correlation between the AVISO sea surface height anomaly (SSHA) and chlorophyll is found during the wintertime bloom. On a regional scale, interannual variability of the wintertime bloom is thus dominated by cyclonic eddies which vary in location from one year to another. These results were compared against the outputs from three different 3-D Earth System models. We show that two coarse (1°) models with the relatively complex biogeochemistry (TOPAZ) capture the annual cycle but neither eddies nor the interannual variability. An eddy-resolving model (GFDL CM2.6) with a simpler biogeochemistry (miniBLING) displays larger interannual variability, but overestimates the wintertime bloom and captures eddy-bloom coupling in the south but not in the north. The southern part of the domain is a region with a much sharper thermocline and nutricline relatively close to the surface, in which eddies modulate diffusive nutrient supply to the surface (a mechanism not previously emphasized in the literature). We suggest that for the model to simulate the observed wintertime blooms within cyclones, it will be necessary to represent this relatively unusual nutrient structure as well as the cyclonic eddies. This is a challenge in the Northern Arabian Sea as it requires capturing the details of the outflow from the Persian Gulf.
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.
Gnanadesikan, Anand, John P Dunne, and Rym Msadek, May 2014: Connecting Atlantic Temperature Variability and Biological Cycling in two Earth System Models. Journal of Marine Systems, 133, DOI:10.1016/j.jmarsys.2013.10.003. Abstract
Connections between the interdecadal variability in North Atlantic temperatures and biological cycling have been widely hypothesized. However, it is unclear whether such connections are due to small changes in basin-averaged temperatures indicated by the Atlantic Multidecadal Oscillation (AMO) Index, or whether both biological cycling and the AMO index are causally linked to changes in the Atlantic Meridional Overturning Circulation (AMOC). We examine interdecadal variability in the annual and month-by-month diatom biomass in two Earth System Models with the same formulations of atmospheric, land, sea ice and ocean biogeochemical dynamics but different formulations of ocean physics and thus different AMOC structure and variability. In the isopycnal-layered ESM2G, strong interdecadal changes in surface salinity associated with changes in AMOC produce spatially heterogeneous variability in convection, nutrient supply and thus diatom biomass. These changes also produce changes in ice cover, shortwave absorption and temperature and hence the AMO Index. Off West Greenland, these changes are consistent with observed changes in fisheries and support climate as a causal driver.. In the level-coordinate ESM2M, nutrient supply is much higher and interdecadal changes in diatom biomass are much smaller in amplitude and not strongly linked to the AMO index.
Despite slow rates of ocean mixing, observational and modeling studies suggest that buoyancy is redistributed to all depths of the ocean on surprisingly short interannual to decadal time scales. The mechanisms responsible for this redistribution remain poorly understood. This work uses an Earth System Model to evaluate the global steady state ocean buoyancy (and related steric sea level) budget, its interannual variability, and its transient response to a doubling of CO2 over 70 years, with a focus on the deep ocean. At steady state, the simple view of vertical advective-diffusive balance for the deep ocean holds at low- to mid-latitudes. At higher latitudes, the balance depends on a myriad of additional terms, namely mesoscale and submesoscale advection, convection and overflows from marginal seas, and terms related to the nonlinear equation of state. These high-latitude processes rapidly communicate anomalies in surface buoyancy forcing to the deep ocean locally; the deep, high-latitude changes then influence the large-scale advection of buoyancy to create transient deep buoyancy anomalies at lower latitudes. Following a doubling of atmospheric carbon dioxide concentrations, the high latitude buoyancy sinks are suppressed by a slowdown in convection and reduced dense water formation. This change is accompanied by a slowing of both upper and lower cells of the global meridional overturning circulation, reducing the supply of dense water to low latitudes beneath the pycnocline and the commensurate flow of light waters to high latitudes above the pycnocline. By this mechanism, changes in high latitude buoyancy are communicated to the global deep ocean on relatively fast advective timescales.
The impact of climate warming on the upper layer of the Bering Sea is investigated by using a high-resolution coupled global climate model. The model is forced by increasing atmospheric CO2 at a rate of 1% per year until CO2 reaches double its initial value (after 70 years), after which it is held constant. In response to this forcing, the upper layer of the Bering Sea warms by about 2�C in the southeastern shelf and by a little more than 1�C in the western basin. The wintertime ventilation to the permanent thermocline weakens in the western Bering Sea. After CO2 doubling, the southeastern shelf of the Bering Sea becomes almost ice-free in March, and the stratification of the upper layer strengthens in May and June. Changes of physical condition due to the climate warming would impact the pre-condition of spring bio-productivity in the southeastern shelf.
Baughman, E, Anand Gnanadesikan, A DeGaetano, and Alistair Adcroft, November 2012: Investigation of the Surface and Circulation Impacts of Cloud Brightening Geoengineering. Journal of Climate, 25(21), DOI:10.1175/JCLI-D-11-00282.1. Abstract
Projected increases in greenhouse gases have prompted serious discussion on geoengineering the climate system to counteract global climate change. Cloud albedo enhancement has been proposed as a feasible geoengineering approach, but previous research suggests undesirable consequences of globally uniform cloud brightening. The present study uses GFDL�s CM2G global coupled model to simulate cloud albedo enhancement via increases in cloud condensation nuclei (CCN) to 1000 cm−3 targeted at the marine stratus deck of the Pacific Ocean, where persistent low clouds suggest a regional approach to cloud brightening. We investigate the impact of this regional geoengineering on global circulation and climate in the presence of a 1% annual increase of CO2. Surface temperatures returned to near Pre-Industrial levels over much of the globe with cloud modifications in place. In the first 40 years and over the 140 year mean, significant cooling over the Equatorial Pacific, continued Arctic warming, and large precipitation changes over the western Pacific, and a westward compression and intensification of the Walker Circulation were observed in response to cloud brightening. The cloud brightening caused a persistent La Nina condition associated with an increase in hurricane maximum potential intensity and genesis potential index, and decreased vertical wind shear between July and November in the tropical Atlantic, South China Sea, and to the east of Japan. Responses were similar with CCN = 500 cm−3.
Gnanadesikan, Anand, John P Dunne, and Jasmin G John, March 2012: Understanding why the volume of suboxic waters does not increase over centuries of global warming in an Earth System Model. Biogeosciences, 9(3), DOI:10.5194/bg-9-1159-2012. Abstract
Global warming is expected to reduce oxygen solubility
and vertical exchange in the ocean, changes which
would be expected to result in an increase in the volume of
hypoxic waters. A simulation made with a full Earth System
model with dynamical atmosphere, ocean, sea ice and biogeochemical
cycling (the Geophysical Fluid Dynamics Laboratory’s
Earth System Model 2.1) shows that this holds true
if the condition for hypoxia is set relatively high. However,
the volume of the most hypoxic (i.e., suboxic) waters does
not increase under global warming, as these waters actually
become more oxygenated. We show that the rise in dissolved
oxygen in the tropical Pacific is associated with a drop in
ventilation time. A term-by-term analysis within the least
oxygenated waters shows an increased supply of dissolved
oxygen due to lateral diffusion compensating an increase in
remineralization within these highly hypoxic waters. This
lateral diffusive flux is the result of an increase of ventilation
along the Chilean coast, as a drying of the region under
global warming opens up a region of wintertime convection
in our model. The results highlight the potential sensitivity of
suboxic waters to changes in subtropical ventilation as well
as the importance of constraining lateral eddy transport of
dissolved oxygen in such waters.
Goldberg, D N., Christopher M Little, Olga V Sergienko, Anand Gnanadesikan, Robert Hallberg, and M Oppenheimer, June 2012: Investigation of land ice-ocean interaction with a fully coupled ice-ocean model, Part 2: Sensitivity to external forcings. Journal of Geophysical Research: Earth Surface, 117, F02038, DOI:10.1029/2011JF002247. Abstract
A coupled ice stream-ice shelf-ocean cavity model is used to assess the sensitivity of the coupled system to far-field ocean temperatures, varying from 0.0 to 1.80C, as well as sensitivity to the parameters controlling grounded ice flow. A response to warming is seen in grounding line retreat and grounded ice loss that cannot be inferred from the response of integrated melt rates alone. This is due to concentrated thinning at the ice shelf lateral margin, and to processes that contribute to this thinning. Parameters controlling the flow of grounded ice have a strong influence on the response to sub-ice shelf melting, but this influence is not seen until several years after an initial perturbation in temperatures. The simulated melt rates are on the order of that observed for Pine Island Glacier in the 1990s. However, retreat rates are much slower, possibly due to unrepresented bedrock features.
Goldberg, D N., Christopher M Little, Olga V Sergienko, Anand Gnanadesikan, Robert Hallberg, and M Oppenheimer, June 2012: Investigation of land ice-ocean interaction with a fully coupled ice-ocean model, Part 1: Model description and behavior. Journal of Geophysical Research: Earth Surface, 117, F02037, DOI:10.1029/2011JF002246. Abstract
Antarctic ice shelves interact closely with the ocean cavities beneath them, with ice shelf geometry influencing ocean cavity circulation, and heat from the ocean driving changes in the ice shelves, as well as the grounded ice streams that feed them. We present a new coupled model of an ice stream-ice shelf-ocean system that is used to study this interaction. The model is capable of representing a moving grounding line and dynamically responding ocean circulation within the ice shelf cavity. Idealized experiments designed to investigate the response of the coupled system to instantaneous increases in ocean temperature show ice-ocean system responses on multiple timescales. Melt rates and ice shelf basal slopes near the grounding line adjust in 1-2 years, and downstream advection of the resulting ice shelf thinning takes place on decadal timescales. Retreat of the grounding line and adjustment of grounded ice takes place on a much longer timescale, and the system takes several centuries to reach a new steady state. During this slow retreat, and in the absence of either an upward-or downward-sloping bed or long-term trends in ocean heat content, the ice shelf and melt rates maintain a characteristic pattern relative to the grounding line.
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).
Air pollution (ozone and particulate matter in surface air) is strongly linked to synoptic weather and thus is likely sensitive to climate change. In order to isolate the responses of air pollutant transport and wet removal to a warming climate, we examine a simple carbon monoxide (CO)–like tracer (COt) and a soluble version (SAt), both with the 2001 CO emissions, in simulations with the GFDL chemistry-climate model (AM3) for present (1981-2000) and future (2081-2100) climates. In 2081-2100, projected reductions in lower tropospheric ventilation and wet deposition exacerbate surface air pollution as evidenced by higher surface COt and SAt concentrations. However, the average horizontal general circulation patterns in 2081-2100 are similar to 1981-2000, so the spatial distribution of COt changes little. Precipitation is an important factor controlling soluble pollutant wet removal, but the total global precipitation change alone does not necessarily indicate the sign of the soluble pollutant response to climate change. Over certain latitudinal bands, however, the annual wet deposition change can be explained mainly by the simulated changes in large-scale (LS) precipitation. In regions such as North America, differences in the seasonality of LS precipitation and tracer burdens contribute to an apparent inconsistency of changes in annual wet deposition versus annual precipitation. As a step towards an ultimate goal of developing a simple index that can be applied to infer changes in soluble pollutants directly from changes in precipitation fields as projected by physical climate models, we explore here a “Diagnosed Precipitation Impact” (DPI) index. This index captures the sign and magnitude (within 50%) of the relative annual mean changes in the global wet deposition of the soluble pollutant. DPI can only be usefully applied in climate models in which LS precipitation dominates wet deposition and horizontal transport patterns change little as climate warms. Our findings support the need for tighter emission regulations, for both soluble and insoluble pollutants, to obtain a desired level of air quality as climate warms.
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.
Gnanadesikan, Anand, John P Dunne, and Jasmin G John, July 2011: What ocean biogeochemical models can tell us about bottom-up control of ecosystem variability. ICES Journal of Marine Science, 68(6), DOI:10.1093/icesjms/fsr068. Abstract
Processes included in earth system models amplify the impact of climate variability on phytoplankton biomass and, therefore, on
upper trophic levels. Models predict much larger relative interannual variability in large phytoplankton biomass compared with
total phytoplankton biomass, supporting the goal of better constraining size-structured primary production and biomass from
remote sensing. The largest modelled variability in annually averaged large phytoplankton biomass is associated with changes in
the areal extent of relatively productive regions. Near the equator, changes in the areal extent of the high-productivity zone are
driven by large-scale shifts in nutrient fields, as well as changes in currents. Along the poleward edge of the Subtropical Gyres,
changes in physical mixing dominate. Finally, models indicate that high-latitude interannual variability in large phytoplankton
biomass is highest during spring. Mechanisms for producing such variability differ across biomes with internal ocean processes,
such as convection complicating efforts to link ecosystem variability to climate modes defined using sea surface temperature
alone. In salinity-stratified subpolar regions, changes in bloom timing driven by salinity can produce correlations between low
surface temperatures and high productivity, supporting the potential importance of using coupled atmosphere–ocean reanalyses,
rather than simple forced ocean reanalyses, for attributing past ecosystem shifts.
This paper documents time mean simulation characteristics from the ocean and sea ice components in a new coupled climate model developed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). The climate model, known as CM3, is formulated with effectively the same ocean and sea ice components as the earlier GFDL climate model, CM2.1, yet with extensive developments made to the atmosphere and land model components. Both CM2.1 and CM3 show stable mean climate indices, such as large scale circulation and sea surface temperatures (SSTs). There are notable improvements in the CM3 climate simulation relative to CM2.1, including a modified SST bias pattern and reduced biases in the Arctic sea ice cover. We anticipate SST differences between CM2.1 and CM3 in lower latitudes through analysis of the atmospheric fluxes at the ocean surface in corresponding Atmospheric Model Intercomparison Project (AMIP) simulations. In contrast, SST changes in the high latitudes are dominated by ocean and sea ice effects absent in AMIP simulations. The ocean interior simulation in CM3 is generally warmer than CM2.1, which adversely impacts the interior biases.
Marinov, I, and Anand Gnanadesikan, February 2011: Changes in ocean circulation and carbon storage are decoupled from air-sea CO2 fluxes. Biogeosciences, 8(2), DOI:10.5194/bg-8-505-2011. Abstract
The spatial distribution of the air-sea flux of carbon dioxide is a poor indicator of the underlying ocean circulation and of ocean carbon storage. The weak dependence on circulation arises because mixing-driven changes in solubility-driven and biologically-driven air-sea fluxes largely cancel out. This cancellation occurs because mixing driven increases in the poleward residual mean circulation result in more transport of both remineralized nutrients and heat from low to high latitudes. By contrast, increasing vertical mixing decreases the storage associated with both the biological and solubility pumps, as it decreases remineralized carbon storage in the deep ocean and warms the ocean as a whole.
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.
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.
De Boer, A M., Anand Gnanadesikan, N R Edwards, and A J Watson, February 2010: Meridional density gradients do not control the Atlantic overturning circulation. Journal of Physical Oceanography, 40(2), DOI:10.1175/2009JPO4200.1. Abstract
A wide body of modeling and theoretical scaling studies support the concept that changes to the Atlantic meridional overturning circulation (AMOC), whether forced by winds or buoyancy fluxes, can be understood in terms of a simple causative relation between the AMOC and an appropriately defined meridional density gradient (MDG). The MDG is supposed to translate directly into a meridional pressure gradient. Here two sets of experiments are performed using a modular ocean model coupled to an energy–moisture balance model in which the positive AMOC–MDG relation breaks down. In the first suite of seven model integrations it is found that increasing winds in the Southern Ocean cause an increase in overturning while the surface density difference between the equator and North Atlantic drops. In the second suite of eight model integrations the equation of state is manipulated so that the density is calculated at the model temperature plus an artificial increment ΔT that ranges from −3° to 9°C. (An increase in ΔT results in increased sensitivity of density to temperature gradients.) The AMOC in these model integrations drops as the MDG increases regardless of whether the density difference is computed at the surface or averaged over the upper ocean. Traditional scaling analysis can only produce this weaker AMOC if the scale depth decreases enough to compensate for the stronger MDG. Five estimates of the depth scale are evaluated and it is found that the changes in the AMOC can be derived from scaling analysis when using the depth of the maximum overturning circulation or estimates thereof but not from the pycnocline depth. These two depth scales are commonly assumed to be the same in theoretical models of the AMOC. It is suggested that the correlation between the MDG and AMOC breaks down in these model integrations because the depth and strength of the AMOC is influenced strongly by remote forcing such as Southern Ocean winds and Antarctic Bottom Water formation.
Galbraith, Eric D., Anand Gnanadesikan, John P Dunne, and M R Hiscock, March 2010: Regional impacts of iron-light colimitation in a global biogeochemical model. Biogeosciences, 7(3), DOI:10.5194/bg-7-1043-2010. Abstract
Laboratory and field studies have revealed that
iron has multiple roles in phytoplankton physiology, with
particular importance for light-harvesting cellular machinery.
However, although iron-limitation is explicitly included
in numerous biogeochemical/ecosystem models, its implementation
varies, and its effect on the efficiency of light harvesting
is often ignored. Given the complexity of the ocean
environment, it is difficult to predict the consequences of applying
different iron limitation schemes. Here we explore
the interaction of iron and nutrient cycles in an ocean general
circulation model using a new, streamlined model of
ocean biogeochemistry. Building on previously published
parameterizations of photoadaptation and export production,
the Biogeochemistry with Light Iron Nutrients and Gasses
(BLING) model is constructed with only three explicit tracers
but including macronutrient and micronutrient limitation,
light limitation, and an implicit treatment of community
structure. The structural simplicity of this computationally inexpensive
model allows us to clearly isolate the global effect
that iron availability has on maximum light-saturated
photosynthesis rates vs. the effect iron has on photosynthetic
efficiency. We find that the effect on light-saturated photosynthesis
rates is dominant, negating the importance of photosynthetic
efficiency in most regions, especially the cold
waters of the Southern Ocean. The primary exceptions to
this occur in iron-rich regions of the Northern Hemisphere,
where high light-saturated photosynthesis rates allow photosynthetic
efficiency to play a more important role. In other
words, the ability to efficiently harvest photons has little effect
in regions where light-saturated growth rates are low.
Additionally, we speculate that the small phytoplankton cells
dominating iron-limited regions tend to have relatively high
photosynthetic efficiency, due to reduced packaging effects.
If this speculation is correct, it would imply that natural communities
of iron-stressed phytoplankton may tend to harvest
photons more efficiently than would be inferred from iron limitation
experiments with larger phytoplankton. We suggest
that iron limitation of photosynthetic efficiency has a
relatively small impact on global biogeochemistry, though it
is expected to impact the seasonal cycle of plankton as well
as the vertical structure of primary production.
Because ocean color alters the absorption of sunlight, it can produce changes in sea surface temperatures with further impacts on atmospheric circulation. These changes can project onto fields previously recognized to alter the distribution of tropical cyclones. If the North Pacific subtropical gyre contained no absorbing and scattering materials, the result would be to reduce subtropical cyclone activity in the subtropical Northwest Pacific by 2/3, while concentrating cyclone tracks along the equator. Predicting tropical cyclone activity using coupled models may thus require consideration of the details of how heat moves into the upper thermocline as well as biogeochemical cycling.
We overview problems and prospects in ocean circulation models, with emphasis on certain developments aiming to
enhance the physical integrity and flexibility of large-scale models used to study global climate. We also consider elements
of observational measures rendering information to help evaluate simulations and to guide development priorities.
http://www.oceanobs09.net/blog/?p=88
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.
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.
The role of the penetration length scale of shortwave radiation into the surface ocean and its impact on tropical Pacific variability is investigated with a fully coupled ocean, atmosphere, land and ice model. Previous work has shown that removal of all ocean color results in a system that tends strongly towards an El Niño state. Results from a suite of surface chlorophyll perturbation experiments show that the mean state and variability of the tropical Pacific is highly sensitive to the concentration and distribution of ocean chlorophyll. Setting the near-oligotrophic regions to contain optically pure water warms the mean state and suppresses variability in the western tropical Pacific. Doing the same above the shadow zones of the tropical Pacific also warms the mean state but enhances the variability. It is shown that increasing penetration can both deepen the pycnocline (which tends to damp El Niño) while shifting the mean circulation so that the wind response to temperature changes is altered. Depending on what region is involved this change in the wind stress can either strengthen or weaken ENSO variability.
Fang, Y, Arlene M Fiore, Larry W Horowitz, Anand Gnanadesikan, Hiram Levy II, Y Hu, and A G. Russell, December 2009: Estimating the contribution of strong daily export events to total pollutant export from the United States in summer. Journal of Geophysical Research: Atmospheres, 114, D23302, DOI:10.1029/2008JD010946. Abstract
While the export of pollutants from the United States exhibits notable variability from day to day and is often considered to be “episodic,” the contribution of strong daily export events to total export has not been quantified. We use carbon monoxide (CO) as a tracer of anthropogenic pollutants in the Model of OZone And Related Tracers (MOZART) to estimate this contribution. We first identify the major export pathway from the United States to be through the northeast boundary (24–48°N along 67.5°W and 80–67.5°W along 48°N), and then analyze 15 summers of daily CO export fluxes through this boundary. These daily CO export fluxes have a nearly Gaussian distribution with a mean of 1100 Gg CO day−1 and a standard deviation of 490 Gg CO day−1. To focus on the synoptic variability, we define a “synoptic background” export flux equal to the 15 day moving average export flux and classify strong export days according to their fluxes relative to this background. As expected from Gaussian statistics, 16% of summer days are “strong export days,” classified as those days when the CO export flux exceeds the synoptic background by one standard deviation or more. Strong export days contributes 25% to the total export, a value determined by the relative standard deviation of the CO flux distribution. Regressing the anomalies of the CO export flux through the northeast U.S. boundary relative to the synoptic background on the daily anomalies in the surface pressure field (also relative to a 15 day running mean) suggests that strong daily export fluxes are correlated with passages of midlatitude cyclones over the Gulf of Saint Lawrence. The associated cyclonic circulation and Warm Conveyor Belts (WCBs) that lift surface pollutants over the northeastern United States have been shown previously to be associated with long-range transport events. Comparison with observations from the 2004 INTEX-NA field campaign confirms that our model captures the observed enhancements in CO outflow and resolves the processes associated with cyclone passages on strong export days. “Moderate export days,” defined as days when the CO flux through the northeast boundary exceeds the 15 day running mean by less than one standard deviation, represent an additional 34% of summer days and 40% of total export. These days are also associated with migratory midlatitude cyclones. The remaining 35% of total export occurs on “weak export days” (50% of summer days) when high pressure anomalies occur over the Gulf of Saint Lawrence. Our findings for summer also apply to spring, when the U.S. pollutant export is typically strongest, with similar contributions to total export and associated meteorology on strong, moderate and weak export days. Although cyclone passages are the primary driver for strong daily export events, export during days without cyclone passages also makes a considerable contribution to the total export and thereby to the global pollutant budget.
Gnanadesikan, Anand, and Whit G Anderson, February 2009: Ocean water clarity and the ocean general circulation in a coupled climate model. Journal of Physical Oceanography, 39(2), DOI:10.1175/2008JPO3935.1. Abstract
Ocean water clarity affects the
distribution of shortwave heating in the water column. In a one-dimensional
time-mean sense, increased clarity would be expected to cool the surface and
heat subsurface depths as shortwave radiation penetrates deeper into the
water column. However, wind-driven upwelling, boundary currents, and the
seasonal cycle of mixing can bring water heated at depth back to the
surface. This warms the equator and cools the subtropics throughout the year
while reducing the amplitude of the seasonal cycle of temperature in polar
regions. This paper examines how these changes propagate through the climate
system in a coupled model with an isopycnal ocean component focusing on the
different impacts associated with removing shading from different regions.
Increasing shortwave penetration along the equator causes warming to the
south of the equator. Increasing it in the relatively clear gyres off the
equator causes the Hadley cells to strengthen and the subtropical gyres to
shift equatorward. Increasing shortwave penetration in the less clear
regions overlying the oxygen minimum zones causes the cold tongue to warm
and the Walker circulation to weaken. Increasing shortwave penetration in
the high-latitude Southern Ocean causes an increase in the formation of mode
water from subtropical water. The results suggest that more attention be
paid to the processes distributing heat below the mixed layer.
Little, Christopher M., Anand Gnanadesikan, and M Oppenheimer, December 2009: How ice shelf morphology controls basal melting. Journal of Geophysical Research: Oceans, 114, C12007, DOI:10.1029/2008JC005197. Abstract
The response of ice shelf basal melting to climate is a function of ocean temperature, circulation, and mixing in the open ocean and the coupling of this external forcing to the sub–ice shelf circulation. Because slope strongly influences the properties of buoyancy-driven flow near the ice shelf base, ice shelf morphology plays a critical role in linking external, subsurface heat sources to the ice. In this paper, the slope-driven dynamic control of local and area-integrated melting rates is examined under a wide range of ocean temperatures and ice shelf shapes, with an emphasis on smaller, steeper ice shelves. A 3-D numerical ocean model is used to simulate the circulation underneath five idealized ice shelves, forced with subsurface ocean temperatures ranging from −2.0°C to 1.5°C. In the sub–ice shelf mixed layer, three spatially distinct dynamic regimes are present. Entrainment of heat occurs predominately under deeper sections of the ice shelf; local and area-integrated melting rates are most sensitive to changes in slope in this “initiation” region. Some entrained heat is advected upslope and used to melt ice in the “maintenance” region; however, flow convergence in the “outflow” region limits heat loss in flatter portions of the ice shelf. Heat flux to the ice exhibits (1) a spatially nonuniform, superlinear dependence on slope and (2) a shape- and temperature-dependent, internally controlled efficiency. Because the efficiency of heat flux through the mixed layer decreases with increasing ocean temperature, numerical simulations diverge from a simple quadratic scaling law.
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
Gnanadesikan, Anand, and I Marinov, 2008: Export is not enough: Nutrient cycling and carbon sequestration. Marine Ecology Progress Series, 364, DOI:10.3354/meps07550. Abstract
The question of whether ocean iron fertilization (OIF) can yield verifiable carbon
sequestration is often cast in terms of whether fertilization results in enhanced particle export. However,
model studies show that oceanic carbon storage is only weakly related to global particle
export—depending instead on an increase in the carbon associated with the pool of remineralized
nutrients. The magnitude of such an increase depends on circulation, stoichiometric ratios and gas
exchange. We argue that this puts serious challenges before efforts to properly credit OIF that must
be taken into account at the design stage.
Little, Christopher M., Anand Gnanadesikan, and Robert Hallberg, October 2008: Large-scale oceanographic constraints on the distribution of melting and freezing under ice shelves. Journal of Physical Oceanography, 38(10), DOI:10.1175/2008JPO3928.1. Abstract
Previous studies suggest that ice shelves experience asymmetric melting and freezing. Topography may constrain oceanic circulation (and thus basal melt–freeze patterns) through its influence on the potential vorticity (PV) field. However, melting and freezing induce a local circulation that may modify locations of heat transport to the ice shelf. This paper investigates the influence of buoyancy fluxes on locations of melting and freezing under different bathymetric conditions. An idealized set of numerical simulations (the “decoupled” simulations) employs spatially and temporally fixed diapycnal fluxes. These experiments, in combination with scaling considerations, indicate that while flow in the interior is governed by large-scale topographic gradients, recirculation plumes dominate near buoyancy fluxes. Thermodynamically decoupled models are then compared to those in which ice–ocean heat and freshwater fluxes are driven by the interior flow (the “coupled” simulations). Near the southern boundary, strong cyclonic flow forced by melt-induced upwelling drives inflow and melting to the east. Recirculation is less evident in the upper water column, as shoaling of meltwater-freshened layers dissipates the dynamic influence of buoyancy forcing, yet freezing remains intensified in the west. In coupled simulations, the flow throughout the cavity is relatively insensitive to bathymetry; stratification, the slope of the ice shelf, and strong, meridionally distributed buoyancy fluxes weaken its influence.
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 impact of the penetration length scale of shortwave radiation into the surface ocean is investigated with a fully coupled ocean, atmosphere, land and ice model. Oceanic shortwave radiation penetration is assumed to depend on the chlorophyll concentration. As chlorophyll concentrations increase the distribution of shortwave heating becomes shallower. This change in heat distribution impacts mixed-layer depth. This study shows that removing all chlorophyll from the ocean results in a system that tends strongly towards an El Niño state—suggesting that chlorophyll is implicated in maintenance of the Pacific cold tongue. The regions most responsible for this response are located off-equator and correspond to the oligotrophic gyres. Results from a suite of surface chlorophyll perturbation experiments suggest a potential positive feedback between chlorophyll concentration and a non-local coupled response in the fully coupled ocean-atmosphere system.
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.
In
many global ocean climate models, mesoscale eddies are parameterized as
along isopycnal diffusion and eddy-induced advection (or equivalently
skew-diffusion). The eddy-induced advection flattens isopycnals and acts as
a sink of available potential energy, whereas the isopycnal diffusion mixes
tracers along neutral directions. While much effort has gone into estimating
diffusivities associated with this closure, less attention has been paid to
the details of how this closure (which tries to flatten isopycnals)
interacts with the mixed layer (in which vertical mixing tries to drive the
isopycnals vertical). In order to maintain numerical stability, models often
stipulate a maximum slope Smax which in combination with
the thickness diffusivity Agm defines a maximum
eddy-induced advective transport Agm*Smax.
In this paper, we examine the impact of changing Smax
within the GFDL global coupled climate model. We show that this parameter
produces significant changes in wintertime mixed layer depth, with
implications for wintertime temperatures in key regions, the distribution of
precipitation, and the vertical structure of heat uptake. Smaller changes
are seen in details of ventilation and currents, and even smaller changes as
regards the overall hydrography. The results suggest that not only the value
of the coefficient, but the details of the tapering scheme, need to be
considered when comparing isopycnal mixing schemes in models.
Gnanadesikan, Anand, Joellen L Russell, and Fanrong Zeng, 2007: How does ocean ventilation change under global warming?Ocean Science, 3(1), 43-53. Abstract PDF
Since the upper ocean takes up much of the heat added to the earth system by anthropogenic global warming, one would expect that global warming would lead to an increase in stratification and a decrease in the ventilation of the ocean interior. However, multiple simulations in global coupled climate models using an ideal age tracer which is set to zero in the mixed layer and ages at 1 yr/yr outside this layer show that the intermediate depths in the low latitudes, Northwest Atlantic, and parts of the Arctic Ocean become younger under global warming. This paper reconciles these apparently contradictory trends, showing that the decreases result from changes in the relative contributions of old deep waters and younger surface waters. Implications for the tropical oxygen minimum zones, which play a critical role in global biogeochemical cycling are considered in detail.
Gnanadesikan, Anand, A M De Boer, and B K Mignone, 2007: A simple theory of the Pycnocline and overturning revisited In Ocean Circulation: Mechanisms and Impacts, Geophysical Monograph Series 173, Washington, DC, American Geophysical Union, 19-32. Abstract
A simple theory linking the pycnocline depth and volume transport of the thermohaline overturning to winds, eddies, surface densities and diapycnal diffusion was proposed by Gnanadesikan (Science, 283: 2077-2079, 1999. This paper revisits this theory, with eye to understanding which predictions are robust, and which may be limited by the geometric simplification required to derive such a simple theory. We show that the theory works extremely well for diagnostic models, in which surface density is fixed. It thus appears that the model can be used as a diagnostic framework for understanding mechanisms behind circulation changes. The key insight of the theory that the Southern Ocean, rather than the tropics, can serve as a pathway for transformation of dense water to light water is supported by the observed distribution of radiocarbon. However, we demonstrate that changes in forcing do more than simply scale the magnitude of the circulation up and down, producing changes in pycnocline shape and circulation geometry. In particular, the roles of buoyancy forcing and stationary eddies are more complicated than would be expected from the simple theory. Such changes must be taken into account when interpreting measurements at individual locations.
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.
The formulation and simulation characteristics of two new global coupled climate models developed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) are described. The models were designed to simulate atmospheric and oceanic climate and variability from the diurnal time scale through multicentury climate change, given our computational constraints. In particular, an important goal was to use the same model for both experimental seasonal to interannual forecasting and the study of multicentury global climate change, and this goal has been achieved.
Two versions of the coupled model are described, called CM2.0 and CM2.1. The versions differ primarily in the dynamical core used in the atmospheric component, along with the cloud tuning and some details of the land and ocean components. For both coupled models, the resolution of the land and atmospheric components is 2° latitude × 2.5° longitude; the atmospheric model has 24 vertical levels. The ocean resolution is 1° in latitude and longitude, with meridional resolution equatorward of 30° becoming progressively finer, such that the meridional resolution is 1/3° at the equator. There are 50 vertical levels in the ocean, with 22 evenly spaced levels within the top 220 m. The ocean component has poles over North America and Eurasia to avoid polar filtering. Neither coupled model employs flux adjustments.
The control simulations have stable, realistic climates when integrated over multiple centuries. Both models have simulations of ENSO that are substantially improved relative to previous GFDL coupled models. The CM2.0 model has been further evaluated as an ENSO forecast model and has good skill (CM2.1 has not been evaluated as an ENSO forecast model). Generally reduced temperature and salinity biases exist in CM2.1 relative to CM2.0. These reductions are associated with 1) improved simulations of surface wind stress in CM2.1 and associated changes in oceanic gyre circulations; 2) changes in cloud tuning and the land model, both of which act to increase the net surface shortwave radiation in CM2.1, thereby reducing an overall cold bias present in CM2.0; and 3) a reduction of ocean lateral viscosity in the extratropics in CM2.1, which reduces sea ice biases in the North Atlantic.
Both models have been used to conduct a suite of climate change simulations for the 2007 Intergovernmental Panel on Climate Change (IPCC) assessment report and are able to simulate the main features of the observed warming of the twentieth century. The climate sensitivities of the CM2.0 and CM2.1 models are 2.9 and 3.4 K, respectively. These sensitivities are defined by coupling the atmospheric components of CM2.0 and CM2.1 to a slab ocean model and allowing the model to come into equilibrium with a doubling of atmospheric CO2. The output from a suite of integrations conducted with these models is freely available online (see http://nomads.gfdl.noaa.gov/).
Manuscript received 8 December 2004, in final form 18 March 2005
The current generation of coupled climate models run at the Geophysical Fluid Dynamics Laboratory (GFDL) as part of the Climate Change Science Program contains ocean components that differ in almost every respect from those contained in previous generations of GFDL climate models. This paper summarizes the new physical features of the models and examines the simulations that they produce. Of the two new coupled climate model versions 2.1 (CM2.1) and 2.0 (CM2.0), the CM2.1 model represents a major improvement over CM2.0 in most of the major oceanic features examined, with strikingly lower drifts in hydrographic fields such as temperature and salinity, more realistic ventilation of the deep ocean, and currents that are closer to their observed values. Regional analysis of the differences between the models highlights the importance of wind stress in determining the circulation, particularly in the Southern Ocean. At present, major errors in both models are associated with Northern Hemisphere Mode Waters and outflows from overflows, particularly the Mediterranean Sea and Red Sea.
Gnanadesikan, Anand, and Ronald J Stouffer, 2006: Diagnosing atmosphere-ocean general circulation model errors relevant to the terrestrial biosphere using the Köppen climate classification. Geophysical Research Letters, 33, L22701, DOI:10.1029/2006GL028098. Abstract PDF
Coupled atmosphere-ocean-land-sea ice climate models (AOGCMs) are often tuned using physical variables like temperature and precipitation with the goal of minimizing properties such as the root-mean-square error. As the community moves towards modeling the earth system, it is important to note that not all biases have equivalent impacts on biology. Bioclimatic classification systems provide means of filtering model errors so as to bring out those impacts that may be particularly important for the terrestrial biosphere. We examine one such diagnostic, the classic system of Köppen, and show that it can provide an “early warning” of which model biases are likely to produce serious biases in the land biosphere. Moreover, it provides a rough evaluation criterion for the performance of dynamic vegetation models. State-of-the art AOGCMs fail to capture the correct Köppen zone in about 20–30% of the land area excluding Antarctica, and misassign a similar fraction to the wrong subzone.
Hallberg, Robert, and Anand Gnanadesikan, 2006: The role of eddies in determining the structure and response of the wind-driven Southern Hemisphere overturning: Results from the modeling eddies in the Southern Ocean (MESO) project. Journal of Physical Oceanography, 36(12), 2232-2252. Abstract PDF
The Modeling Eddies in the Southern Ocean (MESO) project uses numerical sensitivity studies to examine the role played by Southern Ocean winds and eddies in determining the density structure of the global ocean and the magnitude and structure of the global overturning circulation. A hemispheric isopycnal-coordinate ocean model (which avoids numerical diapycnal diffusion) with realistic geometry is run with idealized forcing at a range of resolutions from coarse (2°) to eddy-permitting (1/6°). A comparison of coarse resolutions with fine resolutions indicates that explicit eddies affect both the structure of the overturning and the response of the overturning to wind stress changes. While the presence of resolved eddies does not greatly affect the prevailing qualitative picture of the ocean circulation, it alters the overturning cells involving the Southern Ocean transformation of dense deep waters and light waters of subtropical origin into intermediate waters. With resolved eddies, the surface-to-intermediate water cell extends farther southward by hundreds of kilometers and the deep-to-intermediate cell draws on comparatively lighter deep waters. The overturning response to changes in the winds is also sensitive to the presence of eddies. In noneddying simulations, changing the Ekman transport produces comparable changes in the overturning, much of it involving transformation of deep waters and resembling the mean circulation. In the eddy-permitting simulations, a significant fraction of the Ekman transport changes are compensated by eddy-induced transport drawing from lighter waters than does the mean overturning. This significant difference calls into question the ability of coarse-resolution ocean models to accurately capture the impact of changes in the Southern Ocean on the global ocean circulation.
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.
A coupled climate model with poleward-intensified westerly winds simulates significantly higher storage of heat and anthropogenic carbon dioxide by the Southern Ocean in the future when compared with the storage in a model with initially weaker, equatorward-biased westerlies. This difference results from the larger outcrop area of the dense waters around Antarctica and more vigorous divergence, which remains robust even as rising atmospheric greenhouse gas levels induce warming that reduces the density of surface waters in the Southern Ocean. These results imply that the impact of warming on the stratification of the global ocean may be reduced by the poleward intensification of the westerlies, allowing the ocean to remove additional heat and anthropogenic carbon dioxide from the atmosphere.
Russell, Joellen L., Keith W Dixon, Anand Gnanadesikan, J R Toggweiler, and Ronald J Stouffer, August 2006: The once and future battles between Thor and the Midgard Serpent: The Southern Hemisphere Westerlies and the Antarctic Circumpolar Current. Geochimica et Cosmochimica Acta, 70(18 Supp 1), DOI:10.1016/j.gca.2006.06.1010. PDF
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.
A number of recent papers have argued that the mechanical energy budget of the ocean places constraints on how the thermohaline circulation is driven. These papers have been used to argue that climate models, which do not specifically account for the energy of mixing, potentially miss a very important feedback on climate change. This paper reexamines the question of what energetic arguments can teach us about the climate system and concludes that the relationship between energetics and climate is not straightforward. By analyzing the buoyancy transport equation, it is demonstrated that the large-scale transport of heat within the ocean requires an energy source of around 0.2 TW to accomplish vertical transport and around 0.4 TW (resulting from cabbeling) to accomplish horizontal transport. Within two general circulation models, this energy is almost entirely supplied by surface winds. It is also shown that there is no necessary relationship between heat transport and mechanical energy supply.
This paper summarizes the formulation of the ocean component to the Geophysical Fluid Dynamics Laboratory's (GFDL) climate model used for the 4th IPCC Assessment (AR4) of global climate change. In particular, it reviews the numerical schemes and physical parameterizations that make up an ocean climate model and how these schemes are pieced together for use in a state-of-the-art climate model. Features of the model described here include the following: (1) tripolar grid to resolve the Arctic Ocean without polar filtering, (2) partial bottom step representation of topography to better represent topographically influenced advective and wave processes, (3) more accurate equation of state, (4) three-dimensional flux limited tracer advection to reduce overshoots and undershoots, (5) incorporation of regional climatological variability in shortwave penetration, (6) neutral physics parameterization for representation of the pathways of tracer transport, (7) staggered time stepping for tracer conservation and numerical efficiency, (8) anisotropic horizontal viscosities for representation of equatorial currents, (9) parameterization of exchange with marginal seas, (10) incorporation of a free surface that accommodates a dynamic ice model and wave propagation, (11) transport of water across the ocean free surface to eliminate unphysical "virtual tracer flux" methods, (12) parameterization of tidal mixing on continental shelves. We also present preliminary analyses of two particularly important sensitivities isolated during the development process, namely the details of how parameterized subgridscale eddies transport momentum and tracers.
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.
The impact of changes in shortwave radiation penetration depth on the global ocean circulation and heat transport is studied using the GFDL Modular Ocean Model (MOM4) with two independent parameterizations that use ocean color to estimate the penetration depth of shortwave radiation. Ten to eighteen percent increases in the depth of 1% downwelling surface irradiance levels results in an increase in mixed layer depths of 3-20 m in the subtropical and tropical regions with no change at higher latitudes. While 1D models have predicted that sea surface temperatures at the equator would decrease with deeper penetration of solar irradiance, this study shows a warming, resulting in a 10% decrease in the required restoring heat flux needed to maintain climatological sea surface temperatures in the eastern equatorial Atlantic and Pacific Oceans. The decrease in the restoring heat flux is attributed to a slowdown in heat transport (5%) from the Tropics and an increase in the temperature of submixed layer waters being transported into the equatorial regions. Calculations were made using a simple relationship between mixed layer depth and meridional mass transport. When compared with model diagnostics, these calculations suggest that the slowdown in heat transport is primarily due to off-equatorial increases in mixed layer depths. At higher latitudes (5°-40°), higher restoring heat fluxes are needed to maintain sea surface temperatures because of deeper mixed layers and an increase in storage of heat below the mixed layer. This study offers a way to evaluate the changes in irradiance penetration depths in coupled ocean-atmosphere GCMs and the potential effect that large-scale changes in chlorophyll a concentrations will have on ocean circulation.
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.
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.
Gnanadesikan, Anand, Richard D Slater, and Bonita L Samuels, September 2003: Sensitivity of water mass transformation and heat transport to subgridscale mixing in coarse-resolution ocean models. Geophysical Research Letters, 30(18), 1967, DOI:10.1029/2003GL018036. Abstract
This paper considers the impact of the parameterization of subgridscale mixing on ocean heat transport in coarse-resolution ocean models of the type used in coupled climate models. Increasing the vertical diffusion increases poleward heat transport in both hemispheres. Increasing lateral diffusion associated with transient eddies increases poleward heat transport in the southern hemisphere while decreasing it in the northern hemisphere. The results are interpreted in the context of a simple analytical model.
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
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.
Gnanadesikan, Anand, and Robert Hallberg, 2002: Physical oceanography, thermal structure and general circulation In Encyclopedia of Physical Science and Technology, Vol. 12, New York, NY, Academic Press, 189-210. Abstract
Physical oceanography is concerned with the study of the physical processes which control the spatiotemporal structure of such fields as density, temperature, and velocity within the ocean. A major thrust of this field is the development of an understanding of the general circulation, namely the circulation of the ocean on large scales (of order 100-10,000 km) and over long times (decades to millennia). The general circulation is what determines the large-scale chemical and thermal structure of the ocean and plays a major role in global climate and biogeochemistry. The large-scale circulation can be broken down into three cells, a surface cell where wind driving is important, a deep cell where mixing is important, and an intermediate cell where potentially both wind and mixing are important. This article discusses the physical framework necessary to understand these circulations and presents standard models which explain some key features of the large-scale structure.
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.
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.
Hallberg, Robert, and Anand Gnanadesikan, 2001: An exploration of the role of transient eddies in determining the transport of a zonally reentrant current. Journal of Physical Oceanography, 31(11), 3312-3330. Abstract PDF
The meridional Ekman transport in a zonally reentrant channel may be balanced by diabatic circulations, standing eddies associated with topography, or by Lagrangian mean eddy mass fluxes. A simple model is used to explore the interaction between these mechanisms. A key assumption of this study is that diabatic forcing in the poleward edge of the channel acts to create lighter fluid, as is the case with net freshwater fluxes into the Southern Ocean. For weak wind forcing or strong diabatic constraint, a simple scaling argument accurately predicts the level of baroclinic shear. However, given our understanding of the relative magnitudes of Ekman flux and deep upwelling, this is not the appropriate parameter range for the Antarctic Circumpolar Current. With stronger wind stresses, eddies are prominent, with baroclinic instability initially developing in the vicinity of large topography. Arguments have been advanced by a number of authors that baroclinic instability should limit the velocity shear, leading to a stiff upper limit on the transport of the current. However, in the simulations presented here baroclinic instability is largely confined to the region of topographic highs, and the approach to a current that is independent of the wind stress occurs gradually. Several recent parameterizations of transient eddy fluxes do not reproduce key features of the observed behavior.
Gnanadesikan, Anand, and Robert Hallberg, 2000: On the relationship of the Circumpolar Current to Southern Hemisphere winds in coarse-resolution ocean models. Journal of Physical Oceanography, 30(8), 2013-2034. Abstract PDF
The response of the Circumpolar Current to changing winds has been the subject of much debate. To date, most theories of the current have tried to predict the transport using various forms of momentum balance. This paper argues that it is also important to consider thermodynamic as well as dynamic balances. Within large-scale general circulation models, increasing eastward winds within the Southern Ocean drive a northward Ekman flux of light water, which in turn produces a deeper pycnocline and warmer deep water to the north of the Southern Ocean. This in turn results in much larger thermal wind shear across the Circumpolar Current, which, given relatively small near-bottom velocities, results in an increase in Antarctic Circumpolar Current (ACC) transport. The Ekman flux near the surface is closed by a deep return flow below the depths of the ridges. A simple model that illustrates this picture is presented in which the ACC depends most strongly on the winds at the northern and southern edges of the channel. The sensitivity of this result to the formulation of buoyancy forcing is illustrated using a second simple model. A number of global general circulation model runs are then presented with different wind stress patterns in the Southern Ocean. Within these runs, neither the mean wind stress in the latitudes of Drake Passage nor the wind stress curl at the northern edge of Drake Passage produces a prediction for the transport of the ACC. However, increasing the wind stress within the Southern Ocean does increase the ACC transport.
Gnanadesikan, Anand, 1999: Connecting the Southern Ocean with the rest of the World: Results from large-scale ocean models. International WOCE Newsletter, 35, 30-31.
Gnanadesikan, Anand, 1999: A global model of silicon cycling: Sensitivity to eddy parameterization and dissolution. Global Biogeochemical Cycles, 13(1), 199-220. Abstract PDF
A simple model of silicon cycling has been embedded in a coarse-resolution ocean general circulation model. The modeled distribution of silicate was extremely sensitive to the parameterization of the effects of mesoscale eddies, and was somewhat sensitive to the distribution of dissolution within the water column. The modeled export flux of biogenic silica varied by a factor of 2 with respect to both the range of dissolution and eddy parameterizations. The best results were found when eddies were parameterized as mixing along isopycnals while simulataneously driving an advective flux so as to homogenize the depth of isopycnal surfaces (the so-called Gent-McWilliams parameterization). The Gent-McWilliams scheme produced the most realistic stratification field in the Southern Ocean, reducing vertical exchange and hence export fluxes of biogenic silica. This scheme did not solve the model's tendency to underpredict the formation of the North Atlantic Deep Water and to overpredict the importance of Antarctic source waters in the abyss, especially in the Atlantic. When used with a temperature-dependent dissolution scheme, this model predicted a global production of 89 Tmol yr SUP -1 with about 40% occurring in both the Southern Ocean and the tropics. In both the North Pacific and Southern Oceans, local silicate maxima are maintained by near-surface inflow of silicate with outflow of depth. The modeled production of biogenic silica showed little sensitivity to polar freshening when the Gent-McWilliams parameterization was used. The results emphasize the impact of including both the diffusive and advective effects of eddies, as in the Gent-McWilliams parameterization.
Gnanadesikan, Anand, 1999: Numerical issues for coupling biological models with isopycnal mixing schemes. Ocean Modelling, 1, 1-15. Abstract PDF
In regions of sloping isopycnals, isopycnal mixing acting in conjunction with biological cycling can produce patterns in the nutrient field which have negative values of tracer in light water and unrealistically large values of tracer in dense water. Under certain circumstances, these patterns can start to grow unstably. This paper discusses why such behavior occurs. Using a simple four-box model, it demonstrates that the instability appears when the isopycnal slopes exceed the grid aspect ration (Δ z / Δ x). In contrast to other well known instabilities of the CFL type, this instability does not depend on the time step or time-stepping scheme. Instead it arises from a fundamental incompatibility between two requirements for isopycnal mixing schemes, namely that they should produce no net flux of passive tracer across an isopycnal and everywhere reduce tracer extrema. In order to guarantee no net flux of tracer across an isopycnal, some upgradient fluxes across certain parts of an isopycnal are required to balance downgradient fluxes across other parts of the isopycnal. However, these upgradient fluxes can cause local maxima in the nutrient field to become self-reinforcing. Although this is less of a problem in larger domains, there is still a strong tendency for isopycnal mixing to overconcentrate tracer in the dense water. The introduction of eddy-induced advection is shown to be capable of counteracting the upgradient fluxes of nutrient which cause problems, stabilizing the solution. The issue is not simply a numerical curiosity. When used in a GCM, different parameterizations of eddy mixing result in noticeably different distributions of nutrient and large differences in biological production. While much of this is attributable to differences in convection and circulation, the numerical errors described here may also play an important role in runs with isopycnal mixing alone.
Gnanadesikan, Anand, 1999: A simple predictive model for the structure of the oceanic pycnocline. Science, 283(5410), 2077-2079. Abstract PDF
A simple theory for the large-scale oceanic circulation is developed, relating pycnocline depth, Northern Hemisphere sinking, and low-latitude upwelling to pycnocline diffusivity and Southern Ocean winds and eddies. The results show that Southern Ocean processes help maintain the global ocean structure and that pycnocline diffusion controls low-latitude upwelling.
Gnanadesikan, Anand, and J R Toggweiler, 1999: Constraints placed by silicon cycling on vertical exchange in general circulation models. Geophysical Research Letters, 26(13), 1865-1868. Abstract PDF
The flux of biogenic silica out of the oceanic mixed layer (the export flux) must balance the import of high-silicate deep waters associated with mass exchange between the surface and deep ocean. Recent regional estimates of the export flux limit it to 50-80 Tmol yr-1. In order to reproduce such low export fluxes, coarse-resolution general circulation models must have low pycnocline diffusivity and a lateral exchange scheme which involves advection of tracers by unresolved mesoscale eddies. Failure to capture low rates of vertical exchange will result in climate models which overpredict the uptake of anthropogenic carbon dioxide and fail to capture the proper locations and rates of density transformation.
Griffies, Stephen M., Anand Gnanadesikan, Ronald C Pacanowski, V D Larichev, J K Dukowicz, and R D Smith, 1998: Isoneutral diffusion in a z-coordinate ocean model. Journal of Physical Oceanography, 28(5), 805-830. Abstract PDF
This paper considers the requirements that must be satisfied in order to provide a stable and physically based isoneutral tracer diffusion scheme in a z-coordinate ocean model. Two properties are emphasized: 1) downgradient orientation of the diffusive fluxes along the neutral directions and 2) zero isoneutral diffusive flux of locally referenced potential density. It is shown that the Cox diffusion scheme does not respect either of these properties, which provides an explanation for the necessity to add a nontrivial background horizontal diffusion to that scheme. A new isoneutral diffusion scheme is proposed that aims to satisfy the stated properties and is found to require no horizontal background diffusion.
Holt, B, A K Liu, D W Wang, Anand Gnanadesikan, and H S Chen, 1998: Tracking storm-generated waves in the northeast Pacific Ocean with ERS-1 synthetic aperture radar imagery and buoys. Journal of Geophysical Research, 103(C4), 7917-7929. Abstract PDF
This paper examines the capability of synthetic aperture radar imagery from ERS-1 and buoys to track the wave field emanating from an intense storm over a several-day period. The first part of the study is a validation component that compares SAR-derived wave length and direction with buoy data from two locations over 10 different dates in late 1991 and National Oceanic and Atmospheric Administration (NOAA) wave model (WAM) wave direction results. When the SAR is linear (8 out of 10 cases), mean wavelength is within 5% of the buoy measurements and mean wave direction is within 1 degree of direction derived from the wave model (albeit with a large standard deviation of 27°), indicating close agreement. The wave field generated from the intense storm in late December 1991 was measured by three separate ERS-1 SAR passes over a 3-day period. A simple kinematic model was used for waves propagating from a storm. Comparing the model results with both SAR and buoy data indicates that SAR-derived peak wavelength and direction measurements can be reliably used to predict arrival times and propagation direction over a several-day period and considerable distances. The measurements can also be used to derive estimated wave generation source regions about the storm as well. Such measurements are useful for comparing with wave model results, which perform less accurately for direction than wave height for example, and for predicting hazardous conditions for ship navigation and coastal regions.
Pacanowski, Ronald C., and Anand Gnanadesikan, 1998: Transient response in a Z-level ocean model that resolves topography with partial cells. Monthly Weather Review, 126(12), 3248-3270. Abstract PDF
Ocean simulations are in part determined by topographic waves with speeds and spatial scales dependent on bottom slope. By their very nature, discrete z-level ocean models have problems accurately representing bottom topography when slopes are less than the grid cell aspect ratio delta z/delta x. In such regions, the dispersion relation for topographic waves is inaccurate. However, bottom topography can be accurately represented in discrete z-level models by allowing bottom-most grid cells to be partially filled with land. Consequently, gently sloping bottom topography is resolved on the scale of horizontal grid resolution and the dispersion relation for topographic waves is accurately approximated. In contrast to the standard approach using full cells, partial cells imply that all grid points within a vertical level are not necessarily at the same depth and problems arise with pressure gradient errors and the spurious diapycnal diffusion. However, both problems have been effectively dealt with. Differences in flow fields between simulations with full cells and partial cells can be significant, and simulations with partial cells are more robust than with full cells. Partial cells provide a superior representation of topographic waves when compared to the standard method employing full cells.
An important component of the ocean's thermohaline circulation is the sinking of dense water from continental shelves to abyssal depths. Such downslope flow is thought to be a consequence of bottom stress retarding the alongslope flow of density-driven plumes. In this paper the authors explore the potential for explicitly simulating the simple mechanism in z-coordinate models. A series of experiments are performed using a twin density-coordinate model simulation as a standard of comparison. The adiabatic nature of the experiments and the importance of bottom slope make it more likely that the density-coordinate model will faithfully reproduce the solution. The difficulty of maintaining the density signal as the plume descends the slope is found to be the main impediment to accurate simulation in the z-coordinate model. The results of process experiments suggest that the model solutions will converge when the z-coordinate model has sufficient vertical resolution to resolve the bottom viscous layer and horizontal grid spacing equal to its vertical grid spacing divided by the maximum slope. When this criterion is met it is shown that the z-coordinate model converges to an analytical solution for a simple two-dimensional flow.
Gnanadesikan, Anand, and Ronald C Pacanowski, 1997: Improved representation of flow around topography in the GFDL modular ocean model MOM 2. International WOCE Newsletter, 27, 23-25.
Gnanadesikan, Anand, 1996: Mixing driven by vertically variable forcing: an application to the case of Langmuir circulation. Journal of Fluid Mechanics, 322, DOI:10.1017/S0022112096002716. Abstract
Two-dimensional mixing driven by an instability mechanism which is concentrated near one of the boundaries is considered, with particular application to Langmuir circulations driven by a wave spectrum. The question of how to define the equivalent of the Rayleigh number is attacked using the energy balance equations and simple truncated models of the instability. Given a particular horizontal wavelength for the disturbance, the strength of the forcing on the cells, and thus the growth rate, is determined by a tradeoff between maximizing the depth-averaged forcing and maximizing the depth of penetration. As a result of this tradeoff, long-wavelength cells grow more slowly, but penetrate more deeply and have a larger equivalent Rayleigh number. At finite amplitude, these long-wavelength cells come to dominate the flow field. The depth of penetration of, and density transport accomplished by Langmuir cells is considered as a function of the mean stratification and diffusion. An application to oceanic mixed layers is considered assuming the Mellor-Yamada 2-1/2-level turbulence closure model to define the background level of turbulent mixing. For many realistic cases, Langmuir cells are predicted to dominate the vertical transport of momentum and density.
Gnanadesikan, Anand, 1996: Modeling the diurnal cycle of carbon monoxide: Sensitivity to physics, chemistry, biology, and optics. Journal of Geophysical Research, 101(C5), DOI:10.1029/96JC00463. Abstract
As carbon monoxide within the oceanic surface layer is produced by solar radiation, diluted by mixing, consumed by biota, and outgassed to the atmosphere, it exhibits a diurnal cycle. The effect of dilution and mixing on this cycle is examined using a simple model for production and consumption coupled to three different mixed layer models. The magnitude and timing of the peak concentration, the magnitude of the average concentration, and the air-sea flux are considered. The models are run through a range of heating and wind stress and compared to experimental data reported by Kettle [1994]. The key to the dynamics is the relative size of four length scales; D mix, the depth to which mixing occurs over the consumption time; L, the length scale over which production occurs; L out, the depth to which the mixed layer is ventilated over the consumption time; and L comp, the depth to which the diurnal production can maintain a concentration in equilibrium with the atmosphere. If Dmix >> L, the actual model parameterization can be important. If the mixed layer is maintained by turbulent diffusion, Dmix can be substantially less than the mixed layer depth. If the mixed layer is parameterized as a homogeneous slab, Dmix is equivalent to the mixed layer depth. If Dmix > Lout. production is balanced by consumption rather than outgassing. The ratio between Dmix and Lcomp determines whether the ocean is a source or a sink for CO. The main thermocline depth H sets an upper limit for Dmix and hence Dmix/L,Dmix/Lout and Dmix/Lcomp. The models are run to simulate a single day of observations. The mixing parameterization is shown to be very important, with a model which mixes using small-scale diffusion producing markedly larger surface concentrations than models which homogenize the mixed layer completely and instantaneously.
Gnanadesikan, Anand, and R A Weller, December 1995: Structure and instability of the Ekman spiral in the presence of surface gravity waves. Journal of Physical Oceanography, 25(12), DOI:10.1175/1520-0485(1995)025<3148:SAIOTE>2.0.CO;2. Abstract
The physical processes responsible for maintaining the mixed layer are examined by considering the velocity structure. The low-frequency Ekman response in the interior of unstratified mixed layers is much less sheared than is predicted using eddy viscosity models that reproduce the temperature structure. However, the response is more sheared than predicted by models that parameterize the mixed layer as a slab. An explanation is sought by considering the effect of an infinite train of surface gravity waves on the mean Ekman spiral. For some realistic conditions, the Ekman spiral predicted by assuming small-scale diffusion alone is strongly unstable to Langmuir cells driven by wave-current interaction. In the Northern Hemisphere, these cells are oriented to the right of the wind, the result of a balance between maximizing the wave- current forcing, maximizing the efficiency of this forcing in producing cells, and minimizing the crosscell shear. The cells are capable of replacing small-scale turbulent diffusion as the principal transport mechanism within the mixed layer. Finite-difference code runs that include infinite-length trains of surface gravity waves qualitatively explain the reduction in shear within the mixed layer relative to that predicted by small-scale mixing. However, the theory also predicts an Eulerian return flow balancing the Stokes drift that has not been observed.
