Arteaga, Lionel, M Pahlow, Seth M Bushinsky, and Jorge L Sarmiento, August 2019: Nutrient controls on export production in the Southern Ocean. Global Biogeochemical Cycles, 33(8), DOI:10.1029/2019GB006236. Abstract
We use observations from novel biogeochemical profiling floats deployed by the SOCCOM program to estimate annual net community production (ANCP) (associated with carbon export) from the seasonal drawdown of mesopelagic oxygen and surface nitrate in the Southern Ocean. Our estimates agree with previous observations in showing an increase in ANCP in the vicinity of the polar front (~3 mol C m−2 y−1), compared to lower rates in the subtropical zone (≤1 mol C m−2 y−1) and the seasonal ice zone (<2 mol C m−2 y−1). Paradoxically, the increase in ANCP south of the subtropical front is associated with elevated surface nitrate and silicate concentrations, but decreasing surface iron. We hypothesize that iron limitation promotes silicification in diatoms, which is evidenced by the low silicate to nitrate ratio of surface waters around the Antarctic polar front. High diatom silicification increases the ballasting effect of POC and overall ANCP in this region. A model‐based assessment of our methods shows a good agreement between ANCP estimates based on oxygen and nitrate drawdown and the modeled downward organic carbon flux at 100 m. This agreement supports the presumption that net biological consumption is the dominant process affecting the drawdown of these chemical tracers, and that, given sufficient data, ANCP can be inferred from observations of oxygen and/or nitrate drawdown in the Southern Ocean.
Bushinsky, Seth M., et al., September 2019: Observing Changes in Ocean Carbonate Chemistry: Our Autonomous Future. Current Climate Change Reports, 5(3), DOI:10.1007/s40641-019-00129-8. Abstract
Purpose of Review
We summarize recent progress on autonomous observations of ocean carbonate chemistry and the development of a network of sensors capable of observing carbonate processes at multiple temporal and spatial scales.
Recent Findings
The development of versatile pH sensors suitable for both deployment on autonomous vehicles and in compact, fixed ecosystem observatories has been a major development in the field. The initial large-scale deployment of profiling floats equipped with these new pH sensors in the Southern Ocean has demonstrated the feasibility of a global autonomous open-ocean carbonate observing system.
Summary
Our developing network of autonomous carbonate observations is currently targeted at surface ocean CO2 fluxes and compact ecosystem observatories. New integration of developed sensors on gliders and surface vehicles will increase our coastal and regional observational capability. Most autonomous platforms observe a single carbonate parameter, which leaves us reliant on the use of empirical relationships to constrain the rest of the carbonate system. Sensors now in development promise the ability to observe multiple carbonate system parameters from a range of vehicles in the near future.
Bushinsky, Seth M., P Landschützer, C Rödenbeck, Alison R Gray, D F Baker, Matthew R Mazloff, Laure Resplandy, K S Johnson, and Jorge L Sarmiento, November 2019: Reassessing Southern Ocean air‐sea CO2 flux estimates with the addition of biogeochemical float observations. Global Biogeochemical Cycles, 33(11), DOI:10.1029/2019GB006176. Abstract
New estimates of pCO2 from profiling floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project have demonstrated the importance of wintertime outgassing south of the Polar Front, challenging the accepted magnitude of Southern Ocean carbon uptake (Gray et al. 2018). Here, we put 3.5 years of SOCCOM observations into broader context with the global surface carbon dioxide database (Surface Ocean CO2 Atlas, SOCAT) by using the two interpolation methods currently used to assess the ocean models in the Global Carbon Budget (Le Quéré et al. 2018) to create a ship‐only, a float‐weighted, and a combined estimate of Southern Ocean carbon fluxes (< 35°S). In our ship‐only estimate, we calculate a mean uptake of ‐1.14 ± 0.19 Pg C yr‐1 for 2015‐2017, consistent with prior studies. The float‐weighted estimate yields a significantly lower Southern Ocean uptake of ‐0.35 ± 0.19 Pg C yr‐1. Subsampling of high‐resolution ocean biogeochemical process models indicates that some of the difference between float and ship‐only estimates of the Southern Ocean carbon flux can be explained by spatial and temporal sampling differences. The combined ship and float estimate minimizes the root mean square pCO2 difference between the mapped product and both datasets, giving a new Southern Ocean uptake of ‐0.75 ± 0.22 Pg C yr‐1, though with uncertainties that overlap the ship‐only estimate. An atmospheric inversion reveals that a shift of this magnitude in the contemporary Southern Ocean carbon flux must be compensated for by ocean or land sinks within the Southern Hemisphere.
Bushinsky, Seth M., and S Emerson, November 2018: Biological and physical controls on the oxygen cycle in the Kuroshio Extension from an array of profiling floats. Deep-Sea Research, Part I, 141, DOI:10.1016/j.dsr.2018.09.005. Abstract
The Kuroshio Extension (KE) is a current associated with the largest CO2 flux into the Pacific Ocean, a broad region of uptake that extends across the Pacific basin between the subarctic and subtropical regions. The relative importance of the biological and physical processes controlling this sink is uncertain. Because oxygen is stoichiometrically linked to changes in dissolved inorganic carbon due to photosynthesis and respiration and subject to many of the same physical drivers as the CO2 flux, in situ oxygen measurements help to determine the processes driving this large CO2 flux. We analyzed data from eight Argo profiling floats equipped with oxygen sensors to estimate oxygen fluxes in the upper ocean of the KE region (approximate bounds: 25°N to 45°N, 135°E to 155°E). In situ air calibrations of these sensors allowed us to accurately measure air-sea oxygen differences, which largely control the flux of oxygen to and from the atmosphere. To characterize distinct biogeographical regions in the Kuroshio Extension and to accommodate seasonal north-south shifts in the location of the regional boundaries, we averaged oxygen measurements from different floats along isopycnal surfaces into 3 regions based on temperature-salinity relationships: North KE, Central KE, and South KE. Using these regional concentration time series, we determined the physical fluxes using an upper ocean layered model and calculated the residual oxygen flux, a combination of column-integrated net community production and physical processes unexplained by the model. The annual oxygen budget is largely a balance of air-sea exchange and the residual oxygen term. Residual oxygen fluxes are -5.4 ± 1.1, -5.9 ± 0.1, and 2.1 ± 2.2 mol O2 m-2 yr-1 (where negative is a loss from the upper ocean) for the North, Central, and South KE regions, respectively. The North and Central KE are regions of mode water formation, which balances the large air-sea fluxes into the ocean. The South KE oxygen residual indicates a biologically produced flux to the atmosphere in two out of three years that agrees with previous estimates of subtropical annual net community production (ANCP) but exhibits high interannual variability. This study suggests that physical processes are the primary drivers for the annual uptake of the gases oxygen and carbon dioxide in the Central KE region where the annual CO2 uptake is strongest.
Gray, Alison R., K S Johnson, Seth M Bushinsky, S C Riser, Joellen L Russell, Lynne D Talley, R Wanninkhof, N L Williams, and Jorge L Sarmiento, September 2018: Autonomous biogeochemical floats detect significant carbon dioxide outgassing in the high‐latitude Southern Ocean. Geophysical Research Letters, 45(17), DOI:10.1029/2018GL078013. Abstract
Although the Southern Ocean is thought to account for a significant portion of the contemporary oceanic uptake of carbon dioxide (CO2), flux estimates in this region are based on sparse observations that are strongly biased towards summer. Here we present new estimates of Southern Ocean air‐sea CO2 fluxes calculated with measurements from biogeochemical profiling floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project during 2014‐2017. Compared to ship‐based CO2 flux estimates, the float‐based fluxes find significantly stronger outgassing in the zone around Antarctica where carbon‐rich deep waters upwell to the surface ocean. Although interannual variability contributes, this difference principally stems from the lack of autumn and winter ship‐based observations in this high‐latitude region. These results suggest that our current understanding of the distribution of oceanic CO2 sources and sinks may need revision and underscore the need for sustained year‐round biogeochemical observations in the Southern Ocean.
Bushinsky, Seth M., Alison R Gray, K S Johnson, and Jorge L Sarmiento, November 2017: Oxygen in the Southern Ocean From Argo Floats: Determination of Processes Driving Air-Sea Fluxes. Journal of Geophysical Research: Oceans, 122(11), DOI:10.1002/2017JC012923. Abstract
The Southern Ocean is of outsized significance to the global oxygen and carbon cycles with relatively poor measurement coverage due to harsh winters and seasonal ice cover. In this study, we use recent advances in the parameterization of air-sea oxygen fluxes to analyze 9 years of oxygen data from a recalibrated Argo oxygen data set and from air-calibrated oxygen floats deployed as part of the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project. From this combined data set of 150 floats, we find a total Southern Ocean oxygen sink of −183 ± 80 Tmol yr−1 (positive to the atmosphere), greater than prior estimates. The uptake occurs primarily in the Polar-Frontal Antarctic Zone (PAZ, −94 ± 30 Tmol O2 yr−1) and Seasonal Ice Zone (SIZ, −111 ± 9.3 Tmol O2 yr−1). This flux is driven by wintertime ventilation, with a large portion of the flux in the SIZ passing through regions with fractional sea ice. The Subtropical Zone (STZ) is seasonally driven by thermal fluxes and exhibits a net outgassing of 47 ± 29 Tmol O2 yr−1 that is likely driven by biological production. The Subantarctic Zone (SAZ) uptake is −25 ± 12 Tmol O2 yr−1. Total oxygen fluxes were separated into a thermal and nonthermal component. The nonthermal flux is correlated with net primary production and mixed layer depth in the STZ, SAZ, and PAZ, but not in the SIZ where seasonal sea ice slows the air-sea gas flux response to the entrainment of deep, low-oxygen waters.
Yang, B, S Emerson, and Seth M Bushinsky, April 2017: Annual net community production in the subtropical Pacific Ocean from in situ oxygen measurements on profiling floats. Global Biogeochemical Cycles, 31(4), DOI:10.1002/2016GB005545. Abstract
Annual net community production (ANCP) in the subtropical Pacific Ocean was determined by using annual oxygen measurements from Argo profiling floats with an upper water column oxygen mass balance model. ANCP was determined to be from 2.0 to 2.4 mol C m−2 yr−1 in the western subtropical North Pacific, 2.4 mol C m−2 yr−1 in the eastern subtropical North Pacific, and near zero in the subtropical South Pacific. Error analysis with the main sources of uncertainty being the accuracy of oxygen measurements and the parameterization of bubble fluxes in winter suggested an uncertainty of ~0.3 mol C m−2 yr−1 in subtropical Pacific. The results are in good agreement with previous observations in locations where ANCP has been determined before. These are the first results from the western subtropical North Pacific and subtropical South Pacific where ANCP have not been evaluated before. ANCP for the subtropical South Pacific is significantly lower than in all other open ocean locations where it has been determined by mass balance. Comparison of our observations with net biological carbon export estimated from remote sensing algorithms indicates that observations from the subtropical North Pacific are higher than the satellite estimates, but those in the subtropical South Pacific are lower than satellite‐determined carbon export.
Bushinsky, Seth M., and S Emerson, June 2016: The role of bubbles during air‐sea gas exchange. Journal of Geophysical Research: Oceans, 121(6), DOI:10.1002/2016JC011744. Abstract
The potential for using the air‐sea exchange rate of oxygen as a tracer for net community biological production in the ocean is greatly enhanced by recent accuracy improvements for in situ measurements of oxygen on unmanned platforms. A limiting factor for determining the exchange process is evaluating the air‐sea flux contributed by bubble processes produced by breaking waves, particularly during winter months under high winds. Highly accurate measurements of noble gases (Ne, Ar & Kr) and nitrogen, N2, in seawater are tracers of the importance of bubble process in the surface mixed layer. We use measured distributions of these gases in the ventilated thermocline of the North Pacific and an annual time series of N2 in the surface ocean of the NE Subarctic Pacific to evaluate four different air‐water exchange models chosen to represent the range of model interpretation of bubble processes. We find that models must have an explicit bubble mechanism to reproduce concentrations of insoluble atmospheric gases, but there are periods when they all depart from observations. The recent model of Liang et al. (2013) stems from a highly resolved model of bubble plumes and categorizes bubble mechanisms into those that are small enough to collapse and larger ones that exchange gases before they resurface, both of which are necessary to explain the data.
Newsom, E R., Andrea J Fassbender, A E Maloney, and Seth M Bushinsky, September 2016: Increasing the usability of climate science in political decision-making. Elementa: Science of the Anthropocene, 4, DOI:10.12952/journal.elementa.000127. Abstract
As climate-science graduate students at the University of Washington, we had the opportunity to engage in a political process focused on implementing legislation to reduce greenhouse gas emissions in Washington State. Our insights gained from this rare, first-hand, experience may be particularly relevant to other climate scientists. We argue that inflexible research goals within the United States climate-science community limit the relevance of the knowledge our community creates. The mismatch between climate-science research and the information needs of policy makers, while widely acknowledged in certain domains, has yet to be fully appreciated within many earth science disciplines. Broadening the climate-science training of graduate students to include education on the uses of climate information outside of academic settings would both inform and motivate new research directions, and engender validation of non-traditional research within disciplinary cultures.
