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.
Talley, Lynne D., I Rosso, I V Kamenkovich, Matthew R Mazloff, J Wang, E S Boss, Alison R Gray, K S Johnson, Robert M Key, S C Riser, N L Williams, and Jorge L Sarmiento, January 2019: Southern Ocean biogeochemical float deployment strategy, with example from the Greenwich Meridian line (GO‐SHIP A12). Journal of Geophysical Research: Oceans, 124(1), DOI:10.1029/2018JC014059. Abstract
Biogeochemical Argo floats, profiling to 2000 m depth, are being deployed throughout the Southern Ocean by the Southern Ocean Carbon and Climate Observations and Modeling program (SOCCOM). The goal is 200 floats by 2020, to provide the first full set of annual cycles of carbon, oxygen, nitrate and optical properties across multiple oceanographic regimes. Building from no prior coverage to a sparse array, deployments are based on prior knowledge of water mass properties, mean frontal locations, mean circulation and eddy variability, winds, air‐sea heat/freshwater/carbon exchange, prior Argo trajectories, and float simulations in the Southern Ocean State Estimate (SOSE) and Hybrid Coordinate Ocean Model (HYCOM). Twelve floats deployed from the 2014‐2015 Polarstern cruise from South Africa to Antarctica are used as a test case to evaluate the deployment strategy adopted for SOCCOM's 20 deployment cruises and 126 floats to date. After several years, these floats continue to represent the deployment zones targeted in advance: (1) Weddell Gyre sea ice zone, including the Antarctic Slope Front, Maud Rise, and the open gyre; (2) Antarctic Circumpolar Current (ACC) including the topographically‐steered Southern zone ‘chimney' where upwelling carbon/nutrient‐rich deep waters produce surprisingly large carbon dioxide outgassing; (3) Subantarctic and Subtropical zones between the ACC and Africa; and (4) Cape Basin. Argo floats and eddy‐resolving HYCOM simulations were the best predictors of individual SOCCOM float pathways, with uncertainty after 2 years on the order of 1000 km in the sea ice zone and more than double that in and north of the ACC.
Chamberlain, P M., Lynne D Talley, Matthew R Mazloff, S C Riser, K Speer, and Alison R Gray, et al., November 2018: Observing the Ice‐Covered Weddell Gyre With Profiling Floats: Position Uncertainties and Correlation Statistics. Journal of Geophysical Research: Oceans, 123(11), DOI:10.1029/2017JC012990. Abstract
Argo‐type profiling floats do not receive satellite positioning while under sea ice. Common practice is to approximate unknown positions by linearly interpolating latitude‐longitude between known positions before and after ice cover, although it has been suggested that some improvement may be obtained by interpolating along contours of planetary‐geostrophic potential vorticity. Profiles with linearly interpolated positions represent 16% of the Southern Ocean Argo data set; consequences arising from this approximation have not been quantified. Using three distinct data sets from the Weddell Gyre—10‐day satellite‐tracked Argo floats, daily‐tracked RAFOS‐enabled floats, and a particle release simulation in the Southern Ocean State Estimate—we perform a data withholding experiment to assess position uncertainty in latitude‐longitude and potential vorticity coordinates as a function of time since last fix. A spatial correlation analysis using the float data provides temperature and salinity uncertainty estimates as a function of distance error. Combining the spatial correlation scales and the position uncertainty, we estimate uncertainty in temperature and salinity as a function of duration of position loss. Maximum position uncertainty for interpolation during 8 months without position data is 116 ± 148 km for latitude‐longitude and 92 ± 121 km for potential vorticity coordinates. The estimated maximum uncertainty in local temperature and salinity over the entire 2,000‐m profiles during 8 months without position data is 0.66 ∘C and 0.15 psu in the upper 300 m and 0.16 ∘C and 0.01 psu below 300 m.
In this paper we study upwelling pathways and timescales of Circumpolar Deep Water (CDW) in a hierarchy of models using a Lagrangian particle tracking method. Lagrangian timescales of CDW upwelling decrease from 87 years to 31 years to 17 years as the ocean resolution is refined from 1° to 0.25° to 0.1°. We attribute some of the differences in timescale to the strength of the eddy fields, as demonstrated by temporally degrading high resolution model velocity fields. Consistent with the timescale dependence, we find that an average Lagrangian particle completes 3.2 circumpolar loops in the 1° model in comparison to 0.9 loops in the 0.1° model. These differences suggest that advective timescales and thus inter-basin merging of upwelling CDW may be overestimated by coarse resolution models, potentially affecting the skill of centennial scale climate change projections.
Gray, Alison R., K S Johnson, Seth M Bushinsky, S C Riser, Joellen L Russell, Lynne D Talley, R Wanninkhof, N L Williams, and Jorge L Sarmiento, September 2018: Autonomous biogeochemical floats detect significant carbon dioxide outgassing in the high‐latitude Southern Ocean. Geophysical Research Letters, 45(17), DOI:10.1029/2018GL078013. Abstract
Although the Southern Ocean is thought to account for a significant portion of the contemporary oceanic uptake of carbon dioxide (CO2), flux estimates in this region are based on sparse observations that are strongly biased towards summer. Here we present new estimates of Southern Ocean air‐sea CO2 fluxes calculated with measurements from biogeochemical profiling floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project during 2014‐2017. Compared to ship‐based CO2 flux estimates, the float‐based fluxes find significantly stronger outgassing in the zone around Antarctica where carbon‐rich deep waters upwell to the surface ocean. Although interannual variability contributes, this difference principally stems from the lack of autumn and winter ship‐based observations in this high‐latitude region. These results suggest that our current understanding of the distribution of oceanic CO2 sources and sinks may need revision and underscore the need for sustained year‐round biogeochemical observations in the Southern Ocean.
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.
Upwelling of global deep waters to the sea surface in the Southern Ocean closes the global overturning circulation and is fundamentally important for oceanic uptake of carbon and heat, nutrient resupply for sustaining oceanic biological production, and the melt rate of ice shelves. However, the exact pathways and role of topography in Southern Ocean upwelling remain largely unknown. Here we show detailed upwelling pathways in three dimensions, using hydrographic observations and particle tracking in high-resolution models. The analysis reveals that the northern-sourced deep waters enter the Antarctic Circumpolar Current via southward flow along the boundaries of the three ocean basins, before spiraling southeastward and upward through the Antarctic Circumpolar Current. Upwelling is greatly enhanced at five major topographic features, associated with vigorous mesoscale eddy activity. Deep water reaches the upper ocean predominantly south of the Antarctic Circumpolar Current, with a spatially nonuniform distribution. The timescale for half of the deep water to upwell from 30° S to the mixed layer is ~60–90 years.
