Ship-based CO2 flux estimates of the contemporary air-sea flux of CO2 showed that the Southern Ocean (south of 35oS) plays an important role as a significant carbon sink, with a net uptake at the rate of −0.8 ~ −1.0 Pg C/year (Takahashi et al., 2009; Landschützer et al., 2014) largely consistent with climate model-based estimates (e.g., Nevison et al., 2016). However, a recent study based on measurements from biogeochemical profiling floats estimated a much smaller net CO2 uptake rate of −0.08 Pg C/year for the Southern Ocean south of 35oS (Gray et al., 2018). This is because ship-based CO2 flux estimates underestimate CO2 outgassing in the poorly observed area around Antarctica where carbon-rich Circumpolar Deep Waters upwell to the surface ocean, particularly in the winter season. A new study published in Science used aircraft-based measurements of the vertical atmospheric CO2 gradient to provide a revised estimate of –0.53 Pg C/year for the Southern Ocean south of 45oS. The new estimate indicates stronger summertime uptake and weaker wintertime outgassing compared to the recent profiling float-based estimate and confirms the role of the Southern Ocean as a significant carbon sink.
Figure 4 from Long et al., (2021): Observationally based estimates of Southern Ocean air-sea fluxes. (A) The seasonal cycle of air-sea CO2 flux south of 45°S estimated from aircraft campaigns (black points, labels), plotted at the center of the 90-day window for which the emergent flux constraint was calibrated. Whiskers show the standard deviation derived from propagating analytical and statistical uncertainties; the black line shows a two-harmonic fit used to estimate the annual mean flux. The colored lines give the seasonal cycle from atmospheric inversion systems as well as the neural network extrapolation of the Surface Ocean CO2 Atlas (SOCAT) pCO2 observations and profiling float observations from the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project. Fluxes are averaged over the period 2009–2018, except for the three neural network–based flux estimates incorporating SOCCOM observations, which are averaged over the period 2015–2017. (B) Annual mean flux estimated in this study (leftmost bar) including uncertainty (whisker), along with the mean and standard deviation (whiskers) across the inversion systems shown in (A) as well as the surface-ocean pCO2-based methods; averaging time periods are noted in the axis labels (both SOCAT flux estimates were derived using neural network training over the full observational period).
Long, M. C., Stephens, B. B., McKain, K., Sweeney, C., Keeling, R. F., Kort, E. A., … & Wofsy, S. C. (2021). Strong Southern Ocean carbon uptake evident in airborne observations. Science, 374(6572), 1275-1280. https://doi.org/10.1126/science.abi4355
Takahashi, T., Sutherland, S. C., Wanninkhof, R., Sweeney, C., Feely, R. A., Chipman, D. W., et al. (2009). Climatological mean and decadal change in surface ocean pCO2 and net sea-air CO2 flux over the global oceans. Deep Sea Research, Part II, 56(8-10), 554-577. https://doi.org/10.1016/j.dsr2.2008.12.009
Landschützer, P., Gruber, N., Bakker, D. C. E., & Schuster, U. (2014). Recent variability of the global ocean carbon sink. Global Biogeochemical Cycles, 28(9), 927-949. https://doi.org/10.1002/2014GB004853
Nevison, C. D., Manizza, M., Keeling, R. F., Stephens, B. B., Bent, J. D., Dunne, J., et al. (2016). Evaluating CMIP5 ocean biogeochemistry and Southern Ocean carbon uptake using atmospheric potential oxygen: Present-day performance and future projection. Geophysical Research Letters, 43, 2077-2085. https://doi.org/10.1002/2015GL067584
Gray, A., Johnson, K. S., Bushinsky, S. M., Riser, S. C., Russell, J. L., Talley, L. D., et al. (2018). Autonomous biogeochemical floats detect significant carbon dioxide outgassing in the high-latitude Southern Ocean. Geophysical Research Letters, 45, 9049-9057. https://doi.org/10.1029/2018GL078013
Ocean to Climate 
From Rik Wanninkhof (NOAA AOML)
Biogeochemical profiling floats sample year-round and thus void a wintertime observation gap. However, several lines of evidence suggest that the biogeochemical profiling floats estimate of Gray et al. (2018) is too high for the following reasons
1) Based on observations only in the Pacific sector of the Southen Ocean;
2) Profiling floats were concentrated near the subpolar front where pCO2 is relatively high;
3) Profiling floats do not measure pCO2 but pH instead;
4) Possible bias in pH – pCO2 conversion can account for flux bias of up to 0.7 Pg C/year
More recent updates (e.g. Bushinsky et al. 2019) give better agreement with atmospheric constraints showing that the Southern Ocean is a significant CO2 sink.
Bushinsky, S. M., Landschützer, P., Rödenbeck, C., Gray, A. R., Baker, D., Mazloff, M. R., et al. (2019). Reassessing Southern Ocean air-sea CO2 flux estimates with the addition of biogeochemical float observations. Global Biogeochemical Cycles, 33, 1370– 1388. https://doi.org/10.1029/2019GB006176
The revised estimate of -0.16Pg C/yr south of 44S in Bushinsky et al. (2019) is still much smaller than the aircraft-based estimate of -0.53Pg C/yr south of 45S. In any case, according to this study (Bushinsky et al., 2019), the CO2 flux between 44S and 35S (-59Pg C/yr) is much larger than that south of 44S (-0.16Pg C/yr) as expected from a global CO2 inventory map (Gruber et al., 20919). Unfortunately, the aircraft-based estimate (Long et al., 2021) is available only for the south of 45S; thus, it missed a big chunk of CO2 flux that is eventually injected into the subtropical mode water and Antarctic intermediate waters.
Gruber, N., D. Clement, B. R. Carter et al. (2019) The oceanic sink for anthropogenic CO2 from 1994 to 2007. Science 363 (6432), 1193 – 1199, https://doi.org/10.1126/science.aau5153
Hauck et al. (2023) reiterate that winter-averaged (June-August) mean surface CO2 fluxes (FCO2) in the Southern Ocean derived from various products are very different. In particular, the BGC Argo-derived product shows nearly 0 net FCO2 into the Southern Ocean (in winter) due to a near cancelation between the outgassing in the subpolar region (i.e., wind-driven upwelling region) and the sinking in the subtropical region (mode & intermediate water subduction region). Models (GOBMs & B-SOSE) are not getting the outgassing right. See the above comments from Rik Wanninkhof on the potential bias in pH – pCO2 conversion, which was used to derive FCO2 from BGC Argo drifters.
Hauck, J., Gregor, L., Nissen, C., Patara, L., Hague, M., Mongwe, P., et al. (2023). The Southern Ocean carbon cycle 1985–2018: Mean, seasonal cycle, trends, and storage. Global Biogeochemical Cycles, 37, e2023GB007848. https://doi.org/10.1029/2023GB007848