Reassessing Southern Ocean Air-Sea CO2 Flux Estimates With the Addition of Biogeochemical Float Observations

New estimates of pCO(2) 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 Quere et al., 2018, ) to create a ship-only, a float-weighted, and a combined estimate of Southern Ocean carbon fluxes (<35 degrees S). In our ship-only estimate, we calculate a mean uptake of -1.14 0.19 Pg C/yr 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. Subsampling of high-resolution ocean biogeochemical process models indicates that some of the differences 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 pCO(2) difference between the mapped product and both data sets, giving a new Southern Ocean uptake of -0.75 0.22 Pg C/yr, 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. Plain Language Summary The Southern Ocean is thought to take up a significant amount of carbon dioxide each year but is a difficult region to observe due to its remote location and harsh winter weather. Recently, autonomous robots deployed by the Southern Ocean Carbon and Climate Observations and Modeling project have been making year-round measurements of ocean carbonate chemistry, from which we can estimate surface carbon dioxide. These provide new data at times and locations where we previously had very little. We found that combining the float observations with traditional shipboard data reduced our estimate for the amount carbon that the Southern Ocean takes up each year, though by less than had been previously estimated when considering float observations alone. We also show that some of the new signals is likely due to the differences in when and where floats make measurements. The magnitude of difference between prior estimates of the Southern Ocean carbon flux and our new approach is significant, similar to 20% of the contemporary global ocean carbon flux. It is therefore crucial to understand how this may impact the global carbon cycle, and we show that a compensating flux must be found somewhere within the Southern Hemisphere.