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

New estimates of CO 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, https://doi:10.1029/2018GL078013). Here, we put 3.5 years of SOCCOM observations into broader context with the global surface carbon dioxide database (Surface Ocean CO 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, https://doi:10.5194/essd-10-2141-2018) 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 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 CO 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.