Winter Air‐Sea CO 2 Fluxes Constructed From Summer Observations of the Polar Southern Ocean Suggest Weak Outgassing

The Southern Ocean plays an important role in the global oceanic uptake of CO2. Estimates of the air-sea CO2 flux are made using the partial pressure of CO2 at the sea surface (), but winter observations of the region historically have been sparse, with almost no coverage in the Pacific or Indian ocean sectors south of the Polar front in the period 2004–2017. Here, we use summertime observations of relevant properties in this region to identify subsurface waters that were last in contact with the atmosphere in the preceding winter, and then reconstruct “pseudo observations” of the wintertime . These greatly improve wintertime coverage south of the Polar Front in all sectors, improving the robustness of flux estimates there. We add the pseudo observations to other available observations of and use a multiple linear regression to produce a gap-filled time-evolving estimate of from which we calculate the air-sea flux. The inclusion of the pseudo observations increases outgassing at the beginning of the period, but the effect reduces with time. We estimate a 2004–2017 long-term mean flux of −0.02 ± 0.02 Pg C yr−1 for the Southern Ocean south of the Polar Front, similar to comparable studies based on shipboard data. However, we diverge somewhat from an estimate which utilized autonomous float data for recent years: we find a small sink in 2017 of −0.08 ± 0.03 Pg C yr−1 where the float-based estimate suggested a small source. Plain Language Summary The Southern Ocean is an important region where carbon dioxide (CO2) gets absorbed into the ocean, however, the observations that allow us to calculate the flux are lacking. Estimates of the atmosphere-ocean flux of CO2 rely on observations of surface CO2 concentrations collected on board ships, which are especially sparse in the winter and in the most southerly parts of the Southern Ocean. In this study, we have used observations from below the surface taken in summertime to reconstruct estimates of the wintertime surface CO2 concentrations, which we then use to estimate the flux. Focusing on the period 2004–2017, we estimate CO2 fluxes between the ocean and atmosphere in the most southerly parts of the Southern Ocean that are broadly in line with other studies over the long-term mean, but which differ somewhat for recent years