Winter Air-Sea CO2 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 (pCO2surf), 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 pCO2surf. 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 pCO2surf and use a multiple linear regression to produce a gap-filled time-evolving estimate of pCO2surf 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 pCO2surf 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.

Automated summary

In the Southern Ocean south of the Polar Front, utilizing summertime observations to reconstruct wintertime pCO2surf greatly improved coverage and enhanced accuracy of flux estimates. The addition of pseudo observations initially increased outgassing, which gradually decreased over time.