A new estimate of the wintertime Southern Ocean CO2 flux from subsurface summer observations

The Southern Ocean plays a disproportionate role in the global oceanic uptake of CO₂, but observations of the region historically have been sparse. One method for estimating the air-sea CO₂ flux relies on observations of the partial pressure of CO₂ (pCO₂) at the sea surface. In this study we focus on the period 2004-2010, during which wintertime observations of surface pCO₂ south of the Antarctic polar front are only found in the South Atlantic, with no coverage in the Indian or Pacific sectors. Making use of profiles of relevant properties (temperature, salinity, dissolved inorganic carbon, oxygen) with depth from the Global Ocean Data Analysis Project for carbon (GLODAP), we identify subsurface waters in summer south of the polar front that were last in contact with the atmosphere in the preceding winter. We then combine the winter water properties with wintertime observations of temperature and salinity from autonomous Argo floats to reconstruct the wintertime surface pCO₂. The reconstructed observations add wintertime coverage south of the polar front in all sectors of the Southern Ocean. When added to the available surface pCO₂ observations, the reconstructed pCO₂ significantly decreases the estimated wintertime air-sea CO₂ flux south of 35S, and consequently decreases the estimated annual flux by ~0.1 petagrams of carbon per year.