Summer and winter air–sea CO 2 fluxes in the Southern Ocean

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The seasonal variability of the carbon dioxide (CO 2) system in the Southern Ocean, south of 50°S, is analysed from observations obtained in January and August 2000 during OISO cruises conducted in the Indian Antarctic sector. In the seasonal ice zone, SIZ (south of 58°S), surface ocean CO 2 concentrations are well below equilibrium during austral summer. During this season, when sea-ice is not obstructing gas exchange at the air–sea interface, the oceanic CO 2 sink ranges from −2 to −4 mmol/m 2/d in the SIZ. In the permanent open ocean zone, POOZ (50–58°S), surface oceanic fugacity fCO 2 increases from summer to winter. The seasonal fCO 2 variations (from 10 to 30 μatm) are relatively low compared to seasonal amplitudes observed in the subtropics or the subantarctic zones. However, these variations in the POOZ are large enough to cross the atmospheric level from summer to winter. Therefore, this region is neither a permanent CO 2 sink nor a permanent CO 2 source. In the POOZ, air–sea CO 2 fluxes calculated from observations are about −1.1 mmol/m 2/d in January (a small sink) and 2.5 mmol/m 2/d in August (a source). These estimates obtained for only two periods of the year need to be extrapolated on a monthly scale in order to calculate an integrated air–sea CO 2 flux on an annual basis. For doing this, we use a biogeochemical model that creates annual cycles for nitrate, inorganic carbon, total alkalinity and fCO 2. The changing pattern of ocean CO 2 summer sink and winter source is well reproduced by the model. It is controlled mainly by the balance between summer primary production and winter deep vertical mixing. In the POOZ, the annual air–sea CO 2 flux is about −0.5 mol/m 2/yr, which is small compared to previous estimates based on oceanic observations but comparable to the small CO 2 sink deduced from atmospheric inverse methods. For reducing the uncertainties attached to the global ocean CO 2 sink south of the Polar Front the regional results presented here should be synthetized with historical and new observations, especially during winter, in other sectors of the Southern Ocean.