Anthropogenic CO₂ in the Southern Ocean: distribution and inventory at the Indo-Atlantic boundary (WOCE line I6)

International audienceThe Southern Ocean, where various water masses are formed (mode, intermediate, deep, and bottom waters), has a high potential to absorb anthropogenic CO2 (Cant). However, most data-based and model estimates indicate low Cant inventories south of 50°S. In order to investigate this paradox, the distribution of Cant is estimated between South Africa and Antarctica (World Ocean Circulation Experiment (WOCE) line I6) based on a back-calculation technique previously used in the North Atlantic (Körtzinger et al., 1998) and adapted here for application in the Southern Ocean. At midlatitudes (30°-50°S), formation and spreading of mode and intermediate waters results in a deep penetration of Cant (down to 2000 m). South of 50°S, significant concentrations of Cant were estimated in Circumpolar Deep Water (>10 μmol/kg) and Antarctic Bottom Water (AABW) (20-25 μmol/kg). Higher concentrations are detected along the continental slope in AABW presumably formed in Prydz Bay compared to AABW of Weddell Sea origin. The distribution of Cant obtained north of 50°S compares well with previous data-based and model estimates, but large disagreements are found in the south. However, although transient tracers are not used in the back-calculation technique employed here, the distribution of Cant is remarkably well correlated with CFCs. We reevaluated the column inventories of Cant for the Southern Ocean and found higher values at high latitudes (70-90 mol/m2) compared to the subtropical/subantarctic region (40-80 mol/m2). These results support the idea that deep and bottom water formation in the Southern Ocean is a key process in the natural sequestration of anthropogenic CO2