Anomalous Summertime CO2 sink in the subpolar SouthernOcean promoted by early 2021 sea ice retreat

The physical and biogeochemical processes governing the air-sea CO2 flux in the Southern Ocean are still widely debated. This presentation focus on an anomalously large sink of CO2 observed north of the Weddell Sea in Summer 2022. The processes behind this anomalous situation are analyzed based on the combination of in situ observations, various satellite parameters (altimetric currents, Chl-a, ice concentration and sea surface salinity) and ocean model reanalysis.

The “Southern Ocean Carbon and Heat Impact on Climate” cruise in Summer 2022 aimed at studying physical and biogeochemical processes in the Weddell Sea and in its vicinity. A “CARbon Interface OCean Atmosphere” (CARIOCA) drifting buoy was deployed in January 2022 in the subpolar Southern Ocean, providing hourly surface ocean observations of fCO2 (fugacity of CO2), dissolved oxygen, salinity, temperature and chlorophyll-a fluorescence for 17 months. An underwater glider was piloted with the buoy for the first 6 weeks of the deployment to provide vertical ocean profiles of hydrography and biogeochemistry. These datasets reveal an anomalously strong ocean carbon sink for over 2 months occuring in the region of Bouvet Island and associated with large plumes of chlorophyll-a (Chl-a). Based on Lagrangian backward trajectories reconstructed using various surface currents fields, we identified that the water mass reaching the Bouvet Island region originated from the south-west, from the vicinity of sea ice edge in Spring 2021. We suggest that a strong phytoplankton bloom developed there in November 2021 favoured by early sea ice melt in 2021 in the Weddell Sea. These waters, depleted in carbon, then travelled to the position of the CARIOCA buoy. The very low values of ocean fCO2, measured by the buoy (down to 310 μatm), are consistent with net community production previously observed during blooms occurring near the sea ice edge, partly compensated by air-sea CO2 flux along the water mass trajectory. Early sea ice retreat might therefore have caused a large CO2 sink farther north than usual in Summer 2022, in the Atlantic sector of the subpolar Southern Ocean. Such events might become more frequent in the future as a result of climate change.