Abstract

Interactions between the atmosphere and ocean are mediated by the mixed layer at the ocean surface through the ventilation process. We explore an approximate mass (volume) budget in the surface layer to reveal the intensity and regional variability of the ventilation process, and quantify the role of eddies. Ventilation resulting from Ekman pumping is estimated from satellite winds, the geostrophic mean component is assessed from a climatology strengthened with Argo data and the eddy-induced advection is included via the parameterization of Gent and McWilliam (1990) together with eddy mixing estimates. All three components contribute significantly to ventilation. While the circumpolar-average upper cell structure is consistent with the average surface fluxes, it hides strong longitudinal regional variations and does not represent any local regime.

The eddy-induced contribution is important for the water-mass subduction in the vicinity of the Antarctic Circumpolar Current. It balances the horizontal northward Ekman transport as well as the vertical Ekman pumping. We investigate the pattern of eddy mixing and its control by the mean flow using virtual particle advected by altimetry. While, the mean flow tends to reduce eddy mixing and therefore eddy-induced transport in the vicinity of the ACC , it increases mixing on its equatorward flank. Eddy-mixing has a large interannual variability.

Subduction shows strong regional variability with bathymetrically constrained hotspots of large subduction. This constraint is mainly driven by lateral induction across large mixed-layer gradients. Therefore variations in mixed-layer depth affect the rate of exchange between the atmosphere and deeper ocean. We analyse temperature and salinity data from Argo profiling floats to show that the Southern Annular Mode, the dominant mode of atmospheric variability in the southern hemisphere, leads to large-scale anomalies in mixed-layer depth that are zonally asymmetric. Our result suggest that changes in the Southern Annular Mode, including recent and projected trends attributed to human activity, drive variations in Southern Ocean mixed-layer depth, with consequences for air-sea exchange, ocean sequestration of heat and carbon, and biological productivity.