Deep winter mixed layer anchored by the meandering Antarctic Circumpolar Current: Cross-basin variations

The Southern Ocean features some of the deepest winter mixed layers on Earth, crucial for water mass formation and the storage of anthropogenic heat. The winter mixed layer depth (MLD) significantly varies across basins, exceeding 300 m in the Indian and Pacific sectors but less than 150 m in the Atlantic. Current climate models simulate a distribution that is too broad and struggle to accurately represent this inter-basin variation. Using observational data and a global atmospheric model, this study investigates the contribution of surface buoyancy flux and background stratification to inter-basin MLD variations. The surface heat flux is decomposed into broad-scale and frontal-scale variations, both of which are influenced by the Antarctic Circumpolar Current’s (ACC) structure. At the broad-scale, the meandering ACC path is accompanied by a zonal wavenumber-1 structure of sea surface temperature with a warmer Pacific than Atlantic; under the prevailing westerly winds, this temperature contrast results in larger surface heat loss facilitating deeper MLD in the Indian and Pacific than the Atlantic. At the frontal-scale, intensified ACC fronts in the Indian sector further strengthen heat loss to the north. Surface freshwater flux pattern largely follows that of evaporation and reinforces the heat flux pattern, especially in the southeast Pacific. Background stratification also significantly varies across the Southern Ocean, influencing MLD pattern. In the Atlantic and western Indian oceans where the ACC is at a low latitude (45°S), solar heating, intrusions of subtropical gyres and energetic mesoscale eddies together maintain strong stratification. In the southeast Pacific, in comparison, the ACC reaches its southernmost latitude (56°S), far away from the Subtropical Front. This creates a weaker stratification that allows deep mixed layers to form.