Edward Doddridge, John Marshall, Hajoon Song, Jean-Michel Campin, & Maxwell Kelley
The observational record shows a substantial 40-year upward trend in summertime westerly winds over the Southern Ocean, as characterised by the Southern Annular Mode (SAM) index. Enhanced summertime westerly winds have been linked to cold summertime sea surface temperature (SST) anomalies. Previous studies have suggested that an Ekman transport mechanism is responsible for this seasonal cooling. Here, another equally important process is presented in which cooling, driven by summertime wind-induced enhanced vertical mixing, moves heat downwards, cooling the sea surface and warming subsurface waters. The anomalously cold SSTs draw heat from the atmosphere into the ocean, leading to enhanced depth-integrated ocean heat content. The subsurface heat is returned to the surface mixed layer during the autumn and winter as the mixed layer deepens, leading to anomalously warm SSTs and potentially reducing sea ice cover. Observational analyses and numerical experiments support this mechanism, showing that enhanced vertical mixing drives subsurface warming and cools the surface mixed layer. Anomalous advection also contributes to the surface cooling, but the relative importance of advective and mixing contributions is model dependent. Modeling results suggest that sea ice volume is more sensitive to summertime winds than sea ice extent, implying that enhanced summertime westerly winds may lead to thinner sea ice in the following winter, if not lesser ice extent. Thus, strong summertime winds could precondition the sea ice cover for a rapid retreat in the following melt season.