Drivers of Interannual Variability of Summer Mixed Layer Depth in the Southern Ocean Between 2002 and 2011

Climate change projections indicate that there will be warming and an intensification of westerly winds in the Southern Ocean (SO) in the future. These two forcings potentially have opposing effects on the depth of the surface mixed layer. Here we investigate how interannual to decadal variability of atmospheric surface air temperature (SAT) and zonal wind speed (uwind) impact mixed layer depth (MLD) in the SO (south of 30 degrees S) during summer, the season of main biological activity. We use gridded MLD data from observations and atmospheric reanalysis data of uwind and SAT in the SO to assess summer MLD variability and its potential drivers. With a model-based sensitivity experiment, we quantify the relative contributions of uwind versus SAT forcing on the summer MLD in the decade 2002-2011. Wind-induced changes dominate over temperature-induced changes of the MLD between 2002 and 2011. We find a positive trend of summer MLD in the Antarctic Zone of the Atlantic and Indian Ocean sectors. Our model-based sensitivity study suggests that the summer MLD shows a zonally asymmetric response to recent atmospheric forcing. In the Pacific and Australian sectors, cooling and intensification of uwind jointly result in a deepening of the mixed layer. In the Atlantic and Indian sectors, the MLD responds differently north and south of the Antarctic Polar Front (APF). A deepening south of the APF is caused by the increase in uwind, whereas the decrease in uwind and warming act in concert to result in a shoaling of the MLD north of the APF.