ALBERT

Description: Observations show a significant intensification of the Southern Hemisphere westerlies, the prevailing winds between the latitudes of 30° and 60° S, over the past decades. A continuation of this intensification trend is projected by climate scenarios for the twenty-first century. The response of the Antarctic Circumpolar Current and the carbon sink in the Southern Ocean to changes in wind stress and surface buoyancy fluxes is under debate. Here we analyse the Argo network of profiling floats and historical oceanographic data to detect coherent hemispheric-scale warming and freshening trends that extend to depths of more than 1,000 m. The warming and freshening is partly related to changes in the properties of the water masses that make up the Antarctic Circumpolar Current, which are consistent with the anthropogenic changes in heat and freshwater fluxes suggested by climate models. However, we detect no increase in the tilt of the surfaces of equal density across the Antarctic Circumpolar Current, in contrast to coarse-resolution model studies. Our results imply that the transport in the Antarctic Circumpolar Current and meridional overturning in the Southern Ocean are insensitive to decadal changes in wind stress.

Description: Changes in the large-scale hydrographic structure of the Southern Ocean are examined by utilizing the rapidly- expanding ARGO float dataset, in comparison to a historical hydrograhic database (BOA). By invoking and comparing different schemes of considering the historical and modern profiles in narrow meridional bins over both regional and global extent, and by contrasting zonal averages on isobaric and isopycnal surfaces with averages taken along approximate streamlines of the ACC, the study contributes to a synthesis and refinement of previous suggestions of a warming trend in the upper-to-mid-depth Southern Ocean, elucidating the 3D structure of this signal and shedding some light on possible origins. The inspection of the tendencies in meridional cross-sections reveal several, distinct patterns of change during the last four decades. The first two, a deep-reaching warming of uCDW at the southern flank of the ACC, and a pronounced decrease of density (i.e., subsidence and southward shift of isopycnals) in the northern portion of the ACC, appear consistent with a dynamical response of the ACC and its associated meridional overturning to an increasing SAM index and westerly wind stress. The other main signal seen in the regional and global averages is a prominent and extended warming trend at constant depth in the mode waters north of the ACC, corresponding to a cooling and freshening on isopycnal surfaces. Inspection of the temporal tendencies of this process suggests the presence of the mid-depth warming trend at least since the 1960-70s, and give some indications of deviations from a monotonous trend.