Southern Ocean mixed layer seasonal and interannual variations from combined satellite and in situ data

The depth of the upper ocean mixed layer provides fundamental information on the amount of sea water that directly interacts with the atmosphere. Its space-time variability modulates water mass formation and carbon sequestration processes related to both the physical and biological pumps. These processes are particularly relevant in the Southern Ocean, where surface mixed layer depth estimates are generally obtained either as climatological fields derived from in situ observations or through numerical simulations. Here, we demonstrate that weekly observation-based reconstructions can be used to describe the variations of the mixed layer depth in the upper ocean over a range of space and time scales. We compare and validate four different products obtained by combining satellite measurements of the sea surface temperature, salinity and dynamic topography and in situ Argo profiles. We also compute an ensemble mean and use the corresponding spread to estimate mixed layer depth uncertainties and to identify the more reliable products. The analysis points out the advantage of synergistic approaches that include in input the sea surface salinity observations obtained through a multivariate optimal interpolation. Corresponding data allow to assess mixed layer depth seasonal and interannual variability. Specifically, the maximum correlations between mixed layer anomalies and the Southern Annular Mode are found at different time lags, related to distinct summer/winter responses in the Antarctic Intermediate Water and Sub-Antartic Mode Waters main formation areas.