Abstract

Observations reveal significant changes in both Antarctic sea-ice cover and in the hydrography of the underlying Southern Ocean over the past decades. However, our understanding on the processes that cause these changes and the connection between the oceanic and sea-ice changes is strongly limited by sparse observations and the failure of global models to reproduce these changes. Here, we provide a synthesis of observation and model data to investigate recent changes in the sea-ice-ocean system that are driven by changes in atmospheric circulation. Based on observational data, we demonstrate that the recent sea-ice expansion is driven by an enhanced meridional transport of sea ice resulting from an intensified atmospheric circulation. Combining these observational evidences with experiments with a global climate model (MPI-ESM), we infer an anthropogenic influence on the near-surface circulation and the enhanced sea-ice transport. We estimate, on the basis of a sea-ice mass budget from satellite data, that this enhanced northward sea-ice transport caused up to two thirds of the observed freshening of the Southern Ocean surface and intermediate waters in recent decades. Using an eddy-resolving regional ocean model (ROMS), we trace the influence of changes in the observation based sea-ice fluxes on the Southern Ocean hydrography. First results suggest that these changes, together with increased glacial melt fluxes from Antarctica, can explain most of the recent stabilization of the high-latitude Southern Ocean surface waters. As a consequence, our findings also imply that sea-ice processes have a strong potential to alter the overturning circulation and, thus, the deep to surface ocean exchange of heat, carbon, and nutrients in the Southern Ocean.