Modeling the Influence of Land and Sea Ice on Southern Ocean Salinity and its Recent Trends

Due to its low temperatures, the stratification of the Southern Ocean (SO) is especially sensitive to salinity variations. In recent decades, waters along the continental shelves of Antarctica representing newly formed Antarctic Bottom Water as well as surface waters in the northern sea ice regions substantially freshened. Recent observation-based studies suggested that increased melting of glacial ice is likely responsible for the freshening of the AABW, while changes in sea ice transport were proposed as the main cause for the freshening of the waters at the northern sea-ice edge. However, a full quantitative attribution of these salinity changes has remained elusive so far.

With the help of an eddy-resolving regional ocean model (ROMS) of the SO, we are able to reproduce these freshening trends and confirm the importance of these proposed mechanisms. We achieve this by determining all components of the SO freshwater balance, and in particular those associated with the formation and melting of sea and glacial ice through a careful analysis of the available observations. Doing so circumvents many of the common artifacts present in current models and enables us to reproduce the complex temperature and salinity structure of the Southern Ocean much better than hitherto possible. Our simulations reveal that the observed surface ocean freshening in the Ross and Amundsen Seas is mostly caused by the intensified southerly winds that increase the northward flow of sea ice and lead to an enhanced freshwater input into the surface ocean at the northern ice-edge. On the other hand, the increased northward flow entails a reduction of the sea ice induced meltwater input near the Antarctic continent, which partially compensates the observed enhanced freshwater input through glacial melt, bringing the modeled salinity changes in the freshly formed Bottom Waters into agreement with observations.