Impacts of ice-shelf melting on water mass transformation in the Southern Ocean from E3SM simulations

The Southern Ocean overturning circulation is driven by both winds and buoyancy from freshwater sources, and among these sources of freshwater, Antarctic sea-ice formation and melting play the dominant role (followed by precipitation). Even though sub-ice shelf melt is relatively small in magnitude, it is located close to regions of convection, where it may also have an influence on dense water formation. Here, we explore the impacts of sub-ice shelf melting on Southern Ocean water mass transformation (WMT) using simulations from the Energy Exascale Earth System Model (E3SM) both with and without the explicit representation of melt fluxes from beneath Antarctic ice shelves. We find that ice-shelf melting produces upwelling of Upper Circumpolar Deep Water (UCDW) and this upwelled water is directly converted to lower density values. While the overall differences in Southern Ocean WMT between the two simulations are moderate, freshwater fluxes produced by ice-shelf melting have a further, indirect impact on the Southern Ocean overturning circulation through their interaction with sea-ice formation and melting, which also cause considerable upwelling. We further find that surface freshening by ice-shelf melting causes stronger density stratification near the Antarctic coast, and hence reduced vertical heat transport from the deeper ocean, trapping warmer water at depth and resulting in increased Antarctic sea-ice production. Although the addition of ice-shelf melting processes leads to no significant changes in Southern Ocean WMT, the simulations and analysis conducted here imply that increased Antarctic ice-shelf melting in recent decades has likely increased the role of sea ice in Southern Ocean overturning.