The Impact of Antarctic Sea Ice on Southern Ocean Water Mass Transformation in Coupled Climate Models

Abstract

The Southern Ocean is a critical heat and carbon sink due to the interaction between the atmosphere and deep ocean that occurs there. Antarctic sea ice is essential for maintaining this interaction by transforming ventilated deep-water into both denser and lighter water masses. However, the representation of sea ice in climate models is variable, and its impact on water mass transformation remains unclear. We evaluate the contribution of sea ice to surface water mass transformation in the Southern Ocean in 16 models from Phase 6 of the Coupled Model Intercomparison Project (CMIP6). We find that sea ice redistributes freshwater from the Antarctic coast to offshore, acting as a pump, driving 10.2 $\pm$ 8.8 Sv of deep-water ventilation across the ensemble. Variations in ventilation between models are driven by biases both in sea ice production and the ocean state. Firstly, the models' sea ice biases directly affect the net freshwater flux into the ocean. Secondly, the impact of surface freshwater flux on water mass transformation depends also on the ocean's surface density and salinity, so that biases in the model ocean state also play a role. We find that heat fluxes can partially compensate for variations in the sea ice contribution, so that models with lower sea-ice driven ventilation have higher heat flux driven ventilation. Therefore, sea ice is an important but not the sole determinant of the interaction between the atmosphere and deep ocean in models of the Southern Ocean.