The Effect of the Sea Ice Freshwater Flux on Southern Ocean Temperatures in CCSM3: Deep Ocean Warming and Delayed Surface Warming

We explore the role of sea ice freshwater and salt fluxes in modulating 21 century surface warming in the Southern Ocean via analysis of sensitivity experiments in the Community Climate System Model version 3 (CCSM3). In particular, we investigate whether a change in these fluxes can cause surface cooling in the Southern Ocean, expand sea ice, and increase deep oceanic storage of heat. Our results indicate that in response to the doubling of CO2 concentrations in the atmosphere in CCSM3, net freshwater input from sea ice to the ocean increases south of 58°S (owing to less growth) and decreases from 48-58°S (owing to less melt). The freshwater source from changing precipitation in the model is considerably less than from sea ice south of 58°S, but serves to compensate for the reduction in sea ice melt near the ice edge, leaving almost no net freshwater flux change between about 48-58°S. As a result, freshwater input principally from sea ice reduces ocean convection, which in turns reduces entrainment of heat into the mixed layer and reduces upward heat transport along isopycnals below about 1000m. The reduced upward heat transport (from all sources) causes deep ocean heating south of 60°S and below 500m depth, with a corresponding surface cooling in large parts of the Southern Ocean in the model. These results indicate that changing sea ice freshwater and salt fluxes are a major component of the 21 century delay in surface warming of the Southern Ocean and weak reduction in Antarctic sea ice in model projections.