Juicier Southern Ocean Clouds Over the 21st Century?

In 21st century transient climate change experiments completed with the Community Earth System Model (CESM), there is a 50% increase in the cloud liquid water content (liquid water path, LWP) over the Southern Ocean: the clouds get juicier. In fact, the zonal grid-box mean cloud LWP change at 60 degrees South is the largest LWP change on the globe. Many mechanisms could generate these juicier 21st century Southern Ocean CESM clouds. First, Southern Ocean sea ice extents decrease and sea surface temperatures increase over the 21st century. Increased expanses of warm open water promote moisture transfer from the ocean to the atmosphere and juicier clouds. Second, large-scale circulation changes induced by greenhouse gas forcing also affect Southern Ocean clouds. The large-scale circulation response to increased greenhouse gas forcing is a poleward shift of the storm tracks and a tendency for a positive Southern Annular Mode (SAM) index. Circulation changes may also enhance Southern Ocean cloud liquid water content. For example, stronger westerly winds associated with a positive SAM index promote evaporation and could increase cloud LWP. In our presentation, we will diagnose mechanisms for juicier 21st century Southern Ocean CESM clouds and their association with climate feedbacks, sea ice, precipitation/evaporation, and poleward heat transport. We will also assess the realism of the CESM Southern Ocean cloud response and its influence on Southern Ocean climate. For example, CESM has excessive Antarctic sea ice when compared to modern day observations. Thus, CESM cloud LWP increases associated with sea ice decline may be larger than we might expect to observe over the 21st century.