Linkages between Southern Ocean Cloud-Radiative Processes and the Large-Scale Southern Hemisphere Circulation, and Their Implications for Climate Model Projections

Thursday, 18 December 2014: 5:46 PM
Kevin M Grise, University of Virginia Main Campus, Department of Environmental Sciences, Charlottesville, VA, United States and Lorenzo M Polvani, Columbia University, New York, NY, United States
Southern Ocean cloud cover is strongly linked to extratropical weather systems, and thus to the position of the Southern Hemisphere (SH) storm track and the mid-latitude eddy-driven jet stream. Consequently, if the jet moves poleward (either as a result of natural variability or anthropogenic forcing), a notable change in cloud-radiative processes might be expected. In this study, we examine the cloud-radiative anomalies associated with interannual variability in the latitude of the SH mid-latitude eddy-driven jet, using two satellite data sets (ISCCP-FD and CERES) and 20 global climate models from Phase 5 of the Coupled Model Intercomparison Project (CMIP5). Two distinct model types are found. In the first class of models (“type I models”), the total cloud fraction is reduced at SH mid-latitudes as the jet moves poleward, contributing to enhanced shortwave radiative warming. In the second class of models (“type II models”), this dynamically-induced cloud-radiative warming effect is largely absent. Type I and type II models have distinct deficiencies in their representation of observed Southern Ocean clouds, but comparison with the two satellite data sets indicates that the cloud-dynamics behavior of type II models is more realistic.

Because the SH mid-latitude jet shifts poleward in response to CO2 forcing, the cloud-dynamics biases uncovered from interannual variability are directly relevant for climate change projections. In CMIP5 model experiments with abruptly quadrupled atmospheric CO2 concentrations, the global-mean surface temperature initially warms more in type I models, even though their equilibrium climate sensitivity is not significantly larger. In type I models, this larger initial warming is linked to the rapid adjustment of the circulation and clouds to CO2 forcing in the SH, where a nearly instantaneous poleward shift of the mid-latitude jet is accompanied by a reduction in the reflection of solar radiation by clouds. In type II models, the SH jet also shifts rapidly poleward with CO2 quadrupling, but it is not accompanied by cloud-radiative warming anomalies, resulting in a smaller initial global-mean surface temperature warming.