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

Abstract:

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.