Precipitation response to 21st-century global warming: a model-by-model theory-model comparison

We examine the spatially and seasonally distributed precipitation response to 21st century global warming in the CMIP3 suite of comprehensive models, focusing on the robust declines in precipitation found in the model subtropics. Current theories of this response include: circulation-independent changes directly driven by the thermodynamic water vapor increase due to warming; and changes due to dynamic shifts in the atmospheric circulation. In this study, we attempt to evaluate the relevance of these two mechanisms model-by-model. We consider each model's particular, biased, seasonally and zonally varying mean state and its spatial relationship to that model's predicted changes.

We find that almost every model has a general tendency to shift its existing mid-latitude cyclonic precipitation belts poleward in most seasons, leading to rainfall reductions on their subtropical flanks. This broad result agrees with the dynamic theory, and with a storm-track shift in particular. In addition, many of the models tend to reduce precipitation when and where actual evaporation exceeds precipitation, as predicted by the thermodynamic theory, but this is not as common nor as spatially widespread as the former tendency.