Precipitation response to 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 declines in precipitation, and on the area outside of each season's ITCZ proper. Current theories of this response include: changes directly driven by the thermodynamic water vapor increase due to warming; changes due to dynamic shifts in the atmospheric circulation; and feedbacks to water vapor from soil-moisture depletion on land. In this study, we attempt to evaluate the relevance of these theories model-by-model. We consider each model's particular, biased, seasonally varying mean state and its relationship to that model's predicted changes.

We find that almost every model has a general tendency to shift its mid-latitude cyclonic precipitation belts poleward in most seasons. This broad result agrees with the dynamic theory, and with a storm-track shift in particular. In addition, many of the models generally reduce precipitation when and where actual evaporation exceeds precipitation, as predicted by the thermodynamic theory. The responses occasionally contain warm-season continental drying independent of either of these patterns, as suggested by the soil-moisture theory, but this is not as common.