How realistic is precipitation over the western U.S. and Mexico in IPCC AR4 GCMs?

Successful water management in the Desert Southwest and surrounding areas hinges on the timing and distribution of precipitation. Previous studies predict a more arid climate, more extreme precipitation events, and an earlier peak in springtime streamflow as the region warms. This study aims to assess the year-round skill with which general circulation models (GCMs) simulate precipitation and related dynamics over this region, a necessary precursor to reliable hydroclimate projections.

Thirty-year climatologies of several GCMs in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) were statistically evaluated for their skill in representing the location, timing, variability, character, and large-scale forcing of precipitation over the southwestern United States and northwestern Mexico. With an emphasis on the summer monsoon, GCM output was compared between models, to observations, and to model-derived indices of large-scale variability (such as ENSO, tropical easterly waves, midlatitude disturbances, and intraseasonal variability).

The results of this study will lend greater credence to more detailed, higher-resolution studies, based on the IPCC AR4 models, of the region's future hydrology. Ultimately, decision-makers will be able to more confidently plan future water management.