23 August 2012

U.S. Southwestern Drought Projected by Regional Models in Warmer Climate


Getting an accurate projection of water cycle changes for the southwestern (SW) United States is becoming increasingly more urgent in light of regional drought trends and changes in the Colorado River flow.


A research team, including a DOE scientist from Pacific Northwest National Laboratory, analyzed the control and future climate from four pairs of regional and global climate models (RCMs and GCMs). The water cycle of the region is dominated by winter storms that maintain a positive annual net precipitation (precipitation minus evapotranspiration). The research team found that the RCMs simulate a higher fraction of transient eddy moisture fluxes because the hydrodynamic instabilities associated with flow over complex terrain are better resolved. Under global warming, this enables the RCMs to capture the response of transient eddies to increased atmospheric stability that allows more moisture to converge on the windward side of the mountains by blocking.


Their analysis shows that compared to GCMs, RCMs simulate enhanced transient eddy moisture convergence in the SW, although both robustly simulate drying due to enhanced moisture divergence by the divergent mean flow in a warmer climate. This enhanced convergence leads to reduced susceptibility to hydrological change in the RCMs compared to GCMs.

J. E. Kay