PNNL

Abstract

The mean precipitation along the U.S. West Coast exhibits a pronounced seasonality change under warming. Here we explore the mechanisms for the seasonality change based on 37 climate models using moisture budget framework. The multi-model simulations show a robust enhancement of wet season precipitation over Washington-Oregon and a sharpened seasonal cycle with a stronger and narrower wet season over California. Both changes are predominantly caused by changes in the mean moisture convergence. Thermodynamic effect due to the divergence of increased moisture dominates the ‘wet-get-wetter’ seasonal enhancement over Washington-Oregon, where the climatological lower tropospheric convergence is strong. In contrast, mean zonal moisture advection due to seasonally-dependent changes in moisture gradient dominates the sharpened wet season over California. The seasonally-dependent changes in zonal moisture gradient originate from the effects of both the climatological and changes of land-sea thermal contrast on the changes in land-sea moisture contrast in different seasons under warming through the nonlinear Clausius–Clapeyron relation. More specifically, the stronger climatological land-sea thermal contrast in winter with warmer ocean than land dominates the change in land-sea moisture contrast, resulting in more moisture increase over ocean than land under warming and hence wet advection to California. However, in fall and spring, the future change of land-sea thermal contrast with larger warming over land than ocean dominates the changes in land-sea moisture contrast, inducing an opposite moisture gradient and hence dry advection to California. The sharpening seasonality adds to the complexity to water resource management in the future.