Due to the coarse resolution (∼1◦) of most operational GCMs, regional climate models (RCMs) have, until recently, been the primary tools for assessing regional changes to precipitation. However, variable-resolution enabled global climate models (VRGCMs) have now emerged as an important downscaling technique within the regional climate modeling community. In this study, we use the recently developed variable-resolution option in the Community Earth System Model (VR-CESM) to assess the change of mean precipitation and associated extremes through the end of the 21^st century over the western U.S. (WUS). This region deserves particular attention due to its vulnerability to hydrological extreme events. This study is based on long-term ensemble simulations conducted at a grid resolution of 0.25◦, following the RCP (representative concentration pathway) 8.5 scenario.

The central hypothesis of this study is that changes to mean precipitation and associated extremes under climate change vary strongly by region. These changes are impacted in part due to localized differences in warming under climate change. Teleconnections also play an important role here, particularly the El Niño-Southern Oscillation (ENSO), which is strongly tied to the precipitation features over particular area through teleconnections driven by tropical sea surface temperatures (SSTs).

Our results show that although the mean precipitation shows a weak, increasing trend from past to future, the frequency distribution of precipitation events shows a more dynamic character. In particular, there is a robust increase in winter precipitation extremes toward the latter half of the 21st century, especially over the northwest coast and the Cascades and its eastern flank mainly due to the increased vapor influx from the Pacific Ocean. The magnitude of ENSO’s influence on the character of precipitation is comparable or larger than the impacts of climate forcing.