Changes in Extreme Integrated Water Vapor Transport on the U.S. West Coast in NA-CORDEX, and Their Relationship to Mountain and Inland Precipitation

Western US rainfall and snowpack has large interannual and decadal variability, and this, paired with its importance to water resources, makes future projections of these variables extremely societally relevant. Previous studies have shown that precipitation events in the western US are influenced by the timing, positioning, and duration of extreme integrated water vapor transport (IVT) events (e.g., atmospheric rivers) at the coast, and also by the pathways which this moisture-rich air takes through the complex terrain of the western US.

We investigate projections of western US precipitation and IVT in a collection of regional climate models (RCMs) forced by several global climate models (GCMs) from the North American Coordinated Regional Downscaling Experiment (NA-CORDEX). We briefly explore how well present-day precipitation and IVT are represented by NA-CORDEX. We then document projected changes in precipitation and extreme IVT statistics at the end of the 21st century in the RCP8.5 CMIP5 emissions pathway. Several of the NA-CORDEX RCMs project a decrease in precipitation at high elevation (e.g., across the Sierra Nevada) with a corresponding increase in the Great Basin of the US. We explore the causes of this terrain-related precipitation change in a subset of the NA-CORDEX RCMs using diagnostics such as the lifting condensation level and drying ratio.