The role of atmospheric rivers in precipitation regime change projected for western North America

Climate change in California is projected to result in decreased frequency of daily precipitation, while wet extremes are projected to increase in magnitude and frequency. Atmospheric rivers (ARs) are responsible for the lion's share of West-Coast precipitation extremes. A seven-decade-long record of historical AR activity has, meanwhile, been shown to exhibit a gradual increase in the intensity of land-falling ARs upon the west coast of North America in conjunction with warming North Pacific sea surface temperatures. We apply our AR detection methodology to historical and projected climate epochs in a suite of eighteen global climate models (GCMs) from Phase 5 of the Coupled Model Intercomparison Project (CMIP5). After assessing the ability of these GCMs to realistically reproduce the climatology of land-falling ARs as well as their contribution to total annual precipitation, we quantify the projected changes in AR landfalls and AR contributions to projected precipitation, which has been statistically downscaled to resolve orographic effects. This approach allows us to constrain the uncertainty of CMIP5 model projections by highlighting results from the most realistic GCMs. A clear role for ARs in projected precipitation regime change along the North American West Coast emerges. We show that while the decrease in precipitation frequency is mostly due to non-AR events, the increase in heavy and extreme precipitation is almost entirely due to ARs. This research highlights the mounting role of atmospheric rivers in Western water resources via increasing contribution of ARs to future precipitation along the west coast and some inland regions where AR influences penetrate. It also provides nuanced expectations of future precipitation regimes and emphasizes the key role of atmospheric rivers in increasing the natural volatility of Western water resources in a warming climate.