Atmospheric Rivers of Varying Intensity in the Western United States: Variability in the Present and Future Climate

Under a warming climate, the characteristics of landfalling atmospheric rivers (ARs) over the United States West Coast are expected to change. Recent work using a variable-intensity AR-identification method showed that the end-of-21st-century changes in West Coast AR landfall frequency depended on their intensity: extreme ARs (Integrated water Vapor Transport greater than the 98th percentile) increased in both frequency and intensity, whereas moderate ARs (IVT between 90th and 98th percentile) decreased in frequency by as much as 10%. Until now, this methodology has been applied only to a small set of regional climate models; however, in this study, we apply this methodology to a large set of global climate models (GCMs). We investigate end-of-century changes in AR frequency as a function of AR intensity in the present and future climate over the West Coast in a subset of 11 models within the Coupled Model Intercomparison Project Phase 6 (CMIP6) and a set of 40 members from the CESM2 Large Ensemble Community Project (LENS2). We find that, similar to the previous work, weak to moderate ARs are reduced in frequency and moderate to extreme ARs increase in frequency, but find that the intensity at which this shift occurs varies by GCM. In some GCMs, the intensity at which this shift occurs varies such that a large proportion of IVT events are increasing in frequency. We also see variation in these changes across the members of the CESM-LE, but the ensemble variability is relatively small. These shifts in AR frequency have direct implications for Western U.S. precipitation, with increased extreme precipitation from extreme ARs and decreased moderate precipitation amounts (impacting seasonal total precipitation).