Changes in Winter Atmospheric Rivers along the North American West Coast in CMIP5 Climate Models

Most extreme precipitation events that occur along the North American west coast are associated with winter atmospheric river (AR) events. Global climate models have sufficient resolution to simulate synoptic features associated with AR events, such as high values of vertically integrated water vapor transport (IVT) approaching the coast. From phase 5 of the Coupled Model Intercomparison Project (CMIP5), 10 simulations are used to identify changes in ARs impacting the west coast of North America between historical (1970-99) and end-of-century (2070-99) runs, using representative concentration pathway (RCP) 8.5. The most extreme ARs are identified in both time periods by the 99th percentile of IVT days along a north-south transect offshore of the coast. Integrated water vapor (IWV) and IVT are predicted to increase, while lower-tropospheric winds change little. Winter mean precipitation along the west coast increases by 11%-18% [from 4% to 6% (degrees C)(-1)], while precipitation on extreme IVT days increases by 15%-39% [from 5% to 19% (degrees C)(-1)]. The frequency of IVT days above the historical 99th percentile threshold increases as much as 290% by the end of this century.