The contribution of atmospheric rivers to precipitation in Europe and the United States

Atmospheric rivers (ARs) are narrow corridors within the warm conveyor belt of extratropical cyclones in which the majority of the poleward water vapour transport occurs. These filamentary synoptic features are responsible for extreme precipitation and flooding in Europe and the central and western United States, and also play an essential role for water resources in these areas. Using gridded precipitation products across Europe and the continental United States and the ERA-Interim reanalysis, we investigate the fraction of precipitation from 1979 to 2012 that is related to ARs in these regions. The results are region- and month-dependent, with the largest contribution generally occurring during the winter season and being on the order of 30-50%. This is particularly true for Western Europe, the U.S. West Coast, and the central and northeastern United States. Our results suggest that ARs are important agents for water supply in Europe and the United States. We have also examined whether there have been changes over time in the fractional contribution of ARs to seasonal rainfall using zero-inflated beta regression. We find that there has been a decrease in the average AR-contribution over the Mediterranean region and over the central United States.