Large-scale circulation drives atmospheric river landfall in the western United States

Atmospheric rivers cause extreme precipitation and sustain water resources in the western United States. Their occurrence has often been attributed to extratropical cyclones. Here, we apply a recently proposed multiscale index to atmospheric rivers identified from reanalysis data and show that their landfalls in this region are not merely synoptic-scale phenomena but are also driven by large-scale circulation independent of extratropical cyclones. Specifically, quasi-stationary waves with centers of action along the Eurasian and North American coasts form a circum-North Pacific pattern. This large-scale teleconnection pattern channels subtropical moisture toward the U.S. West Coast on intraseasonal timescales, enabling its constructive interference with extratropical cyclone-induced moisture transport. The resulting “intermediate” atmospheric rivers account for up to twice as much winter precipitation as purely synoptic atmospheric rivers and exhibit a stronger correspondence with high-category events. Recognizing this multiscale process will be critical for the improved understanding of their predictability, variability and projected changes. Atmospheric rivers landfalling on the west coast of the USA are frequently induced by the constructive interference between extratropical cyclone-induced and low-frequency large-scale moisture transports, according to an examination of reanalysis datasets.