Beyond extratropical cyclones: Large-scale control on atmospheric river landfall in the western United States

Atmospheric rivers (ARs) play a crucial role in extreme precipitation and water resources management in the western United States. Their occurrence has been widely attributed to extratropical cyclones. Here, we find that landfalling ARs in this region are not merely synoptic-scale phenomena but are also driven by large-scale circulation. Specifically, quasi-stationary Rossby waves with centers of action along the Eurasian and North American coasts form a newly identified circum-North Pacific pattern. This 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” ARs account for up to twice as much winter precipitation as purely synoptic ARs. Moreover, they exhibit a stronger correspondence with high-category AR events. This previously unrecognized multiscale process deserves greater attention for the skillful prediction of ARs and the improved understanding of their variability and projected changes.