<p>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.</p>