The summer atmospheric river (AR) frequency over East Asia and Western North Pacific (EA-WNP) is investigated by multiple AR detection algorithms based on the Atmospheric River Tracking Method Intercomparison Project (ARTMIP) Tier2 reanalysis dataset. The results show that AR frequency during the recent four decades experienced opposite interdecadal shifts, greatly contributing to the interdecadal equatorward trends of EA ARs and poleward trends of WNP ARs with a boundary around 135°E. The opposite variations are mainly influenced by a zonal dipole of integrated water vapor transport with cyclonic and anticyclonic anomalies centered over Taiwan and the ocean to the southeast of Japan, respectively. A major impact of the Pacific Decadal Oscillation and a reinforcement effect due to a zonal wave train from the North Atlantic jointly modulate the pattern. Considering ARs may curve their pathway over EA-WNP, the algorithms based on historical AR shapes should be cautiously used during AR detection.