The frequency of North Pacific atmospheric rivers (ARs) affects water supply and flood risk over western North America. Thus, understanding factors that affect the variability of landfalling AR frequency is of scientific and societal importance. This study aims at identifying the sources of the moisture for North Pacific ARs and assessing how different modes of variability modulate these sources. To this end, the sources and variability of the background divergent component of the integrated moisture flux (DIVT) in ARs are identified using MERRA reanalysis. It is shown that in the boreal winter, this background DIVT in ARs is related to the outflow from the subsidence over the subtropics that transports moisture northward, while in summer it is related to the Asian monsoon and it transports moisture northwestward. This leads to a seasonal northwest–southeast movement of the AR frequency climatology. At the intraseasonal scale, propagation of the Madden–Julian oscillation introduces an anticlockwise rotation of the background DIVT, with northward transport in phases 1 and 2, westward in 3 and 4, southward in 5 and 6, and eastward in 7 and 8, making landfall over the west coast of North America most likely during the last two phases. Similarly, El Niño–Southern Oscillation variability also affects the frequency of ARs through modulation of the westerly background DIVT, favoring landfall over the U.S. West Coast during strong El Niño phases. It is shown that in general the likelihood of AR landfall over the western United States is correlated with the zonal background DIVT over northeastern Pacific.