The frequency of northern Pacific Atmospheric rivers (ARs) has implications for the water supply and flood risk over western North America. In this study the effects of variations in the background divergent moisture flux associated with the seasonal cycle and tropical intra-seasonal and interannual modes of variability on the regional distribution of AR frequency is examined. This effect is quantified by calculating the contribution of the background circulation to the divergent component of the integrated vapor transport (DIVT) in ARs detected in 38 years of MERRA reanalysis. It is shown that in the boreal winter this contribution is related to the outflow from the subsidence of the strengthened subtropical high transports moisture northward, while in summer it is related to the Asian monsoon which transports moisture northwestward leading to a seasonal northwest/southeast movement of AR frequency climatology. At intra-seasonal scale, the propagation of MJO introduces an anti-clockwise rotation of background DIVT, with northward transport in phases 1 and 2, westward in 3 and 4, southward in 5 and 6 and eastward in phases 7 and 8, making landfall over the west coast of North America most likely during the last two phases. ENSO variability also affects the frequency of ARs through modulation of westerly background DIVT, favoring landfall over the US west coast during strong El-Nino years. It is shown that in general the likelihood of landfall over the US is correlated with the background DIVT over northeastern Pacific. Based on the correlation an index for monitoring such likelihood of landfall is introduced. Finally, using CMIP6 simulations, the response of background DIVT to warming is assessed. The comparison of historical and end of the 21^st century of SSP585 scenario simulations from 25 models indicates a robust increase both in the poleward transport in winter and in the equatorward transport in summer suggesting a likely increase in the seasonality of ARs under warming.