PNNL

Abstract

The frequency of North Pacific atmospheric rivers 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. In this study the effects of 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 are examined. These effects are 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 over the subtropics which transports moisture northward, while in summer it is related to the Asian monsoon and it transports moisture northwestward leading to a seasonal northwest/southeast movement of the AR frequency climatology. At intra-seasonal scale, propagation of the Madden-Julian Oscillation introduces an anti-clockwise 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 westerly background DIVT, favoring landfall over the US west coast during strong El Niño phases. It is shown that in general the likelihood of AR landfall over the western US is correlated with the background DIVT over northeastern Pacific. Based on this correlation, an index for monitoring the likelihood of landfall is introduced.