Lawrence Berkeley National Laboratory

An atmospheric river (AR) event represents strong poleward moisture transport and is defined as a series of spatiotemporally connected instantaneous AR objects. Utilizing an AR tracking algorithm with a depth-first search (a widely used algorithm in computer science), we examine the life cycle characteristics of AR events that make landfall over the U.S. West Coast by their distinct origin locations. Landfalling AR events from the Northwest Pacific (120°E–170°W, WLAR events) temporally last longer (5.3 versus 3.6 days on average) and have stronger intensity of integrated vapor transport (508 versus 388 kg m−1 s−1 on average) than those originating from the Northeast Pacific (125°W–170°W, ELAR events). A persistent tripole geopotential height anomaly pattern over the North Pacific modulates the origin locations and propagation of landfalling AR events. WLAR events are associated with anomalous highs over northeastern Asia and the Northeast Pacific and an anomalous low over the central North Pacific. This pattern provides favorable conditions for WLAR events to start, propagate northeastward, and make landfall in the northwestern West Coast. WLAR events contribute approximately 25% of the total winter precipitation over Washington and British Columbia. ELAR events are associated with a similar tripole pattern to the WLAR events with an eastward shift. The anomalous low over the Northeast Pacific helps ELAR events to start, propagate northeastward, and make landfall in the southwestern West Coast. Precipitation induced by ELAR events contributes up to 30% of total winter precipitation over California.