Impact of Distinct Origin Locations on the Life Cycles of Landfalling Atmospheric Rivers Over the U.S. West Coast
We tracked the life cycle of atmospheric rivers (ARs) that make landfall over the U.S. West Coast and examined their dynamics and impacts. ARs from the Northwest (Northeast) Pacific can induce precipitation over the northwestern (southwestern) U.S. West Coast.
The tracking algorithm that we developed to identify AR life cycles is a new tool and can be applied to process-based studies on different stages (origin, propagation, and termination) of AR life cycles. The empirical relationship between origin and termination locations provides insights on improving forecasts of the temporal propagation of ARs and AR-induced precipitation.
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 days vs. 3.6 days on average) and have stronger intensity of integrated vapor transport (508 kg m-1 s-1 vs. 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 the nearly opposite tripole pattern to the WLAR events. 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.