The Increasingly Moisture-Dominated Flavor of US West Coast Atmospheric Rivers

Abstract:

Atmospheric rivers (ARs) are essential features of the global water cycle. Although ARs are commonly defined using integrated vapor transport (IVT) metrics, ARs of a given IVT can induce a wide range of surface precipitation and wind impacts. IVT and the IVT-based AR-CAT index represent a critical first step in informing stakeholders of the impacts that could result from a given storm, and the wide diversity of AR characteristics suggests that there may be additional aspects of AR transport that are important for impacts. We develop a AR “flavor” metric that partitions AR IVT into moisture-dominant and wind-dominant components. We use this metric to create a climatological catalogue of “wet” and “windy” ARs along the United States West Coast from 1980-2016. Windy ARs are generally associated with stronger surface winds than wet ARs, with the largest differences at low IVT. Differences in precipitation also depend on IVT level: windy ARs are associated with greater precipitation when IVT is low, while wet ARs are associated with greater precipitation when IVT is high--particularly over mountainous regions in California. Furthermore, U.S. West Coast ARs have become increasingly moisture-dominated from 1980-2016, which has important implications for Western U.S. water availability and flood risk.