Large-scale Circulation Context for Atmospheric Rivers: Influence of the North Pacific Oscillation / West Pacific Teleconnection

A database of global atmospheric rivers (AR) is analyzed to quantify the number of winter (December-February) landfalling events from 1980-2018. Guan and Waliser (2015) developed the database from the MERRA-2 reanalysis, focusing on areas of enhanced integrated vapor transport (IVT). A climatology of 5-day (pentad) sums, corresponding to the number of AR events in that time period, is constructed along with pentad anomalies. To advance understanding of the spatiotemporal distribution of ARs, their pentad anomalies were regressed on the leading subseasonal teleconnection patterns of boreal winter, the latter identified from a spatiotemporal analysis of pentad height variations at 200-hPa in the 1979-2019 NCEP Reanalysis; both contemporaneous and lead-lag regressions were computed to document AR origin and evolution, focusing on US West Coast exposure. The positive phase of the North Pacific Oscillation / West Pacific teleconnection (NPO/WP), the pattern that explains the second-most variance in 200-hPa geopotential heights, is found to be particularly influential in modulating the number of landfalling ARs on the US West Coast. The NPO/WP pattern is especially influential in December/January and generally more influential than the Pacific North American (PNA) pattern during winter. Although onshore moisture fluxes are smaller in magnitude in the NPO/WP pattern, the southwesterly fluxes penetrate further inland rather than being parallel to the coast. The lead-lag regressions of ARs on pentad resolution subseasonal teleconnection patterns are being analyzed to assess their potential predictability of US West Coast ARs from the incipient-phase knowledge of the teleconnections. Further analysis is done on landfalling ARs on the west coast of Canada and the southern coast of Alaska. Refined prediction of these moisture-laden events is pivotal for efficient water resource management along the west coast of North America.