Large-Scale Circulation Context for North American West Coast Atmospheric Rivers: Influence of the Subseasonal NPO/WP Teleconnection

Understanding the variability of atmospheric rivers (ARs) on subseasonal time scales is pivotal for efficient water resource management along the west coast of North America. ARs during 1980-2018 based on the Modern-Era Retrospective analysis for Research and Applications, version 2 are analyzed to quantify the modulation of winter (December-February) landfalling ARs in the western US by leading subseasonal teleconnections, focusing on pentad evolution rather than seasonal-mean patterns. The growth phase of the North Pacific Oscillation/West Pacific (NPO/WP) teleconnection-the second leading pattern in 200-hPa geopotential heights in boreal winter-is found to be particularly influential in modulating the number of landfalling ARs in this region. In the positive phase of NPO/WP growth, the presence of anomalous low pressure centered just south of Alaska (i.e., a strengthening of the Aleutian Low) and anomalous high pressure around Hawaii results in moisture convergence in the central and eastern Pacific, bringing southwesterly moisture fluxes to the coast and inland. The modulation by NPO/WP is stronger than by commonly-considered climate variability modes, such as the Pacific/North American (PNA) pattern. Although southwesterly fluxes are stronger over the Pacific Ocean during the positive phase of PNA, they tend to transition to southerly fluxes before extending inland, resulting in smaller overland impacts in the western US. The analysis of temporal evolutions indicates AR activity peaks 5 days after the mature phase of NPO/WP growth, as in the case of PNA. Overall, the study suggests potential subseasonal predictability of US West Coast ARs from incipient-phase knowledge of the leading teleconnection patterns, especially the NPO/WP.