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. Plain Language Summary: Atmospheric rivers (ARs) are "rivers in the sky" that transport vast amounts of water vapor—equivalent to about 27 Mississippi Rivers on average for those observed in the northeastern Pacific. As major precipitation deliverers, flood producers, and drought busters, ARs represent a key meteorological phenomenon of societal importance in the semi‐arid western US. Understanding how large‐scale climate conditions, such as El Niño and La Niña, influence the occurrence and strength of ARs is pivotal for efficient water resource management in the region. Of particular interest to water managers are AR activity variations on the subseasonal‐to‐seasonal time scales, that is, on time scales beyond the typical 1–2‐week period of weather forecasts. Focusing on these time scales, the current study identifies a subseasonal weather pattern that is particularly influential on AR activity in the western US. The North Pacific Oscillation/West Pacific teleconnection pattern (NPO/WP) is more influential than other subseasonal patterns previously linked to AR activity, both in terms of landfall occurrence and inland reach. Our findings advance prospects of subseasonal prediction of winter AR activity over the western US, from the incipient‐phase knowledge of the impactful subseasonal teleconnections operating over the Pacific‐North American region, especially the NPO/WP. Key Points: The modulation of atmospheric rivers (ARs) in the western US by climate variability modes is analyzed, focusing on pentad evolutionThe North Pacific Oscillation/West Pacific (NPO/WP) pattern is more influential on western US AR activity than commonly‐considered modesThe finding advances prospects of subseasonal prediction of ARs over the western US from the NPO/WP's incipient‐phase knowledge [ABSTRACT FROM AUTHOR]