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

September 27, 2023

A new paper titled “Large-Scale Circulation Context for North American West Coast Atmospheric Rivers: Influence of the Subseasonal NPO/WP Teleconnection” by Justin Hicks (University of Maryland), Bin Guan (UCLA, NASA JPL), Sumant Nigam (University of Maryland), and Alfredo Ruiz-Barradas (University of Maryland) was recently published in the American Geophysical Union’s Journal of Geophysical Research. This 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. The results 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. This prospect of improved long-term forecasts of high-impact precipitation events (i.e. ARs) is of particular interest to those in the water resources community in the western US.

ABSTRACT

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.

KEY POINTS

Figure 1: Figure 3 from Hicks et al. (2023): Left Panels: Characteristic pentad atmospheric river (AR) counts and 850-hPa height anomalies associated with the leading subseasonal teleconnection patterns and El Niño variability from Niño3.4 sea surface temperature (SST) index during the extended winter season (November–March), obtained from linear regressions during 1980–2018 and 1982–2018, respectively. From top to bottom: the North Atlantic Oscillation (NAO), the growth phase of North Pacific Oscillation/West Pacific (NPO/WP) pattern, the decay phase of NPO/WP, the Pacific North American (PNA) pattern, the Scandinavian (SCAND) pattern, and El Niño. Positive/negative AR regressions are shaded red/blue at 0.2 AR/pentad interval; 850-hPa height anomalies are contoured at 10 m interval with continuous/dashed lines for positive/negative anomalies. Right Panels: AR regressions are superposed on the AR climatology in the Pacific–North American region, where climatological AR counts are contoured at 0.4 AR/pentad; the 2.0 ARs/pentad anomaly contour is thickened for emphasis. A 9-point spatial smoother is applied twice to the AR regressions and climatology.

Figure 1: Figure 4 from Hicks et al. (2023): Inter-reanalysis comparison of climatological atmospheric river (AR) counts and AR-count regressions on winter teleconnections during 1980–2018: Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2) (right column) and ERA-Interim (left column). From the top: regressions on the North Atlantic Oscillation (NAO), the growth phase of North Pacific Oscillation/West Pacific (NPO/WP) pattern, the decay phase of NPO/WP, the Pacific North American (PNA) pattern, and the Scandinavian (SCAND) pattern. AR regressions are shaded red/blue for positive/negative values at 0.2 AR/pentad/index interval (see color bar). The climatological AR count per pentad is contoured in black at 0.4 intervals, with the 2.0 ARs/pentad isoline thickened for emphasis. A 9-point spatial smoother is applied twice to AR regressions and climatology.

Hicks, J., Guan, B., Nigam, S., & Ruiz-Barradas, A. (2023). Large-scale circulation context for North American west coast atmospheric rivers: Influence of the subseasonal NPO/WP teleconnection. Journal of Geophysical Research: Atmospheres, 128, e2023JD038693. https://doi.org/10.1029/2023JD038693