Exploring the role of tropospheric stationary waves for the North Pacific response to SSWs

Abstract

Stratospheric variability can have a significant impact on surface weather extremes in winter, in particular during stratospheric extreme events, so-called sudden stratospheric warming (SSW) events. The stratospheric downward impact has been shown to be communicated by both planetary-scale and synoptic-scale waves, but their relative roles and interactions are not fully understood. Since there is a strong average response to stratospheric forcing over the North Atlantic but a weak response over the North Pacific, studies of SSW downward impact generally focus on the North Atlantic, where the synoptic eddy-feedback plays a strong role. We here examine the relative roles of planetary-scale waves for the North Pacific after SSW events. By examining the case-by-case response over the North Pacific following SSW onset using ERA5 reanalysis, we identify differences between events in terms of their interactions between the wave anomalies induced by the stratosphere and the tropospheric stationary waves. Specifically, the destructive and constructive interference of zonal wavenumber-1 anomalies plays a dominant role in contributing to different responses over the North Pacific, which are associated with an equatorward and a poleward jet shift, respectively. We suggest that SSW events can exhibit opposite responses over the North Pacific, potentially explaining the generally weak response observed in this region when averaging across all SSW events.