Contrasting the Intraseasonal Precipitation in the Western and Eastern North Pacific During Boreal Winter

Abstract

The wintertime North Pacific exhibits two types of intraseasonal periodicity in daily precipitation, with a period of 10–20 and 30–90 days in the western and eastern North Pacific. This study aims to identify and compare characteristics and mechanisms of intraseasonal precipitation in these two regions using daily observational data. Our results indicate their notable differences in atmospheric circulations and dynamical processes, despite similarities in moisture sources. A wave train associated with the intraseasonal precipitation in the western North Pacific characterized by anomalous cyclones and anticyclones from West Siberia into the North Pacific is observed, accompanied by anomalous wave activity. By diagnosing local finite-amplitude wave activity (LWA) budget, we demonstrate that the baroclinic eddy generation initiates the upstream growth of wave activity, whereas its downstream development is primarily attributed to the zonal advection of wave train. The upstream wave train induces barotropic and baroclinic instabilities in Kuroshio-Oyashio extension (KOE). In contrast, the anomalous atmospheric circulations related to intraseasonal precipitation in the eastern North Pacific feature a tripolar structure, with downstream propagation of wave activity from the eastern North Pacific to the North American continent. Diabatic heating, along with local eddy generation, contributes to the upstream growth of wave activity. Residual term, possibly linked to the wave activity dissipation associated with wave breaking, extends the wave activity downstream. Thermal and baroclinic instabilities, associated with the diabatic heating and local eddy generation, create favorable conditions for the precipitation. This study may contribute to the improvement of intraseasonal precipitation prediction over the pan-North Pacific region.