Connection between weak stratospheric vortex events and the Pacific Decadal Oscillation

Abstract

We investigate the possible impacts of the Pacific Decadal Oscillation (PDO) on the occurrence of weak stratospheric polar vortex (WSV) events in the Northern Hemisphere winter. WSV events, which are defined when polar-cap geopotential height anomalies at 50 hPa fall below the 10th percentile in winter, are observed more frequently during positive PDO phases than during negative PDO phases. Additionally, tropospheric wave forcings that drive WSV events are remarkably different between the two phases of the PDO. During the positive PDO phase, the vertical propagation of wavenumber-one waves plays a predominant role with a rather minor contribution of wavenumber-two waves. This contrasts with the negative PDO phase when the WSV events are primarily caused by wavenumber-two waves. This difference is partly related to the PDO-induced tropospheric circulation anomalies over the North Pacific whose zonal wavenumber-one component constructively (destructively) interferes with climatological planetary-scale waves during positive (negative) PDO winters. The predominant wavenumber-two wave forcings during the negative PDO phase are likely related to the enhanced tropospheric eddy activity over Alaska that results from the poleward shift of the Pacific jet in response to the negative PDO.