Possible connection between weak stratospheric vortex events and the Pacific Decadal Oscillation

The possible effect of the Pacific Decadal Oscillation (PDO) on the morphology of weak stratospheric vortex (WSV) events is examined by analyzing long-term reanalysis data. The WSV events, which are defined by less than 10 percentile of 10-hPa Northern Annular Mode index, are found to occur more frequently during the positive phase of the PDO (+PDO) than during the negative phase (-PDO). More importantly, during +PDO, vortex displacement events are more popular. This contrasts with –PDO during which vortex split events are more common. This difference is closely linked to the difference in tropospheric wave forcing. It is found that while +PDO WSV events are primarily driven by wavenumber-one heat flux, –PDO WSV events are mainly driven by wavenumber-two heat flux.

This difference in wave driving results from the PDO-related tropospheric circulation anomalies over the North Pacific whose wavenumber-one or -two components linearly interfere with climatological planetary-scale waves. The PDO-related mean state also contributes positively to the anomalous wavenumber-one heat flux during +PDO although the opposite is true during -PDO. The enhanced wavenumber-two heat flux during -PDO is likely caused by the intensification of tropospheric intraseasonal variability over the Bering Sea and Alaska due to the poleward shift of the Pacific jet.