Tropospheric Precursors of Anomalous Northern Hemisphere Stratospheric Polar Vortices

Regional extratropical tropospheric variability affects the wave driving of the Northern Hemisphere wintertime stratospheric polar vortex. Simple reasoning is used to understand the nature of the regional variability that reinforces extratropical planetary waves, and thus zonal wavenumber-1 and wavenumber-2 vertical EP flux leaving the troposphere. In the reanalysis record and in WACCM (Whole Atmosphere Community Climate Model), one regional

pathway for enhanced planetary wave driving is a deeper low over the North Pacific, and a second pathway is an enhanced high over Eastern Europe. Perturbations in the vortex induced by the two pathways add linearly.

These two pathways begin to weaken the upper stratospheric vortex immediately, with a peak influence after a lag of some twenty days; the influence then propagates downwards in time, as expected from wave-mean flow interaction theory. These patterns are influenced by the El-Nino Southern Oscillation (ENSO) and

October Eurasian snow cover. Finally, these two patterns and the Quasi-Biennial Oscillation (QBO) explain 40% of wintertime polar vortex variability.