The extratropical transition of TC Dale (1996) and its impact on the early 1996–97 wintertime stratospheric circulation

It has become increasingly accepted that recurving tropical cyclones (TCs) can have substantial impacts on the hemispheric general circulation, as well as on downstream forecast uncertainty. A recurving TC that transitions into an extratropical cyclone can excite a Rossby wave train that is associated with meridional fluxes of heat and momentum. In some cases, meridional fluxes of heat and momentum extend well into the stratosphere, where such fluxes are associated with an upward EP flux from the troposphere into the stratosphere. An environment characterized by EP flux convergence experiences an increase in wave activity and consequently a decrease in the westerly component of the mean zonal wind. When a TC undergoes extratropical transition (ET) during the autumnal spin-up of the Northern Hemisphere wintertime circulation, the EP flux convergence in the stratosphere can delay the establishment of the wintertime stratospheric polar vortex and the associated stratospheric polar night jet.

This talk will explore the hypothesis that the processes associated with the recurvature and ET of TC Dale (1996) were responsible for a warming of the Arctic polar cap and the weakening of the stratospheric polar vortex. After TC Dale recurved and became extratropical, there was a surge in the EP flux from the troposphere to the stratosphere. The EP flux convergence was located along the flank of the stratospheric polar vortex and was associated with stratospheric ridge amplification over Alaska. The ridge amplification occurred at a point in the season when the stratospheric polar night jet was climatologically spinning up for the winter; however, the flux of wave activity from the troposphere to the stratosphere associated with the recurvature and ET of TC Dale resulted in a weakened stratospheric polar vortex and a detectable departure from climatology. The potential implications of this event on the Northern Annular Mode, as well as intraseasonal forecasting, will be highlighted.