Comparing and contrasting extreme stratospheric events, including their coupling to the tropospheric circulation

Recent work has emphasized the importance of stratosphere-troposphere coupling associated with extreme values of the polar vortex strength and stratospheric planetary wave heat flux during Northern Hemisphere winter. Here using ERA-Interim reanalysis data the evolution of the two types of extreme events are compared. First, extreme positive and negative planetary wave-1 heat flux events are shown to exhibit largely equal but opposite-signed impacts producing a net deceleration and acceleration of the polar vortex, respectively. Both events couple with the troposphere via a westward propagating high-latitude zonal wavenumber-1 signal that produces a shift of the North Atlantic jet (equatorward and poleward for positive and negative events, respectively). High-latitude stratospheric vertical zonal wind shear is found to play a key role during the event evolution.

A comparison of extreme stratospheric events reveals that the evolution of vortex events is broadly similar to heat flux events but occurs over a longer time scale. Multiple positive and negative heat flux events produce cumulative irreversible impacts that contribute to the development of weak and strong vortex events, respectively. Clear coupling with the North Atlantic jet stream occurs during vortex events when wave-1 dominates the total eddy heat flux in the lower stratosphere, otherwise interference with wave-2 makes the impacts less clear. The tropospheric impacts in the North Atlantic associated with extreme planetary wave events are found to be comparable if not larger than those associated with extreme vortex events.

Overall the results illustrate a clear dualism between extreme stratospheric events and hence wave-mean flow interaction in the stratosphere. In particular they demonstrate that strong vortex and negative heat flux events are true dynamical events that play an important role in coupling to the tropospheric circulation.