Comparing and contrasting extreme stratospheric events, including their coupling to the tropospheric circulation. Issue 4 (26th February 2015) - Dunn‐Sigouin, Etienne
Abstract 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 events are compared. The life cycle of total (anomaly plus climatology) positive/negative heat flux events are associated with vertically deep high‐latitude planetary wave structures and exhibit largely equal but opposite‐signed impacts, including a net deceleration/acceleration of the polar vortex due to EP flux convergence/divergence and an equatorward/poleward tropospheric jet shift in the North Atlantic. The tropospheric wave pattern is westward propagating. High‐latitude stratospheric vertical zonal wind shear plays a key role during both events. A comparison of the stratospheric events reveals that planetary wave events contribute to the development of vortex events. In particular, total negative heat flux events precede strong vortex events showing that strong vortex events represent true dynamical events involving significant wave‐mean flow interaction. Coupling with the North Atlantic jet occurs preceding vortex events when wave‐1 dominates the total eddy heat flux in the lower stratosphere since interference with wave‐2 makes the impacts less clear. The tropospheric impacts in the North Atlantic associated with planetary wave events are found to be comparable if not larger than those following vortex events.