A new look at stratospheric sudden warmings: vertical structure and coherence

The vertical structure of the polar vortex is investigated for observed stratospheric sudden warmings (SSWs) during the period 1957-2002, using ECMWF ERA-40 reanalysis fields. The observed events are divided into 15 wave-1 (displacement) and 14 wave-2 (splitting) events. Composites of a suitably scaled potential vorticity are used to illustrate typical vortex behaviour with an emphasis on highlighting differences in the evolution of the vertical structure of the vortex during each type of event.

For wave-2 SSWs, the composite potential vorticity fields demonstrate that to a good approximation, the polar vortex split occurs simultaneously over an altitude range of ~25km. In contrast, for wave-1 SSWs the composite potential vorticity fields indicate that the breakdown of the polar vortex is strongly dependent on altitude, with the vortex completely destroyed at upper levels (~35km) during the warming, while it remains at lower levels (~20km). For both types of SSW the composite fields show that in relation to the underlying topography, the orientation of the polar vortex during the breakdown is approximately the same for each event.