Climatology of the stratospheric polar vortex and planetary wave breaking

Abstract

We use the distribution of Ertel's potential vorticity (PV) on the 850K isentropic surface to establish a climatology for the transient evolution of the planetary scale circulation in the Northern Hemisphere winter midstratosphere. We assume that reversals of the latitudinal gradient of PV, localized in longitude and latitude may be regarded as signatures of planetary wave breaking. Wave breaking identified by such signatures tends to occur mainly in the vicinity of the Aleutian anticyclone, with a secondary maximum over Europe. The area of the polar vortex, is strongly influenced by wave breaking. Erosion of the polar vortex due to transport and mixing of PV leads to a preconditioned state, that always occurs prior to major stratospheric warmings. During winters with little PV transport or mixing, the vortex area evolves rather uniformly in response to radiative forcing. During winters with major sudden warmings, the wave breaking signature as defined here first appears at low values of PV, then rapidly moves toward higher values as the vortex area is reduced and the "surf-zone' structure becomes well defined. -from Authors