An Interpretation of Sudden Warmings In Terms of Potential vorticity

Abstract

A simple and concise interpretation of stratospheric sudden warmings is offered in terms Of the transient changes in the potential vorticity pattern. The warming is viewed as a manifestation of the reversal of the mean (zonally averaged) relative potential vorticity distribution over a deep layer of the stratosphere and mesosphere. The reversals of the mean zonal flow and of the latitudinal gradient of the mean temperature are regarded merely as accompanying features of the phenomenon. The linchpin of our interpretation is a particularly simple derived relationship between the wave potential vorticity and the mean relative potential vorticity of a quasi-geostrophic flow. This relationship points to the possibility of inducing a reversal of the mean relative potential vorticity distribution by purely transient wave forcing of the stratosphere by tropospheric flow. Furthermore, it indicates that the duration and amplitude of the wave-forcing, together with the transmission properties of the mean zonal flow structure to upward propagating planetary waves, will be factors that regulate the occurrence of these reversal events. These assertions are further examined and illustrated in a series of experiments undertaken with a severely truncated spectral quasi-geostrophic β-plane model.