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The Role of Stratospheric Polar Vortex Breakdown in Southern Hemisphere Climate Trends

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If you have a question about this talk, please contact Dr Scott Hosking.

Substantial evidence has suggested that the SH summertime circulation trends over the late twentieth century are primarily caused by the anthropogenic Antarctic ozone hole in spring. These circulation changes are characterized by a strengthening of the circumpolar winds in the lower stratosphere and a trend towards the positive Southern Annular Mode (SAM) in the troposphere, associated with a poleward shift of the tropospheric eddy-driven jet.

Given the complex interplay between chemistry, radiation, and dynamics in the tropospheric response to stratospheric ozone depletion, the underlying mechanism(s) have not been fully understood. Particularly, the timing of stratospheric final warming (SFW) has been delayed in the reanalysis data. An examination of several climate model outputs with different climate forcings indicates that climate trends appear when there is a delay in the timing of SFWs.

The role of the SFW is examined using an idealized model of stratosphere and troposphere coupling, in which the ozone depletion is mimicked by a springtime polar stratospheric cooling. The responses of zonal-mean atmospheric circulation in the idealized model are similar to the observed climate trends. The years are further divided into those in which the SFW is delayed and those in which it is not. The responses for the years in which the SFW is delayed are very similar to the overall response, while the stratosphere is only characterized by the localized cooling for those years in which the SFW is not delayed, with no subsequent downward influence into the troposphere. This suggests that, in order to affect the troposphere, ozone depletion must first delay the SFW so as to induce a deep response in planetary wave drag and the associated eddy-driven circulation.

This talk is part of the British Antarctic Survey series.

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