Changes of the Stratospheric Circulation: Relationship to Changes of Ozone and Tropospheric Structure

Changes of stratospheric dynamical structure and ozone have been studied in observed variations over the Northern Hemisphere during the 1980s and 1990s. Interannual changes have structure consistent with a strengthening/weakening of the residual mean circulation of the stratosphere, which regulates polar temperature and ozone transport. Two mechanisms forcing the anomalous residual circulation account for almost all of the interannual variance: Changes of EP flux transmitted upward from the troposphere by planetary waves, along with changes of equatorial wind associated with the QBO, represent nearly all of the observed variation of wintertime temperature. Supported by changes of chlorine and aerosol loading, which represent about 20% of the interannual variance, the same conclusion holds for ozone.

Changes of dynamical structure operating coherently with the anomalous residual circulation share major features with the Arctic Oscillation (AO). Both are marked by a prominent signature of downwelling and adiabatic warming over the Arctic, which is visible well into the troposphere. It's compensated at lower latitudes by coherent changes of opposite sign, which reflect upwelling and adiabatic cooling. Despite their structural similarity, changes operating coherently with the anomalous residual circulation are 2-3 times stronger that changes associated with the AO. Those stratospheric changes are coupled to changes of tropospheric wave structure, specifically, to amplification/weakening of the ridge over the north Pacific and analogous changes of the North Atlantic storm track. The anomalous residual circulation exhibits a trend over the 1980s and 1990s, as does the AO. Each is consistent with the observed decline of temperature and ozone. Those trends, in fact, have the same structure as changes operating coherently with the anomalous residual circulation.

These interannual changes stem from dynamical processes operating on shorter times scales. Analogous behavior is visible in daily variations of dynamical structure. Sporadic amplifications of planetary waves magnify the upward EP flux from the troposphere, which in turn is followed by an enhancement of wave absorption in the stratosphere and finally signatures of anomalous downwelling at high latitudes and upwelling at low latitudes. An animation, to be presented if time is available, illustrates these events in successive surges of upward EP flux, followed by warming over the Arctic that descends from upper levels.