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

Abstract

Interannual changes of stratospheric dynamical structure and ozone are explored in observed variations over the Northern Hemisphere during the 1980s and 1990s. Changes of dynamical structure are consistent with a strengthening and weakening of the residual mean circulation of the stratosphere. It varies with the Eliassen?Palm (E?P) flux transmitted upward from the troposphere and, to a lesser degree, with the quasi-biennial oscillation (QBO). These two influences alone account for almost all of the interannual variance of wintertime temperature over the two decades, even during unusually cold winters. Stratospheric changes operating coherently with anomalous forcing of the residual circulation are coupled to changes of tropospheric wave structure. Those changes of dynamical structure share major features with the Arctic Oscillation. Both involve an amplification of the ridge over the North Pacific and an expansion of the North Atlantic storm track. Changes of tropospheric wave structure lead to a temperature signature of anomalous downwelling in the Arctic stratosphere. Accompanying it at a lower latitude is a temperature signature of anomalous upwelling. That compensating change operates coherently but out of phase with the temperature change over the Arctic. However, it is an order of magnitude smaller, making it difficult to isolate in individual years or in small systematic changes that characterize trends. Interannual changes of dynamical structure are mirrored by changes of total ozone. Like temperature, ozone changes are large at high latitudes. They are accompanied at lower latitudes by coherent changes of opposite sign. Those compensating changes, however, are an order of magnitude smaller?like temperature. Ozone changes operating coherently with anomalous forcing of the residual circulation track observed changes. They account for most of the interannual variance. What remains (about 20%) is largely accounted for by changes of the photochemical environment, associated with volcanic perturbations of aerosol and increasing chlorine. The close relationship between these changes and observed ozone is robust: It is obeyed even during years of unusually low ozone. Total ozone then deviates substantially from climatological-mean levels. However, it remains broadly consistent with the relationship deduced from the overall population of years.