RIDGE FORMATION IN THE LOWER STRATOSPHERE AND ITS INFLUENCE ON OZONE TRANSPORT - A GENERAL-CIRCULATION MODEL STUDY DURING LATE JANUARY 1992

Satellite total ozone measurements showed the development of an ozone mini-hole over northern Europe in late January 1992, during the observational phase of the European Arctic Stratospheric Ozone Experiment (EASOE). During the same period, ozone profiles, recorded with ozonesondes, showed that layers below 20 km were strongly depleted in ozone at some locations over northern and central Europe. The perturbed chemistry involving the reactive species, which play a role in the ozone-destroying catalytic cycles, was the focus of intensive observational investigations during the EASOE campaign. These processes are likely to take place between 15 and 30 km, and the question of mixing between vortex air and midlatitude air in the lower stratosphere, especially at the base of the polar vortex, is a central one. High-resolution (T106) 7-day forecast simulations have been performed with a GCM, in which the ozone field was realistically initialized, in order to study the formation and evolution of the ozone mini-hole, and the nature of large-scale mixing in the lower stratosphere. In particular, we tried to examine whether the model could reproduce with some degree of realism ozone tongues seen in the satellite data. The study has revealed that the formation of the mini-hole was Linked to the poleward extension in the lower stratosphere of an anticyclonic ridge. The effect of tropospheric forcing was evident up to at least 40 hPa. Ozone-poor air from subtropical latitudes was advected toward Scandinavia at the same time as a tongue of polar air extended northeasterly toward central Europe. During the course of the 1-week simulation, the modeled ozone mixing ratio and potential vorticity (PV) revealed strong large-scale isentropic mixing between high, middle, and low latitudes in the lower stratosphere. This mixing may occur through either the formation of narrow ozone/PV tongues, or of more vortex-like blobs of ozone/PV also seen to be peeled off from the vortex. There is good correspondence with the structures seen in the satellite-derived total ozone field at medium and synoptic scales.