The unusual dynamical situation over Europe in the winter 1991/1992 caused an unusual behaviour of the total ozone. The strongest negative deviations from the long-term monthly means occurred in January. A very cold middle stratosphere developed at the edge of the polar vortex above a warm anticyclonic block in the troposphere. The corresponding low temperature in the tropopause region was located just beneath the coldest air in the stratosphere.

A high positive correlation between the temperature and ozone partial pressure was derived at the edge of the polar stratospheric vortex for the tropopause region and for the middle stratosphere. The physical background was vertical motions changing temperature as well as the ozone content. That means unusually low ozone values can be expected when an elevated tropopause is combined with the adiabatically cooled end of the upwelling branch of an enhanced planetary wave in the middle stratosphere. It is shown that in February 1990, when extremely low total ozone was also observed over Scandinavia during a short anticyclonic blocking, this event was caused by the same process. However, during transient events the tropopause temperature is often aniticorrelated with the middle stratospheric temperature (a tropospheric ridge reaching into the stratosphere has a cold high tropopause, but a warm middle stratosphere). The forced vertical motions result in extreme ozone columns only when they are in the same direction in both layers.

The enhanced wave activity was connected with a strong polar warming in the upper stratosphere in both winters. At 30 hPa in the middle stratosphere the warm center situated over Eastern Siberia and the coldest part shifted toward Northern Europe.

The same situation was available in 11 cases during the fourteen years series of TOMS data (1979--1992), when total ozone reached values below 225 DU at the edge of the stratospheric polar vortex: enhanced wave activity in the middle stratosphere, polar warming in the upper stratosphere, shift of the coldest part of the polar vortex towards northern Europe over a cold high tropopause of a tropospheric anticyclone. ¿ American Geophysical Union 1994