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The dynamics of the polar vortex in winter and spring play an important role in explaining observed low ozone values. A quantification of physical and chemical processes is necessary to obtain information about natural and anthropogenic causes of fluctuations of ozone. This paper aims to contribute to answering the question of how permeable the polar vortex is. The transport into and out of the vortex ("degree of isolation") remains the subject of considerable debate. Based on the results of a three-dimensional mechanistic model of the middle atmosphere, the possibility of exchange of air masses across the polar vortex edge is investigated. Additionally the horizontal and vertical structure of the polar vortex is examined. The model simulation used for this study is related to the major stratospheric warming observed in February 1989. The model results show fair agreement with observed features of the major warming of 1989. Complex structures of the simulated polar vortex are illustrated by horizontal and vertical cross sections of potential vorticity and inert tracer. A three-dimensional view of the polar vortex enables a description of the vortex as a whole. During the simulation two vortices and an anticyclone, grouped together in a very stable tripolar structure, and a weaker, more amorphous anticyclone are formed. This leads to the generation of small-scale features. The results also indicate that the permeability of the vortex edges is low because the interior of the vortices remain isolated during the simulation.

The comprehensive chemistry module CHEM has been developed for application in general circulation models (GCMs) describing tropospheric and stratospheric chemistry, including photochemical reactions and heterogeneous reactions on sulphate aerosols and polar stratospheric clouds. It has been coupled to the spectral atmospheric GCM ECHAM3. The model configuration used in the current study has been run in an "off-line" mode, i.e. the calculated chemical species do not affect the radiative forcing of the dynamic fields. First results of a 15-year model integration indicate that the model ECHAM3/CHEM runs are numerically efficient and stable, i.e. that no model drift can be detected in dynamic and chemical parameters. The model reproduces the main features regarding ozone, in particular intra- and interannual variability. The ozone columns are somewhat higher than observed (approximately 10%), while the amplitude of the annual cycle is in agreement with observations. A comparison with HALOE data reveals, however, a serious model deficiency regarding lower-stratosphere dynamics at high latitudes. Contrary to what is concluded by observations, the lower stratosphere is characterized by slight upward motions in the polar regions, so that some of the mentioned good agreements must be considered as fortuitous. Nevertheless, ECHAM3/CHEM well describes the chemical processes leading to ozone reduction. It has been shown that the mean fraction of the northern hemisphere, which is covered by polar stratospheric clouds (PSCs) as well as the temporal appearance of PSCs in the model, is in fair agreement with observations. The model results show an activation of chlorine inside the polar vortex which is stronger in the southern than in the northern winter hemisphere, yielding an ozone hole over the Antarctic; this hole, however, is also caused to a substantial degree by the dynamics. Interhemispheric differences concerning reformation of chlorine reservoir species HCl and ClONO2 in spring have also been well reproduced by the model.Key words. Atmospheric composition and structure · Middle atmosphere · Meteorology and atmospheric dynamics · Climatology · General circulation

Large-scale cross-tropopause mass fluxes are diagnosed globally from 1979 to 1989 for Northern Hemisphere winter conditions (December, January, and February). Results of different methods of approaches with regard to the definition of the tropopause and the way to calculate the mass fluxes are compared and discussed. The general pattern of the mass exchange from the tropopause into the stratosphere and vice versa agrees fairly well when using different methods, but the absolute values can differ up to 100%. An inspection of the temporal development of the mass fluxes for solstice conditions indicates a complex picture. Whereas a permanent significant downward flux from the stratosphere into the troposphere is detected for latitude regions nearly between 25°N and 40°N and between 30°S and 50°S (initiated by the poleward branches of the Hadley cells), a non-uniform behaviour is observed at higher latitude bands. Periods of strong mass exchange from the troposphere into the stratosphere are disrupted by periods of an opposite mass exchange. A comparison of the stratoshere-troposphere (ST) exchange with the exchange at higher altitudes through surfaces, quasi-parallel to the tropopause, excludes a general connection. Only a few strong upward directed ST mass exchange events have counterparts at higher altitudes. The composition of the stratosphere may be influenced directly by the ST exchange only in a thin layer above the tropopause.