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This thesis addresses the parameterisation of chemical and dynamical processes in the Antarctic stratosphere. Statistical models for the inter- and intra-annual variability in Antarctic stratospheric ozone depletion were developed based on theory and an understanding of the coupling of dynamics and chemistry in the atmosphere. It was confirmed that the primary driver of the long-term trend in the severity of the Antarctic ozone hole is halogen loading in the stratosphere. The year-to-year variability in ozone mass deficit, a measure of the severity of Antarctic ozone depletion, could be described by a linear combination of South Pole temperatures and midlatitude wave activity. A time lag of two weeks between wave activity effects and ozone depletion indicates the predictive capability of meteorological parameters for seasonal projections of the severity of the Antarctic ozone hole. The statistical model describing the inter-annual variability in ozone mass deficit was regressed against observations from 1979 to 2004. The resulting regression coefficients were applied to South Pole temperature and wave activity fields from 28 chemistry-climate models. This analysis indicates a slight increase in the year-to-year variability in the severity of Antarctic ozone depletion. As a prelude to analysing the seasonal evolution of Antarctic ozone depletion, an improved ozone mass deficit measure was derived by replacing the constant 220 DU threshold with a seasonal varying pre-ozone hole background which leads to better capturing the true extent of the depleted ozone. Furthermore, it was shown that the new measure represents the chemical ozone loss within the Antarctic vortex provided that no mixing occurs through the vortex boundary. This new measure has many advantages over previous stratospheric ozone depletion indices. The conventional ozone mass deficit omits large amounts of depleted mass of ozone, and the onset of ozone depletion does not coincide with the timing of when sunlight first reaches areas of polar stratospheric clouds as expected from theory. Chemical ozone loss derived with a tracer-tracer correlation technique depends on ozone and passive tracer profile measurements which are not as readily available as the total column ozone fields required for the new ozone mass deficit presented in this thesis. As such, the new ozone depletion measure combines the simplicity of the old ozone mass deficit index with higher accuracy of the actual amount of chemically depleted stratospheric ozone. Furthermore, when applying the new definition of ozone mass deficit to chemistry-climate model outputs, model intercomparisons should become easier to interpret because biases in the models can be avoided. Based on theory and understanding of the coupling of chemistry and dynamics in the Antarctic stratosphere, two semi-empirical models were developed to describe the intra-seasonal evolution of chlorine activation and ozone depletion. Regression of the models against chlorine monoxide and ozone mass deficit from observations results in coefficients that capture key sensitivities in the real atmosphere. The seasonal evolution of ozone mass deficit can be described with these coefficients and readily available meteorological fields (temperature and wind fields). The predictive capability of these models was demonstrated for 2005 and 2006. Given temperature and wind fields, which for example can be obtained from general circulation models, these models can predict the size and depth of the Antarctic ozone hole. Important applications of the semi-empirical models could be chemistry-climate model validation by comparing the sensitivities from observations and models which may provide new insights into potential sources of differences in chemistry-climate model projections of Antarctic ozone depletion. Furthermore, projection of the inter-annual and intra-seasonal evolution of the Antarctic ozone hole into the future can help to assess the potential recovery of the Antarctic ozone hole.

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A decadal variation of tropical lower stratospheric ozone and temperature has previously been identified that correlates positively with the 11 year solar activity cycle. However, the El Niño–Southern Oscillation (ENSO) also influences lower stratospheric ozone and temperature. It is therefore legitimate to ask whether quasi-decadal ENSO variability can contribute to this apparent solar cycle variation, either accidentally because of the short measurement record or physically because solar variability affects ENSO. Here we present multiple regression analyses of available data records to compare differences in results obtained with and without including an ENSO term in the statistical model. In addition, simulations are performed using the NRL NOGAPS-ALPHA GCM for warm/cold ENSO conditions to test for consistency with the ENSO regression results. We find only very minor changes in annual mean solar regression coefficients when an ENSO term is included. However, the observed tropical ENSO response provides useful insights into the origin of the unexpected vertical structure of the tropical solar cycle ozone response. In particular, the ENSO ozone response is negative in the lower stratosphere due to increased upwelling but changes sign, becoming positive in the middle stratosphere (5–10 hPa) due mainly to advective decreases of temperature and NOx, which photochemically increase ozone. A similar mechanism may explain the observed lower stratospheric solar cycle ozone and temperature response and the absence of a significant response in the tropical middle stratosphere.