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6. Discussion and conclusion

This work is new in completing a long (20-year) simulation of the past chemistry and dynamics of the stratosphere in a detailed chemistry-climate model with observed greenhouse gases, halogen loadings, sea surface temperatures and sea ice amounts. Critical comparisons have been made with observations over the period 1980-2000. It can be concluded that the annual ozone variability is reproducible and that the values are reasonably consistent with observations. The globally averaged total ozone trend is also in agreement with observations when the solar cycle is removed from the latter. However, there are some important discrepancies, with Arctic ozone depletion smaller than observed and the impact of the Antarctic ozone hole lasting too long into the summer. Also, although the depth of the Antarctic ozone hole is reasonably well simulated it is too small and subject to too much interannual variability in the model.

The strengths and weaknesses of the modelled ozone are closely related to the temperature trends which in general are consistent with trends observed from SSU/MSU data. Also, the model temperature trends and interannual variability for the coupled chemistry simulation presented here are generally in better agreement with observations (Randel and Wu, 1999a, Scaife et al., 2000a) than a simulation of a similar version of the model without chemistry (Butchart et al., 2000). However, there is a systematic underprediction of the cooling rates in the lower stratosphere which could be related to the underprediction of model water vapour trends (Forster and Shine, 1999). These trends are small in the model (up to 3% increase per decade locally) but in the Antarctic lower stratosphere are negative due to reductions in temperature. Further, the absence of a water vapour trend in the model clearly contradicts observations (e.g. Oltmans and Hofmann, 1995).

To return to the original question of whether the model is good enough to predict accurate stratospheric ozone and temperature trends for the next 20 years, the results suggest first the need for further model development. Such comments apply equally well to more simplified models, which conceivably could provide a misleading picture of future stratospheric trends in view of the complexity of the many conflicting processes.

Acknowledgments. This work was supported by the U.K. Public Met. Service Research and Development Programme, the U.K. Department of Environment Transport and the Regions (contract EPG/1/1/83), and the CEC project `European project on Stratospheric Processes and their Impact on Climate and the Environment' (EuroSPICE) which commenced in March 2000. I would like to thank Jeff Cole (U. Reading, UK) for supplying updated AMIP sea surface temperatures and sea ice and Jeff Knight (The Met. Office) for implementing some of the model improvements. Dave Jackson (The Met. Office) kindly supplied the methane oxidation scheme, John Nash (The Met. Office) provided SSU/MSU temperature anomalies.

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