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5. Water vapour trends

Figure 8 shows the model water vapour concentrations and trends. During the first four years of the integration, the water vapour in the model had not reached steady state and hence this period is ignored in the calculation of the trends. The model water vapour is in good agreement with observations (e.g. Randel et al., 1998) with mesospheric values exceeding 6ppmv and the tropical minimum of about 3.5 ppmv in the annual average. The model also simulates realistically low values over Antarctica (minimum 3.0 ppmv in the annual average) due to the impact of dehydration in the winter and spring seasons. The trends (Fig. 8) are generally very small in the stratosphere with increases of order 1% per decade over most of the domain. Descent over the Arctic combined with methane oxidation increases the water vapour by almost 3% per decade at 10 hPa, while a slight reduction occurs over Antarctica due to the dominance of the cooling. The water vapour trends in this model simulation are controlled by the slight cooling at the tropopause (Fig. 11) rather than any effects due to the increase in strength of the Brewer-Dobson circulation.