The CSIRO nine-level atmospheric general circulation model and its equilibrium present and doubled CO2 climates

The CSIRO nine-level atmospheric general circulation model is described. Due to the use of the flux form of the dynamical equations, the model achieves conservation of mass and global energy making it particularly suitable for climate simulation. Climatologies of 30-year equilibrium simulations by the model, when coupled to a mixed-layer ocean with prescribed heat transport, for present and doubled CO2 conditions are presented. The global means of standard fields in the simulated present climate are generally close to observed values. Root mean square errors of the fields are tabulated. The spatial and seasonal variations of the fields are simulated with an accuracy comparable to that of other models. A strong spatial correlation between simulated cloud cover and soil moisture is also noted. The global mean surface warming induced by the doubling of CO2 in the model is 4.8°C. The close energy balance of the model ensures that the warming is not affected by any spurious energy sources or sinks. Changes to the structure of standard fields are generally similar to those in simulations previously reported. In the tropics there is a shift upwards of the mean meridional circulation. This is related to changes in the atmospheric diabatic heating rates. The mean level of latent heating rises about 30 hPa. There is also a shift poleward of the latitudes of peak ascent and precipitation associated with the Hadley circulation. However, changes in zonal mean relative humidities are generally small.