Uncertainties in regional climate change prediction: a regional analysis of ensemble simulations with the HADCM2 coupled AOGCM

We analyze ensembles (four realizations) of historical and future climate t ransient experiments carried out with the coupled atmosphere-ocean general circulation model (AOGCM) of the Hadley Centre for Climate Prediction and R esearch, version HADCM2, with four scenarios of greenhouse gas (GHG) and su lfate forcing. The analysis focuses on the regional scale, and in particula r on 21 regions covering all land areas in the World (except Antarctica). W e examine seasonally averaged surface air temperature and precipitation for the historical period of 1961-1990 and the future climate period of 2046-2 075. Compared to previous AOGCM simulations, the HADCM2 model shows a good performance in reproducing observed regional averages of summer and winter temperature and precipitation. The model, however, does not reproduce well observed interannual variability. We find that the uncertainty in regional climate change predictions associated with the spread of different realizat ions in an ensemble (i.e. the uncertainty related to the internal model var iability) is relatively low for all scenarios and regions. In particular, t his uncertainty is lower than the uncertainty due to inter-scenario variabi lity and (by comparison with previous regional analyses of AOGCMs) with int er-model variability. The climate biases and sensitivities found for differ ent realizations of the same ensemble were similar to the corresponding ens emble averages and the averages associated with individual realizations of the same ensemble did not differ from each other at the 5% confidence level in the vast majority of cases. These results indicate that a relatively sm all number of realizations (3 or 4) is sufficient to characterize an AOGCM transient climate change prediction at the regional scale.