OBJECTIVE COMPARISON OF PATTERNS OF CO2 INDUCED CLIMATE-CHANGE IN COUPLED GCM EXPERIMENTS

Four transient GCM experiments simulating the climatic response to gra dually increasing CO2, and two equilibrium doubled CO2 experiments are compared. The zonally symmetric and asymmetric features of climate ar e both examined. Surface air temperature, sea level pressure, the 500 mb height and the relative topography between 500 and 1000 mb are anal yzed. In the control simulations, the broad aspects of the present cli mate are in most cases well reproduced, although the stationary eddies tend to be less reliably simulated than the zonal means. However, the agreement between the four transient experiments on the geographical patterns of climate change is less impressive. While some zonally symm etric features, in particular the meridional distribution of surface a ir warming in the boreal winter, are rather similar in all models, the intermodel cross correlations for the zonally asymmetric changes are low. The agreement is largely restricted to some very general features such as more warming over the continents than over the oceans. The la rgest discrepancies between the two equilibrium-doubled CO2 experiment s and the transient experiments are found at the high southern latitud es, in particular in the austral winter. To identify the most robust g eographical patterns of change in the transient experiments, the stand ard t test is used to determine if the four-model mean change is signi ficantly above or below the global mean.