The Dependence of Climate Sensitivity on the Horizontal Resolution of a GCM

The impact of changing the horizontal resolution of a version of the Hadley Centre GCM is investigated in present-day and 2 × CO2 climate simulations. The use of higher resolution generally produces favorable changes. The main improvements are in the Southern Hemisphere in both solstitial seasons and in the Tropics and Northern Hemisphere in June, July, and August (JJA). Aspects of the model that show no improvement include the general coldness of the troposphere in all seasons and the pressure pattern in Northern Hemisphere winter. The global mean annual-mean warming due to increased CO2 is very similar, but there are differences in the latitudinal distribution of the zonally averaged temperature response. In winter these differences can be related to the response of the Northern Hemisphere storm tracks. There is a considerable poleward and downstream shift of the high-frequency variability at higher resolution, which is only weakly simulated at low resolution. In JJA, the response of the tropical circulation and precipitation is very different where there were large differences in the unperturbed simulation.