A time-slice experiment with the ECHAM AGCM at high resolution: The impact of horizontal resolution on the simulation of climate change

A time-slice experiment has been performed with the ECHAM4 AGCM at a horizontal resolution of T106 with each time-slice covering a period of 30 years. The first time-slice represents the present-day climate and the second one the climate at a time, when the carbon dioxide concentration in the atmosphere has doubled. In these time-slices the atmosphere has been forced by monthly mean values of the sea surface temperatures, the sea-ice extent and the sea-ice thickness originating from a transient simulation with the ECHAM4/OPYC AOGCM at a horizontal resolution of T42. In these simulations the concentrations of the important greenhouse gases have been prescribed according to the IPCC scenario IS92a. By comparing the two periods of the time-slice experiment with each other we infer the changes of the future climate predicted by this experiment. And by comparing the time-slice experiment with the climate change simulation, which has provided the lower boundary forcing, we investigate, to which extent the predicted change of climate depends on the model's horizontal resolution. This provides not only an estimate of the uncertainties of the climate changes inferred from the two experiments but also an evaluation of the basic assumption of the time-slice approach, namely that the large-scale circulation patterns in the low and the high resolution model are not markedly different from each other.