Impact of horizontal resolution on the simulation of seasonal climate in the Atlantic/European area for present and future times

The climate response to increasing levels of atmospheric greenhouse gases, prescribed according to the Intergovernmental Panel on Climate Change (IPCC ) scenario IS92a, is studied in 2 model simulations. One is a transient res ponse experiment performed with a medium resolution (T42) coupled general c irculation model of the atmosphere and ocean (ECHAM4/OPYC) developed at the Max Planck Institute for Meteorology. The other one is a time-slice experi ment with the high resolution (T106) ECHAM4 model forced with monthly sea-s urface temperatures and sea-ice from the coupled model. For two 30 yr time- slices, representing the present-day climate (1970-1999) and the future cli mate with an effective doubling of carbon dioxide (2060-2099), the seasonal mean climate statistics obtained from the 2 experiments in the Atlantic/Eu ropean area are compared with each other and, for the simulations of the pr esent-day climate, with observational data in order to evaluate the impact of the model's horizontal resolution on the simulation of regional climate change as well as on the quality of the simulation of regional climate. As the region of special interest, the Atlantic/European area is chosen, The m odel's horizontal resolution has a noticeable impact on the assessment of a nthropogenic climate change as well as on the quality of the climate simula tion at a local level, while at a regional level, that is considering the e ntire Atlantic/European area, the effect of the horizontal resolution is co nsiderably reduced. The level of uncertainty related to the horizontal reso lution actually falls with the range of uncertainty caused by the internal model variability obtained from ensembles of climate predictions. The level of uncertainty at a given location is very small for the near-surface temp erature but somewhat higher for the sea-level pressure, in particular in au tumn and winter, and relatively high for precipitation, with a marked maxim um in summer. While the uncertainties in autumn and winter are mainly accou nted for by the marked impact of the horizontal resolution on the dynamical aspects of the model, the high level of uncertainty in summer is related t o the pronounced effect of the horizontal resolution on the model's physica l parameterizations.