A time-slice experiment with the ECHAM4 AGCM at high resolution: the impact of horizontal resolution on annual mean climate change

The climate response to increasing levels of atmospheric greenhouse gases, prescribed according to the International Panel of Climate Change (IPCC) sc enario IS92a, is studied in two model simulations. The reference simulation is a transient response experiment performed with a medium-resolution (T42 ) coupled general circulation model of the atmosphere and ocean (ECHAM4/OPY C) developed at the Max-Planck-Institute for Meteorology. For two 30-year " time slices", representing the present-day climate and the future climate a t the time of effective CO2 doubling, the annual mean climate states are co mpared with those obtained from the high-resolution (T106) ECHAM4 model for ced with monthly sea surface temperatures and sea-ice from the coupled mode l. The large-scale changes in temperature, zonal wind, sea-level pressure a nd precipitation are broadly similar. This applies, in particular, to the r espective zonal means. In general, except for precipitation, the responses in the time-slice experiments are slightly weaker than those simulated in t he coupled model due to a smaller effect of the horizontal resolution on th e simulations of the future (warmer) period than on the simulations of the present period. On a regional scale, the impact of horizontal resolution is smaller in the Southern than in the Northern Hemisphere, where the respons e differences are caused mainly by changes in the positions of the stationa ry waves. Although the precipitation responses are broadly similar, there a re few notable exceptions such as a mole pronounced maximum over the equato rial oceans in the T106 experiment but a weaker response over low-latitude land areas. Differences in precipitation response are found especially in a reas with strong topographical control such as South America, for example.