REGIONAL CLIMATE CHANGES AS SIMULATED IN TIME-SLICE EXPERIMENTS

Three 30 year long simulations have been performed with a T42 atmosphe re model, in which the sea-surface temperature (SST) and sea-ice distr ibution have been taken from a transient climate change experiment wit h a T21 global coupled ocean-atmosphere model. In this so-called time- slice experiment, the SST values (and the greenhouse gas concentration ) were taken at present time CO2 level, at the time of CO2 doubling an d tripling. The annual cycle of temperature and precipitation has been studied over the IPCC regions and has been compared with observations . Additionally the combination of temperature and precipitation change has been analysed. Further parameters investigated include the differ ence between daily minimum and maximum temperature, the rainfall inten sity and the length of droughts. While the regional simulation of the annual cycle of the near surface temperature is quite realistic with d eviations rarely exceeding 3 K, the precipitation is reproduced to a m uch smaller degree of accuracy. The changes in temperature at the time of CO2 doubling amount to only 30-40% of those at the 3 CO2 level a nd show hardly any seasonal variation, contrary to the 3 CO2 experim ent. The comparatively small response to the CO2 doubling can be attri buted to the cold-start of the simulation, from which the SST has been extracted. The strong change in the seasonality cannot be explained b y internal fluctuations and cold start alone, but has to be caused by feedback mechanisms. Due to the delay in warming caused by the transie nt experiment, from which the SST has been derived, the 3 CO2 experi ment can be compared to the CO2 doubling studies performed with mixed- layer models. The precipitation change does not display a clear signal . However, an increase of the rain intensity and of longer dry periods is simulated in many regions of the globe. The changes in these param eters as well as the combination of temperature- and precipitation cha nge and the changes in the daily temperature range give valuable hints , in which regions observational studies should be intensified and und er which aspects the observational data should be evaluated.