Use of Baltic Sea modelling to investigate the water cycle and the heat balance in GCM and regional climate models

Results from the first simulations with the Rossby Centre regional climate atmosphere (RCA) model were used to force 2 versions of process-oriented mo dels of the Baltic Sea-one time-dependent, the other considering the mean s tate. The purpose was primarily to obtain a first scenario of the future st ate of the Baltic Sea. In addition, we looked at this exercise as a method to evaluate the consistency of the water cycle and the heat balance produce d by atmospheric climate models. The RCA model is a high-resolution atmosph eric regional model which is forced with lateral conditions from a global m odel. A large-scale Baltic drainage basin hydrological model, forced by the RCA model, was used to simulate river runoff. Using RCA model data from th e control run we found that that the temperature and ice conditions in the Baltic Sea were reasonably realistic while the salinity field was poorly re produced. We conclude that the modelling of the water cycle needs considera ble improvement. We also conclude that the time for the Baltic Sea to respo nd to the water cycle is much longer than the integration period so far use d with the RCA model. Forcing the ocean models with RCA model data from a f uture scenario with an enhanced greenhouse effect gives an increased sea-su rface temperature and a much reduced extent of ice in the Baltic Sea due to climate warming. Also the salinity is reduced, which implies possible seri ous effects on the future marine life in the Baltic Sea. The results demons trate that accurate atmospheric modelling of not only the heat balance but also the water cycle is crucial for Baltic Sea climate simulations.