CLIMATE SIMULATIONS WITH NCAR CCM2 FORCED BY GLOBAL SEA-SURFACE TEMPERATURE, 1950-89

A 40-yr integration is conducted using the National Center for Atmosph eric Research (NCAR) Community Climate Model Version 2 (CCM2). The sim ulation was forced by observed monthly global sea surface temperature (SST) changes during 1950-89. The January climates of the model result s are presented in the paper. The modeled means and interannual variab ility are analyzed and compared with observations based on different a ccounts. First, the authors concentrate on the period of 1951-79. The monthly varying SSTs of this period were used to construct the SST cli matology for an earlier 20-yr simulation conducted by NCAR researchers . The difference of the model climatology between the two simulations, respectively, forced by monthly varying SST and annually repeating SS T, is examined. The modeled mean fields do not significantly differ be tween the two simulations especially for the Northern Hemisphere. The magnitude of interannual variability is enhanced in the current simula tion especially for the northern Pacific due to the tropical SST forci ng. The authors then concentrate on the remaining part of the simulati on-the period from 1979 to 1989. The global climate during this period analyzed by the European Centre for Medium-Range Weather Forecasts (E CMWF) has been widely used for validation purposes by various general circulation model (GCM) studies including the CCM2 simulation mentione d above. The model performance in terms of basic circulation features for the period 1979-89 is actually quite impressive. Some earlier reco gnized model deficiencies in the above 20-yr simulation are improved s imply because they were identified based upon mismatched time periods between the ECMWF analysis and the model simulation. The model results of the entire simulation are finally compared with the multidecadal d ata of sea level pressure and 700-mb geopotential height analyzed by t he National Meteorological Center. The decadal analysis of the model r esults reveals that the model has different performance for different decades. It is found that the simulated circulations are in better agr eement with the observations during warmer decades in terms of the evo lution of the El Nino-Southern Oscillation. The analysis of tropical/e xtratropical teleconnection patterns based on the SST index over the c entral equatorial Pacific and the Northern Hemisphere 700-mb height sh ows that the negative correlation between these two fields over the no rthern Pacific takes place somewhat too far west compared with observa tions. The net result is that CCM2 tends to produce a ridge of the hei ght field also too far west from the west coast of North America. This deficiency may well be due to an unrealistic heating anomaly associat ed with condensation processes over the western tropical Pacific as in dicated by earlier CCM2 studies and linear steady-state model results.