Abstract
A 40-yr integration is conducted using the National Center for Atmospheric Research (NCAR) Community Climate Model Version 2 (CCM2). The simulation was forced by observed monthly global sea surface temperature (SST) changes during 1950–89. The January climates of the model results are presented in the paper. The modeled means and interannual variability are analyzed and compared with observations based on different accounts. Firm, the authors concentrate on the period of 1951–79. The monthly varying SSTs of this period were used to construct the SST climatology 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 SST, is examined. The modeled mean fields do not significantly differ between the two simulations especially for the Northern Hemisphere. The magnitude of interannual variability is enhanced in the current simulation especially for the northern Pacific due to the tropical SST forcing. The authors then concentrate on the remaining part of the simulation-the period from 1979 to 1989. The global climate during this period analyzed by the European Centre for Medium-Range Weather Forecasts (ECMWF) has been widely used for validation purposes by various general circulation model (GCM) studies including the CCM2 simulation mentioned above. The model performance in terms of basic circulation features for the period 1979–89 is actually quite impressive. Some earlier recognized model deficiencies in the above 20-yr simulation are improved simply 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 data of sea level pressure and 700-mb geopotential height analyzed by the National Meteorological Center. The decadal analysis of the model results reveals that the model has different performance for different decades. It is found that the simulated circulations are in better agreement with the observations during warmer decades in terms of the evolution of the El Niño-Southern Oscillation. The analysis of tropical/extratropical teleconnection patterns based on the SST index over the central equatorial Pacific and the Northern Hemisphere 700-mb height shows that the negative correlation between these two fields over the northern Pacific takes place somewhat too far west compared with observations. The net result is that CCM2 tends to produce a ridge of the height field also too far west from the west coast of North America. This deficiency may well be due to an unrealistic beating anomaly associated with condensation processes over the western tropical Pacific as indicated by earlier CCCM2 studies and linear steady-state model results.
