A Comparison of the Atmospheric Circulations Simulated by the CCM3 and CSM1*

The atmospheric state simulated by the National Center for Atmospheric Research (NCAR) Community Climate Model, version 3 (CCM3), is compared to that simulated by the NCAR Climate System Model, version 1 (CSM1). CCM3 is an atmospheric general circulation model that uses specified sea surface temperatures (SSTs) for a lower boundary condition. Observed monthly mean SSTs for 1979–93 were used in the present study. CSM1 is a coupled general circulation model in which the SSTs are determined as part of the simulation and CCM3 serves as the atmospheric component. It is found that the differences between CCM3 and CSM1 are quite small in most measures of the atmospheric circulation, consistent with the accurate and drift-free simulation of the SSTs in the coupled model. There are substantial temperature differences near the surface in the Arctic and over the ocean around Antarctica, resulting from different sea-ice distributions. The tropical precipitation also has significant differences, although neither simulation is clearly better and the errors in the two simulations tend to have opposite signs with respect to observations. In response to the change in latent heat release the tropical divergent circulation changes significantly. Middleand high-latitude circulation changes are modest, occurring mostly in winter in association with the sea-ice changes.