The COLA global coupled and anomaly coupled ocean-atmosphere GCM

The Center for Ocean-Land-Atmosphere Studies (COLA) global coupled and anomaly coupled ocean-atmosphere GCM models are described. The ocean and atmosphere components of these coupled models are identical. The only difference between them is in the coupling strategy. The anomaly coupling strategy guarantees that the climatology of the coupled model is close to the observed climatology, whereas the global coupled model suffers from serious climate drift. This climate drift is largest in the eastern tropical Pacific where the coupled model is too warm by as much as 5degreesC. The climate drift in the coupled model can also be seen by the predominance of an erroneous double intertopical convergence zone (ITCZ) in the eastern tropical Pacific. Despite the climate drift, both models exhibit robust interannual variability in the tropical Pacific. Composite analysis, however, reveals that the characteristics of interannual variability in the coupled and the anomaly coupled models are markedly different. For example, the coupled model exhibits a distinct eastward migration of the ENSO events, whereas the anomaly coupled model is dominated by a standing mode, which is too strongly phase-locked to the annual cycle. Based on diagnostic ocean model simulations, it is shown that an erroneous eastward migration of the annual cycle of thermocline depth and upwelling is responsible for the eastward migration of the ENSO events in the coupled model. The anomaly coupled model has a comparatively weak annual cycle in the thermocline depth and upwelling. These calculations emphasize the importance of correctly simulating the mean state in order to capture realistic ENSO variability.