CLIMATE VARIABILITY IN A COUPLED GCM .1. THE TROPICAL PACIFIC

A 26-year integration has been performed with a coupled ocean-atmosphe re general circulation model (CGCM). The oceanic part resolves all thr ee oceans in the latitude band 70-degrees-N-70-degrees-S but is dynami cally active only between 30-degrees-N and 30-degrees-S. The atmospher e is represented by a global low-order spectral model. The coupled mod el was forced by seasonally varying insolation. Although the simulated time-averaged mean conditions in both atmosphere and ocean show signi ficant deviations from the observed climatology, the CGCM realisticall y simulates the interannual variability in the tropical Pacific. In pa rticular, the CGCM simulates an irregular ENSO with a preferred time s cale of about 3 years. The mechanism for the simulated interannual var iability in the tropical Pacific is related to both the ''delayed acti on oscillator'' and the ''slow SST mode.'' It therefore appears likely that either both modes can coexist or they degenerate to one mode wit hin certain locations of the parameter space. This hypothesis is also supported by calculations performed with simplified coupled models, in which the atmospheric GCM was replaced by linear steady-state atmosph ere models. Further, evidence is found for an eastward migration of zo nal wind anomalies over the western Pacific prior to the extremes of t he simulated ENSO, indicating a link to circulation systems over Asia. Because an earlier version of the CGCM did not simulate interannual v ariability in the tropical Pacific, additional experiments with a simp lified coupled model have been conducted to study the sensitivity of c oupled systems to varying mean oceanic background conditions. It is sh own that even modest changes in the background conditions can push the coupled system from one flow regime into another.