THE SEASONAL CYCLE OVER THE TROPICAL PACIFIC IN COUPLED OCEAN-ATMOSPHERE GENERAL-CIRCULATION MODELS

The seasonal cycle over the tropical Pacific simulated by 11 coupled o cean-atmosphere general circulation models (GCMs) is examined. Each mo del consists of a high-resolution ocean GCM of either the tropical Pac ific or near-global oceans coupled to a moderate- or high-resolution a tmospheric GCM, without the use of flux correction. The seasonal behav ior of sea surface temperature (SST) and eastern Pacific rainfall is p resented for each model. The results show that current state-of-the-ar t coupled GCMs share important successes and troublesome systematic er rors. All 11 models are able to simulate the mean zonal gradient in SS T at the equator over the central Pacific. The simulated equatorial co ld tongue generally tends to be too strong, too narrow, and extend too far west. SSTs are generally too warm in a broad region west of Peru and in a band near 10 degrees S. This is accompanied in some models by a double intertropical convergence zone (ITCZ) straddling the equator over the eastern Pacific, and in others by an ITCZ that migrates acro ss the equator with the seasons; neither behavior is realistic. There is considerable spread in the simulated seasonal cycles of equatorial SST in the eastern Pacific. Some simulations do capture the annual har monic quite realistically, although the seasonal cold tongue tends to appear prematurely. Others overestimate the amplitude of the semiannua l harmonic. Nonetheless, the results constitute a marked improvement o ver the simulations of only a few years ago when serious climate drift was still widespread and simulated zonal gradients of SST along the e quator were often very weak.