Simulation of the Tropical-Pacific Climate with a Coupled Ocean-Atmosphere General Circulation Model. Part I: The Seasonal Cycle

A.W. Robertson, C.-C. Ma, C.R. Mechoso and M. Ghil

A multi-year simulation with a coupled ocean-atmosphere general circulation mode l (GCM) is presented. The model consists of the UCLA global atmospheric GCM coup led to the GFDL oceanic GCM; the latter is dynamically active over the tropical Pacific, while climatological time-varying sea surface temperatures (SST) are pr escribed elsewhere. The model successfully simulates the main climatological fea tures associated with the seasonal cycle including the east-west gradient in SST across the equatorial Pacific. The most apparent deficiencies include a systema tic cold bias (~2K) across most of the tropical Pacific, and underestimated wind stress magnitudes in the equatorial band. Multi-channel singular spectrum analy sis is used to describe the multivariate structure of the seasonal cycle at the equator, in both the model and observed data. The annual harmonic in equatorial SST is primarily wind driven, whilst air-sea interaction strongly affects the se mi-annual harmonic.