ANNUAL CYCLE AND ENSO IN A COUPLED OCEAN-ATMOSPHERE GENERAL-CIRCULATION MODEL

Results from multiyear integrations of a coupled ocean-atmosphere gene ral circulation model are described. The atmospheric component is a rh omboidal 15, 18-level version of the Center for Ocean-Land-Atmosphere Studies atmospheric general circulation model. The oceanic component i s the Geophysical Fluid Dynamics Laboratory ocean model with a horizon tal domain extending from 70 degrees S to 65 degrees N. The ocean mode l uses 1.5 degrees horizontal resolution, with meridional resolution i ncreasing to 0.5 degrees near the equator, and 20 vertical levels, mos t in the upper 300 m. No flux adjustments are employed. An initial mul tiyear integration showed significant climate drift in the tropical Pa cific sea surface temperatures, Several modifications were made in the coupled model to reduce these errors, Changes were made to the atmosp heric model cloudiness parameterizations, increasing solar radiation a t the surface in the western equatorial Pacific and decreasing it in t he eastern Pacific, that improved the simulation of tile time-mean sea surface temperature. Large errors in the wind direction near the west ern coast of South America resulted in large mean SST errors in that r egion, A procedure to reduce these errors by extrapolating wind stress values away from the coast to coastal points was devised and implemen ted, Results from the last 17 years of a 62-yr simulation are describe d. The model produces a reasonably realistic annual cycle of equatoria l Pacific sea surface temperature. However, the upper-ocean thermal st ructure has serious errors. Interannual variability for tropical Pacif ic sea surface temperatures, precipitation, and sea level pressure tha t resemble the observed El Nino-Southern Oscillation (ENSO) in structu re and evolution is found. However, differences from observed behavior are also evident. The mechanism responsible for the interannual varia bility appears to be similar to the delayed oscillator mechanism that occurs in the real climate system. The structure of precipitation, sea level pressure, and geopotential anomalies associated with the tropic al Pacific sea surface temperature interannual variability are isolate d and described. The coupled model is capable of producing structures that are similar to those observed. It is concluded that atmosphere-oc ean general circulation models are beginning to capture some of the ob served characteristics of the climatology of the tropical Pacific and the interannual variability associated with the El Nino-Southern Oscil lation. Remaining obstacles to realistic simulations appear interactio ns, and perhaps errors associated with inadequate meridional resolutio n in the atmospheric model equatorial Pacific.