Annual Cycle and ENSO in a Coupled Ocean–Atmosphere General Circulation Model

Results from multiyear integrations of a coupled ocean–atmosphere general circulation model are described. The atmospheric component is a rhomboidal 15, 18-level version of the Center for Ocean–Land–Atmosphere Studies atmospheric general circulation model. The oceanic component is the Geophysical Fluid Dynamics Laboratory ocean model with a horizontal domain extending from 708S to 658N. The ocean model uses 1.58 horizontal resolution, with meridional resolution increasing to 0.58 near the equator, and 20 vertical levels, most in the upper 300 m. No flux adjustments are employed. An initial multiyear integration showed significant climate drift in the tropical Pacific sea surface temperatures. Several modifications were made in the coupled model to reduce these errors. Changes were made to the atmospheric model cloudiness parameterizations, increasing solar radiation at the surface in the western equatorial Pacific and decreasing it in the eastern Pacific, that improved the simulation of the time-mean sea surface temperature. Large errors in the wind direction near the western coast of South America resulted in large mean SST errors in that region. A procedure to reduce these errors by extrapolating wind stress values away from the coast to coastal points was devised and implemented. Results from the last 17 years of a 62-yr simulation are described. The model produces a reasonably realistic annual cycle of equatorial Pacific sea surface temperature. However, the upper-ocean thermal structure has serious errors. Interannual variability for tropical Pacific sea surface temperatures, precipitation, and sea level pressure that resemble the observed El Niño–Southern Oscillation (ENSO) in structure and evolution is found. However, differences from observed behavior are also evident. The mechanism responsible for the interannual variability 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 tropical Pacific sea surface temperature interannual variability are isolated and described. The coupled model is capable of producing structures that are similar to those observed. It is concluded that atmosphere–ocean general circulation models are beginning to capture some of the observed characteristics of the climatology of the tropical Pacific and the interannual variability associated with the El Niño–Southern Oscillation. Remaining obstacles to realistic simulations appear to include ocean model errors in the eastern equatorial Pacific, errors associated with cloud–radiation interactions, and perhaps errors associated with inadequate meridional resolution in the atmospheric model equatorial Pacific.