SIMULATING THE ATMOSPHERIC RESPONSE TO THE 1985-87 EL-NINO CYCLE

The atmospheric response to the evolution of global sea surface temperatures (SSTs) from 1985 to 1987 is studied using the NCAR Community Climate Model (CCM1). Five separate 2-year integrations are performed, and results are presented for the ensemble-averaged response during the pre-El Nino 1985/86 winter and the mature El Nino 1986/87 winter. No skill is found in CCM 1's simulation for 1985/86. The simulation for the following winter, when tropical Pacific SST anomalies approached 2-degrees-C, is more successful. A large-amplitude wave train extends poleward and eastward from the location of anomalous central Pacific convection in CCM 1, although the model's wave train is shifted 30-degrees too far east compared to observations. A linear barotropic stationary wave model is used to diagnose CCM 1's response during 1986/87. The eastward-shifted PNA response is easily excited by a dipole pattern of upper-level forcing with convergence over the western tropical Pacific and divergence over the central tropical Pacific. In contrast, the observed anomaly pattern during 1986/87 is most effectively forced by anomalous subtropical convergence over the central Pacific. Zonally asymmetric features of CCM 1's climate drift, in particular the pattern of zonal wind biases over the tropical and midlatitude North Pacific, are shown to account for these different sensitivites.