Comparison of the Impact of the 1982/83 and 1986/87 Pacific SST Anomalies on Time-Mean Predictions of Atmospheric Circulation

The primary focus of this study is to contrast the impact of the El Niño Pacific sea surface temperature (SST) anomalies observed during the Northern Hemisphere winters of 1982/83 and 1986/87 on predictions with a global general circulation model (GCM). The former event was an El Niño of record magnitude while the latter was of more typical magnitude. For each year, three 60-day control integrations with climatological boundary conditions and three 60-day boundary integrations with the observed SST only in the Pacific were performed with the Goddard Laboratory for Atmospheric Sciences GCM. Among the observed tropical features correctly simulated in both cases are the large positive precipitation anomalies in the Pacific, the Southern Oscillation signal in the sea-level pressure field, and the upper-level easterly wind anomalies. However, the overall simulation of the observed tropical circulation anomalies is considerably better in 1982/83 than in 1987. Some extratropical features are also well simulated, although neither their accuracy nor the differences between the two years are as clear as in the tropics. An analysis of the SST impact on model forecast skill reveals that the impact is mainly on 60-day time means, rather than shorter means. The results of an analysis of the anomalous divergence and effective Rossby wave source in the tropical and subtropical Pacific during these two years suggest that the primary observed and simulated extratropical circulation features may have been forced from the subtropics rather than the tropics. However, the relatively small magnitudes of the SST and precipitation anomalies in the subtropics make it likely that these subtropical divergence and Rossby wave source anomalies were themselves forced from the tropics.