A General Circulation Model Study of January Climate Anomaly Patterns Associated with Interannual Variation of Equatorial Pacific Sea Surface Temperatures

Abstract

A general circulation model has been run in the perpetual January mode to produce several long-term simulations, each distinguished by a different imposed equatorial Pacific sea surface temperature. From each of them simulations we have extracted an eight-member ensemble of 90-day averaged fields. Ensemble-mean difference maps are presented in this paper, together with an estimate of the statistical significance of features which appear in thee maps. These results are compared with observational studies in the literature that present difference maps of Northern Hemisphere winter fields composited according to some index related to the two extremes of equatorial Pacific sea-surface temperature variation. The results show many anomaly patterns of high statistical significance that are also in good agreement with those observed. In the tropics, them include 990 mb wind, sea level pressure and rainfall anomalies constituting the Southern Oscillation, as well as a 200 mb height anomaly at all longitudes. In extratropical latitudes there is a 500 mb height anomaly which agrees closely with the observed Pacific/North American (PNA) pattern. Other manifestations of the simulated PNA pattern which are in good agreement with observations are anomalies of height, zonal wind and temperature all at 700 mb. The model also reproduces large anomalies that are observed in the 10 mb height and zonal wind fields in the polar stratosphere.