The Effect of North Pacific Sea Surface Temperature Anomalies on the January Climate of a General Circulation Model

Abstract

Four perpetual January integrations of an atmospheric general circulation model have been performed, in each of which a different sea surface temperature (SST) anomaly was specified in the North Pacific. The observed SST anomaly for the 1976/77 winter was chosen as the basic anomaly, and 1200-day runs were carded out in which this anomaly was multiplied by ±1 and ±2. A fifth run was performed which combined the basic midlatitude SST anomaly from 1976/77 with a tropical Pacific SST anomaly representative of the mature phase of a warm El Niño/Southern Oscillation (ENSO) episode. An ensemble of eight, independent 90-day averaged realizations was extracted from each simulation. Maps of ensemble-mean differences from the model climatology are presented in this paper, together with estimates of the statistical significance of some of the features which appear on these maps. The model response to the basic SST anomaly and to twice the basic SST anomaly is a midiatitude teleconnection pattern, the Pacific/North American (PNA) pattern, which has been found in previous experiments which used tropical Pacific SST anomalies. The amplitude of the model response increases at a slower than linear rate as the magnitude of the SST anomaly is increased. The model response to the basic midlatitude SST anomaly is compared with the model response to tropical Pacific SST anomalies. When the basic midlatitude anomaly is combined with a tropical Pacific SST anomaly, such as commonly occurs during the mature phase of warm ENSO episodes, we find that the model response to the combined SST anomalies is approximately equal to the sum of the model responses produced by the SST anomalies acting separately. The model response to the basic SST anomaly times ?1 and times ?2 is not a previously described teleconnection pattern. Over the North Pacific, the model response in the upper troposphere is weak, but below 700 mb. the response in heights and temperatures is the opposite of that produced for SST anomalies of the opposite sign. There is also a positive anomalous zonal wind over the southern United States and a negative height anomaly over the eastern United States.