Response of the NCAR General Circulation Model to North Pacific Sea Surface Temperature Anomalies

Abstract

The general circulation model (GCM) of the National Center for Atmospheric Research was used to investigate the effects of anomalous sea surface temperature (SST) patterns in the mid-latitudes of the North Pacific Ocean on atmospheric circulations. One of the most frequently observed SST anomaly (SSTA) patterns, a “cold pool” of water in the west and a “warm pool” in the cast (from 35 to 55°N) with magnitudes ±4°C of the temperature of the surrounding water, was superimposed on the seasonally varying oceanic temperature as the thermal boundary condition in the North Pacific of the atmospheric GCM for the anomaly experiment Three similar experiments, one the control, two the noise and three an exaggerated SSTA, were run simultaneously. Time integrations for all cases were carried out for 120 days, beginning on 15 January. Preliminary results indicate that cyclonic activities are strengthened above the warm SSTA and suppressed above the cold SSTA. A direct thermal circulation is induced in the vertical section: warm air rises and a warm-core low forms above the warm SSTA; cold air sinks and a cold-core high forms above the cold SSTA. The Aleutian low is elongated, shifted northeast and deepened. The planetary wave in midlatitudes shows ridging above the cold SSTA and troughing above the warm SSTA in the lower troposphere. A westward tilting of the trough with increasing altitude above the warm SSTA is also discerned. The westerlies north of the warm SSTA are much strengthened. The westerlies are also stronger in the western as well as eastern North Pacific region, but are much weaker in the central North Pacific region. Some effects on the equatorial circulation due to the existence of midlatitude SSTA's are discussed and a transocean teleconnection is indicated. Certain general features of the experimental results support the hypothesis of Namias (1972). However, they did not conclusively demonstrate the significance of downstream oceanic influences on the weather pattern over a time period longer than one season.