Response of the Atmosphere to Sea Surface Temperature Anomalies over the Equatorial Pacific and the Teleconnections of the Southern Oscillation

Abstract

In order to improve our understanding of the role of equatorial Pacific sea surface temperature (SST) anomalies in the Southern Oscillation and the variability of the Walker Circulation, we have conducted sensitivity studies with warm SSTA's over different parts of the equatorial Pacific using a general circulation model of GFDL. The model response is compared with the variability of the 15-year control run and the statistical significance of the response is assessed. The anomalies produce dominant changes in the vertical circulation in the equatorial longitude-height plane. Besides the large local increase of precipitation near the anomalies and a slight reduction to the west, there are also global-scale anomalies in the equatorial zonal winds (notably in the upper troposphere) and in tropical sea level pressure. The Pacific Walker Circulation is significantly altered. In the case of eastern and central Pacific anomalies these features resemble the observed circulation anomalies during the low index phase of the Southern Oscillation. Thus warm SST anomalies over equatorial eastern and central Pacific are shown to act in such a way as to reinforce the low index phase of the Southern Oscillation. The model response of the low-level zonal winds is such that the maximum westerly anomalies occur to the west of the SST anomaly. This would induce the anomalies to shift westwards. We also propose a self-generating mechanism for the Southern Oscillation. SST anomalies over equatorial central and western Pacific are found to be more efficient in producing atmospheric circulation anomalies compared to equal ones over eastern Pacific. The details of the response are also longitude dependent. The anomalies also produce well-marked subtropical and middle latitude responses, notably in the winter hemisphere. The subtropical anticyclones and the subtropical westerly jet streams in the upper troposphere are strengthened in the longitudes of the anomalies whereas the westerlies in the subpolar latitudes are weakened. In other longitudes there is a weakening of the subtropical anticyclones. The precipitation also increases southeastward and northeastward from the anomaly. There is also a stationary anomaly low (in the middle latitudes) to the southeast of the SST anomaly and an anomaly high further southeast. The diabatic heating anomalies associated with the central and western Pacific anomalies induce circulation changes over south Asia similar to those observed during drought years. The central Pacific anomaly also produces westerly anomalies in the tropical stratosphere, around the globe.