Interannual variability of circulation and climate in the tropical Pacific and Australasia related to the Southern Oscillation

The Southern Oscillation (SO) in Australasia and the western Pacific is studied for the January/February peak of the boreal winter monsoon. The high/cold SO phase is defined by anomalously high pressure at Tahiti and low pressure at Darwin and cold sea surface temperature (SST) anomalies in the equatorial Pacific. Composite surface analyses of contrasting ten-year ensembles during 1948-92 are complemented by case studies of the upper-air circulation during the high/cold SO year 1989 and the low/warm 1992. During the low/warm as compared to the high/cold SO phase, the Australasian low is weak, but the core of positive pressure departure is located somewhat farther North. Over the equatorial Pacific a weakened zonal pressure gradient entails slower easterlies which lead to anomalously warm surface waters. These maintain lower pressure, and enhanced cloudiness, convection, and rainfall over the central equatorial Pacific, where an increase in convective activity is accompanied by a statistically significant northeastward shift of the South Pacific Convergence Zone, enhanced lower-tropospheric convergence, upward motion, and upper-tropospheric divergence. The divergent component of the wind at 200 nib is directed primarily northward into a broad band of upper-tropospheric convergence, which feeds subsidence, lower-tropospheric divergence, and clear skies, and contributes to positive surface pressure departures in the western North Pacific.