Southern Oscillation

The Southern Oscillation is a seesaw of air pressure on the eastern and western parts of the Pacific Ocean. In normal situations, there is a dominate low pressure system above the western equatorial Pacific Ocean and a high pressure system dominating the eastern equatorial Pacific. Winds blow from higher to lower pressure due to the pressure gradient force, so the winds flow from east to west in normal conditions near the equator. These pressure systems are coupled; when pressure rises in the east, it falls in the west.

Sea Level Pressure 1960-1984

sea level pressure from 1960 to 1984
This image is from the U. S. Army Topographic Engineering Center

Measurements are taken at Darwin, Australia, and at Tahiti, and the difference between these two measurements is called the Southern Oscillation index. This difference provides a good indication of whether or not there is a good possibility of El Niño occurring. Since 1985, moored instruments placed across the tropical Pacific ocean, known as the TAO array have been monitoring ocean conditions.

Last reviewed: June 2001
Arlene Guest