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
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