The Southern Oscillation Index

THE SOUTHERN OSCILLATION INDEX (SOI) is calculated using the pressure difference between Tahiti and Darwin. The SOI gives an indication of the stage of El Niño or La Niña events in the Pacific Ocean. The accuracy of the SOI in determining the three-month outlook for rain around Australia is highest between midwinter and midsummer in eastern and northern Australia. A strongly negative SOI (below -10) is characteristic of El Niño, which is often associated with below average rainfall over eastern Australia, and a weaker than normal monsoon in the north. A strongly positive SOI (above +10) is characteristic of La Niña, which is often associated with above average rainfall over parts of tropical and eastern Australia, and an earlier than normal start to the northern monsoon season. Another important term to understand is the Walker Circulation.

Walker Circulation

This is the flow of air across the Pacific Ocean, forming a circulation in a vertical plane at the equator. The trade winds are the clearly identified part of that circulation at the surface. Aloft, the rising air is made visible on satellite photos by the clouds which form when air rises. Where air sinks at the opposite end of the circulation, the skies are cloud-free. See also El Niño and the Walker Circulation.