Southern Oscillation Index [SOI index]

Teleconnective indices: Southern Oscillation Index [SOI index]

The Southern Oscillation Index (SOI) is an index that is based on the difference in air pressure between Darwin (Australia) and Tahiti. The SOI is closely related to the El Niño/La Niña climate phenomenon.
A consistently negative Southern Oscillation (SOI-) often indicates the presence of an El Niño. An El Nino is defined as a prolonged warming of the eastern tropical Pacific Ocean, which leads to a weakening or even a reversal in the trade winds across the Pacific Ocean. Subsequently, this tends to reduce rainfall in eastern and northern Australia. Major El Nino events in the past have included 1997/98 and 1982/83.
A consistently positive Southern Oscillation (SOI+) often indicates the presence of a La Niña. A La Nina occurs when sea surface temperatures to the north of Australia are warmer than normal and trade winds across the Pacific Ocean strengthen. This is the opposite of an El Nino, and subsequently, the results are also opposite. Rainfall probabilities across eastern and northern Australia are higher than normal. Recent strong La Nina events have included 1998/99 and 1988/89.