The Southern Oscillation Index (SOI)

The Southern Oscillation Index (SOI) is a measure of the intensity or strength of the Walker Circulation. It is one of the key atmospheric indices for gauging the strength of El Niño and La Niña events and their potential impacts on the Australian region.

The SOI measures the difference in surface air pressure between Tahiti and Darwin. The index is best represented by monthly (or longer) averages as daily or weekly SOI values can fluctuate markedly due to short-lived, day-to-day weather patterns, particularly if a tropical cyclone is present.

Sustained positive SOI values above about +8 indicate a La Niña event while sustained negative values below about –8 indicate an El Niño.

La Niña events over time

As La Niña events recur on a two to seven-year cycle, there have been many over the last century, varying in strength and impacts. The SOI and sea surface temperatures can be used to compare the intensity of La Niña events. (See graph below for more details.)

Atmospheric and oceanic intensity of La Niña events since 1900. Intensity ranked by SOI values for atmosphere, while oceanic intensity is ranked by sea surface temperature indicators (only available reliably since mid-century). Some multi-year events have two or three La Niña peaks.