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Southern Oscillation Index

The Southern Oscillation Index (SOI) is calculated from the monthly or seasonal fluctuations in the air pressure difference between Tahiti and Darwin.

A strongly and consistently positive SOI pattern (e.g. consistently above about +6 over a two month period) is related to a high probability of above the long-term average (median) rainfall for many areas of Australia, especially areas of eastern Australia (including northern Tasmania) - La Niña.

Conversely, a 'deep' and consistently negative SOI pattern (less than about minus 6 over a two month period, with little change over that period) is related to a high probability of below median rainfall for many areas of Australia at certain times of the year - El Niño.

However, it is important to remember that the pattern of relationship between SOI and rainfall (and temperature) can vary depending on the particular season and region. Additionally, the change in SOI over a specified period can be as important in understanding relationships between SOI and rainfall as is the absolute value in SOI.

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Weather News

Showers ahead for eastern NSW but remaining dry in the west

10:57 EST

Showers and thunderstorms will affect some northern and eastern parts of NSW during the next week, although there's little rain in sight for the state's parched west.

Hospitalisations increase and asphalt hits 60 degrees without Darwin heat mitigation strategy

10:22 EST

Unmitigated heat during hot Darwin days is driving up energy consumption and threatening lives, according to research by the University of New South Wales.

Growers, agronomists and analysts say sector has more ups and downs than a game of snakes and ladders

06:14 EST

It is not too often a board game and the grain sector go hand in hand, but as growers wait to plant a summer crop the season is being likened to "snakes and ladders".