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

Handy rain dampening a dry South Australia

16:37 EST

Parts of South Australia's south have received their first decent rain in months, a welcome dampener to a parched state despite it falling on a weekend.

Mother's Day weather forecast

12:53 EST

Nobody wants a rainy picnic! What will the weather around Australian capitals hold for the celebration day of Mothers this Sunday? 24 hour rainfall expected to fall on Mother’s Day: troughs bringing inland precipitation and a cold front over the Southern Ocean approaching southern states.  Starting in the east, Brisbane, Sydney, Canberra, as well as Hobart are in for a partly cloudy day with generally dry and warm conditions.

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