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

December rain more likely as MJO looms

17:10 EDT

After an exceptionally dry November for most of Australia, there are signs that wet weather could ramp up as we head into December under the influence of the Madden-Julian Oscillation.

Queensland BOM forecast predicts fast and furious thunderstorms for south-east

13:25 EDT

Homeowners are being warned to brace for another series of destructive storms with damaging winds, heavy rain and hail this afternoon across south-east Queensland.