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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

Wild weather set to continue after heavy rain strands scores of motorists

20:18 EDT

Victoria's north-east has been hit hard by torrential rain and flash flooding as intense storms sweep across the state, delivering up to a month's worth of rain in a single day and stranding cars on a major freeway.

Tropical Cyclone Owen reaches category three on NT coast, may develop to category four today

18:50 EDT

Severe Tropical Cyclone Owen is tracking slowly along the Northern Territory's north-east coast, and is expected to batter a number of tiny communities and islands as it moves east towards Queensland.

Storm band to bring rain, wind across northern Tasmania

16:24 EDT

Tasmanians in the state's north are being urged to be ready for heavy rain and strong winds tonight and tomorrow, with a father and son spending an hour in the ocean clinging to their upturned boat off the north-east coast this morning.

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