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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

Queensland braces for Tropical Cyclone Debbie

12:37 EDT

Tropical Cyclone Debbie is strengthening quickly on its journey to make landfall, likely between Lucinda and Proserpine on Monday or Tuesday.

Cyclone Debbie could hit north Queensland as category five, Bureau of Meteorology warns

12:09 EDT

A cyclone off northern Queensland could intensify to category five by the time it crosses the coast somewhere between Cardwell and Bowen, near Townsville, the Bureau of Meteorology (BOM) has warned.

Category four cyclone possible for north Queensland as tropical low strengthens: BOM

11:00 EDT

A tropical low brewing 600 kilometres north-east of the Queensland coast could make landfall somewhere between Cairns and Townsville early next week as a category four cyclone or higher, the Bureau of Meteorology (BOM) says.