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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

Cold weather: Icy morning tipped for NSW as wild winds ease, bureau says

15:43 EST

Icy and windy weather affecting parts of the NSW central-west and Blue Mountains will continue into tomorrow, although strong coastal winds are easing, the Bureau of Meteorology (BoM) says.

Cold front delivers for boardriders

13:47 EST

After a slow start to the snow season and a week of small to no surf, an intense cold front has brought some happiness to snowboarders and surfers in Victoria and New South Wales.

Above average rainfall for WA

13:39 EST

Parts of WA have been receiving persistent showers in recent days, propelling some locations well past their monthly average.