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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

Second surge of heat could challenge records

10:33 EDT

Another round of intense heat is about to sweep across southern and eastern Australia, challenging December heat records for the second time in a week.

Sydney weather: Temperatures hit 40C in western Sydney ahead of late cool change

18:43 EDT

Demand for ambulances has risen by 40 per cent across the Sydney metro area today, with temperatures in the western suburbs hitting 40 degrees Celsius.

How hot does it get in Australia?

14:43 EDT

With summer underway in Australia and heatwave conditions affecting a number of states this week, a lot of temperature observations are being compared to historical records.