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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

Sydney weather: Temperatures forecast to push past 40C in western parts of the city

11:05 EDT

Much of NSW will swelter through its first heatwave of summer today, with temperatures in Sydney's west expected to soar above 40 degrees, but the city's east should be spared thanks to a sea breeze.

Burst of summer heat prompts fire ban in Sydney

10:00 EDT

Authorities have issued a total fire ban for Sydney today as the city's residents brace for the warmest day so far this season.

What is a heatwave?

15:40 EDT

A large portion of Australia is being affected by heatwave conditions this week.