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

Quick change for Canberra

14:37 EST

After an almost summer like day yesterday, Canberra is feeling the cold today, with conditions set to get even colder next week.

Cold and dry winds for Sydney

12:10 EST

A strong front will introduce a much colder and drier airmass across the Harbour City on Monday.

Chilly mornings on the horizon for Brisbane

15:55 EST

The mercury is forecast drop into single figures by Friday morning in Queensland's capital, as cold and very dry air migrates across the region from the southwest.