Fairfax Media Network

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.

Display Your Local Weather

Weather News

Drought bites hard and drought maps questioned

17:46 EST

The accuracy of the current official figure of 66 per cent of Queensland being declared as in drought is being challenged.

Severe storm warning for south-east Queensland cancelled after series of cells sweep through region

16:36 EST

Severe thunderstorm warnings for south-east Queensland have been cancelled after a series of small cells swept through the region.

NSW weather: Record-breaking 40-degree temperature expected this weekend

16:24 EST

Total fire bans have been implemented across half of the state with some parts expected to hit 40 degrees Celsius for the first time in September since records began.