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

Wet week for the Southwest

15:11 EST

While showers and rain have been lingering over southern regions of Western Australia, there are no signs that clear skies are going to return any time soon.

Wild weather sets its sights on New Zealand

12:48 EST

A deep cold front will move over New Zealand, bringing an intense bout of wind, rain and snow.

Tigerair flight turned back to Melbourne after mid-air lightning strike

23:55 EST

An airline passenger has described her terror and ultimate relief after the plane she was on was hit by lightning and forced to return to Melbourne Airport.