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

SA storms: Adelaide residents spend night in darkness during state-wide blackout

10:01 EST

It was a surreal night for many Adelaide residents who had to find alternative ways to entertain themselves by candlelight, after wild weather caused a state-wide blackout.

SA weather: Adelaide power mostly restored but BOM forecasts more storms

10:00 EST

About 70,000 people in South Australia are still without power after wild weather plunged the state into darkness, although the lights are back on in Adelaide's metropolitan area.

SA weather: No link between blackout and renewable energy, experts say

09:58 EST

Linking the statewide blackout in South Australia with the state's heavy reliance on renewable energy is unfounded, energy industry experts say.