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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

Another bout of heat for Perth

16:16 EDT

Another bout of heat for Perth Perth has finally cooled down to about average summer levels over the last few days, and is on track to heat up to a balmy 32 degrees today.

Heat and powerful surf on the way for coastal southeast QLD

12:55 EDT

After a slight dip in temperatures yesterday, warmer days are on the way from the Coolangatta area to the Sunshine Coast region.

Heat put on hold in the southeast

14:24 EDT

Temperatures will cool down for much of the week for South Australia, Victoria and New South Wales.