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

The week ahead for NSW

15:11 EST

A cold front is passing over southern parts of New South Wales, with cold air in its wake and unusually warm air ahead of the system over central and northern areas.

Dry winds elevate the risk of fire in the Top End

13:07 EST

With strong winds and temperatures on the rise, there has been severe fire danger declared for the Darwin and Adelaide River and Northern Fire Protection Area fire forecast areas.

Country towns close to reaching 'day zero', as water supplies dry up in the drought

12:19 EST

Across New South Wales and Queensland's southern downs, country towns are approaching their own 'day zero', as water supplies dry up in the drought.