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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

Clear signs of drought in western NSW

21:05 EST

The prospect of El Nino forming during the second half of 2018 is troubling for parched western NSW, where some places are experiencing their driest year to date in more than five decades.

Young farmers taught 'drought driving' skills as country roads plagued by millions of 'roos

14:26 EST

An initiative by Hay Inc in the New South Wales Riverina town of Hay is teaching young farmers life-saving 'drought driving' lessons on how to drive when faced with kangaroos on the road.

BOM declares a 50 per cent chance of El Nino this spring, possible dry and warm conditions ahead

13:31 EST

Warm and dry — that has been the story of the season, and now there is a good chance that is set to continue for south-east Australia.