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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

Perth's summer set to be a hot one according to the Bureau of Meteorology summer forecast

17:57 EDT

A spring scorcher on Friday of 35 degrees Celsius is shaping up as a taste of things to come, with a hot summer heading our way — in stark contrast to the unusually cool summer of 2017-18.

BOM forecast wet start to summer forecast for NSW, but not drought-breaking

15:48 EDT

There is some good news for New South Wales farmers struggling with drought — the Bureau of Meteorology (BOM) is predicting a wet start to summer.

Australia's summer outlook

15:29 EDT

A dry and hot summer is anticipated for much, but not all of Australia, according to the latest seasonal outlook issued by the Bureau of Meteorology on Thursday.