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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

Flurry of cyclones challenges WA rainfall records

11:19 EDT

Some cyclone-weary parts of Western Australia's north coast have seen more than two years worth of rain in the last two months.

Warnings for NSW beaches and coast amid weekend rescues; 4-metre swells predicted

10:45 EDT

Authorities have issued warnings for dangerous surf conditions, including waves bigger than 4 metres, expected along much of the New South Wales coast today.

Tropical Cyclone Kelvin weakens, but heavy flooding cuts off Great Northern Highway

10:43 EDT

A cyclone which brought damaging winds and heavy rain to northern WA has weakened to a category one system, but there are fears flooding affecting a major supply road could impact food delivery in the region.