Fairfax Media Network

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.

Display Your Local Weather

Weather News

Cyclone Marcus: Surfers gear up for rare super swell along WA's south west coast

11:13 EDT

Western Australian surfers are getting excited about a rare super swell event for Perth and the South West this weekend, fuelled by powerful Tropical Cyclone Marcus.

Tropical Cyclone Marcus clean-up continues, residents remains without power

10:38 EDT

Almost half the Darwin residents who lost power during Tropical Cyclone Marcus have been reconnected, but as clean-up efforts continue, the Bureau of Meteorology (BOM) has warned the Top End could see another cyclone this week.

NSW rain and flooding update

10:22 EDT

More than 300mm of rain has fallen in parts of eastern NSW during the last two days, resulting in flash flooding.