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 Niņa.
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 Niņo.
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.
Three adults and two young children have been rescued by helicopter after becoming trapped in floodwaters in Huonville, south-east of Tasmania.
Parts of New South Wales have been warned to prepare for wild weather overnight as a low-pressure system from Tasmania gets set to batter northern parts of the state.
Power may not be restored to Eyre Peninsula homes until the early hours of the morning after twin tornadoes and wild weather caused significant damage to transmission towers in South Australia.