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.
Adelaide residents are preparing for heavy rain, high seas and strong winds forecast to hit later this week, in what the SES says could be one of the most significant weather events it has seen in South Australia.
Forbes has been declared a natural disaster zone after the region was hit by flooding over the weekend.
Further rain expected in the central west of New South Wales could see the Lachlan River at Forbes reach a higher peak, despite it already being inundated with the greatest water levels in 25 years.