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.
Some homes in the community of Borroloola on the Gulf of Carpentaria have been evacuated with 74 residents sheltering elsewhere, as they wait for Cyclone Alfred to make landfall about sunrise on Tuesday.
Shacks in the fishing village of King Ash Bay in the Northern Territory's Gulf of Carpentaria have been flooded by the McArthur River.
Early today, a tropical low that has been lingering over the Gulf of Carpentaria was upgraded to tropical cyclone.