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.
It's looking more likely that a tropical cyclone will cross the Queensland coast in the coming days.
A slow-moving tropical low has drenched the WA mining town of Port Hedland, which has received more than 270 millimetres of rain this week.
Authorities are preparing for a potential cyclone that could hit the north Queensland coast in coming days, as the Bureau of Meteorology (BOM) tracks a tropical low in the Coral Sea that is expected to strengthen.