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.
Icy and windy weather affecting parts of the NSW central-west and Blue Mountains will continue into tomorrow, although strong coastal winds are easing, the Bureau of Meteorology (BoM) says.
After a slow start to the snow season and a week of small to no surf, an intense cold front has brought some happiness to snowboarders and surfers in Victoria and New South Wales.
Parts of WA have been receiving persistent showers in recent days, propelling some locations well past their monthly average.