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.
After a few cold mornings, the cold days have now started in the ACT.
Snow has fallen across large parts of NSW, including the Blue Mountains, parts of the NSW Central Tablelands, and in the Barrington Tops National Park near Gloucester.
Sydney's run of cold weather has continued today but when can we expect it to warm up? Considering last autumn was the warmest on record the past few wintry days have been a shock to the system for many.