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.
Melbourne has had its coldest day since last September, with heavy rain and flurries in Alpine areas.
For those among us wanting to balance out our participation in Earth Hour, the Vivid Light, Music and Ideas Festival opens in Sydney this on Friday.
Australia's Bureau of Meteorology (BoM) has released modelling showing the latest El Nino cycle is over, but could now lead to a wet winter after a devastating 14 months of drought and famine across the Pacific.