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.
Melbourne has chilled through its coldest pair of days this late in the year since 1983, reaching a footy-season like 13.2 degrees on Saturday and 14.2 on Sunday.
Following a nice Sunday to end the weekend, the mercury is set to plunge on Monday as a cold front welcomes the working week.
Hail storms, heavy rain and gusty winds have lashed Victoria and hundreds of people have called for help from emergency services as a cold front moved across the state.