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.
Winter 2015 varied significantly across Australia, with southeastern parts of Australia shivering through their coldest winter in years, whilst western and northern parts basked in much warmer than average conditions.
Flooded dairy farmers at Berry on the New South Wales south coast are struggling to keep their cows dry and fed in the downpour.
Victoria's Gippsland has experienced its coldest winter in 26 years, playing havoc with horticultural crops.