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.
Flood-affected farmers in the Ravensthorpe shire on the south coast of WA say the State Government has been slow in reacting to their plight, adding there is a lack of comprehension around the scale of the damage.
The seasonal outlook for autumn is hot and dry, but what is causing it? Both maximum and minimum temperatures look to be above average, especially for central Australia, extending down through inland NSW and western VIC.
Wrapping up a hot and humid summer many will be happy to forget, rain is on its way for most of NSW east of the divide.