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.
Less than three weeks into 2017, the desert town of Tennant Creek in the NT is already just 89 millimetres shy of reaching its average yearly rainfall, with the unusual downpour said to be leading to an influx of creepy crawlies.
Rain has continued to fall in parts of Central Australia, ensuring the current greenery continues.
The Great Northern Highway has been closed between Fitzroy Crossing and Halls Creek, as rivers rise across northern Western Australia following a week of steady rain.