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.
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Weather News
Victoria's severe weather sparks Murray River bank collapse warning
23:11 EDT
Authorities are pleading with people living or camping along the Murray River in Victoria's north to stay away from creek and river banks as severe weather again lashes the state.
Winds surging across Victoria
14:08 EDT
Wind has strengthened today across Victoria, with more to come tonight.
Impressive temperature changes across southern parts of WA
13:30 EDT
A band of thunderstorms has been moving over southern parts of WA this morning with the passage of a low pressure trough.
