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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Australian farms have lost on average almost $30,000 each a year in profits over the past 20 years due to climate change, relative to earnings in the latter part of last century, says the Australian Bureau of Agricultural and Resource Economics and Sciences (ABARES).

Latest IOD data backing wet spring outlook for Australia

17:10 EST

The index used to monitor the Indian Ocean Dipole has just reached its lowest value since 2016, bolstering the likelihood of a wet and cool spring in Australia.

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