Fairfax Media Network

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.

Display Your Local Weather

Weather News

Odds favour Perth having drier, warmer winter than average

10:13 EST

Perth residents have been heralding the arrival of winter after heavy rainfall over the weekend and significant fog blanketed Perth city this week.

Another foggy start for Brisbane

15:20 EST

Flights were diverted from Brisbane Airport this morning as fog once again shrouded southeast Queensland.

Sydney wakes to rare rumble

13:00 EST

Sydneysiders were woken up this morning by a loud bang that may have been caused by a rare type of lightning.