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

Victoria weather: Strong winds hit Melbourne, southern parts of state as cold front moves through

18:56 EST

A deep cold front has swept through Melbourne and parts of southern Victoria, uprooting trees and damaging buildings with wind gusts of more than 100 kilometres per hour (kph).

Dry season storm dumps heavy rain on Darwin

18:23 EST

A 90-minute downpour has seen Darwin record more than twice as much rain as usual for the entire month of May.

Wet season wraps up with below average rainfall for many Top End cattle stations

17:53 EST

Despite a promising start to the wet season, many Top End cattle stations are entering the dry season with The bulk of the season's rain came with a monsoon trough which moved over the Northern Territory in , with little follow-up rain throughout January and February.