Rainfall in El Niño years
The information below has been updated. The relevant link is located here.
The Southern Oscillation Index (SOI) is a simple measure of the status of the Walker Circulation, a major wind pattern of the Asia/Pacific region whose variability affects rainfall in Australia and other parts of the world. During El Niño episodes, the Walker circulation weakens and the SOI becomes negative. For example, the 6 month average SOI (April to September) during the drought of 1982 was 16; for 1997 it was 17.8. Other changes during El Niño events include cooling of seas around Australia, as well as a slackening of the Pacific trade winds which in turn feed less moisture into the Australian/Asian region. There is then a high probability that eastern and northern Australia will be drier than normal.
The link between SOI and rainfall across southern and eastern Australia is strongest during the winter and spring periods (June-November). SOI values between April and September are used to produce rainfall outlooks for winter-spring rains, with low values of SOI often linked with below average rainfall in eastern Australia. Average April-September SOI values since 1876 are shown in the graph below, with years when the SOI was less than 10 noted on the graph. It is important to note that these years do not represent an exhaustive or definitive list of El Niños, only the stronger ones. [See El Niño - Detailed Australian Analysis for a more comprehensive look at all the El Niños since 1900 and their impacts on Australia.] Links to rainfall maps for the winter-spring period have been provided below for some of these years. Note that each El Niño event is different with respect to rainfall patterns and areas affected by dry conditions.
The average winter-spring rainfall deciles for the 12 strongest El Niño events have also been calculated.
These conditions are reversed in the opposite phase to El Niño known as La Niña. Positive values of the SOI are associated with stronger Pacific trade winds and warmer sea temperatures to the north of Australia. Together these give a high probability that eastern and northern Australia will be wetter than normal.
For further information on El Niño, see also