Southern Oscillation Index: rapid fall

Diagram showing the probability of rainfall in areas of Australia exceeding their annual median rainfall for June through to August.

The Southern Oscillation Index (SOI) has taken a rather dramatic fall from +1.4 at the end of April to -9.9 at the end of May. Based on this drop in SOI value and the recently measured westerly wind bursts in the central Pacific Ocean region, the pre-conditions required for the possible onset of an El Niño are set. However, at this stage it is still too early to determine when, or even if, an El Niño will develop.

Using the latest information, the chance of getting the median rainfall for the June to August period is a low 20-40 per cent across most of southern Queensland (refer to map). This area includes all or parts of the shires of Murwah, Paroo, Balonne, Booringa, Warroo, Yuleba, Waggamba, Tara, Inglewood, Stanthorpe, Millmerran and Warwick West.

Other areas of low-rainfall probabilities include the coastal strip from Livingstone Shire north, the top of Cape York Peninsula and scattered areas throughout central and western Queensland. The rest of the State has probabilities of 40-70 per cent of receiving at least the median rainfall over the next three months.

These low-rainfall probabilities extend from southern Queensland, through the interior of NSW and well into Victoria and South Australia. The main exception to these low-rainfall probabilities in eastern Australia is in the eastern corner of Victoria and a narrow strip of coastal NSW where probabilities of getting at least the median rainfall vary between 40 and 70 per cent.

As always, when using probability-based forecasts, it is essential to remember that the opposite always applies. For example, if there is a 20-30 per cent chance of getting above the median rainfall at a location, there is also a 70-80 per cent chance of not getting the median rainfall.

For those who like to follow the historical patterns more closely, some of the years in the past that also had a Rapidly Falling Phase in April and May include 1999,1972,1958,1957, 1953,1947,1935,1934, 1929,1928,1925 and 1911.

Will there be an El Niño?

Information is now being gathered to determine whether the falling SOI value is representative of an upcoming El Niño. This includes whether the current passage of the 40-day wave will help generate more of the westerly wind bursts that have been recently recorded in the central equatorial Pacific Ocean region. QCCA staff will also investigate what (if any) changes in sea surface temperature (SST) patterns these westerly wind bursts have generated. Therefore, we strongly encourage producers to utilise the continually updated outlook in the climate and weather section of the QDPI Internet site at and the Longpaddock Internet site at An update outlook incorporating this new information will be released early in July.

A warning for readers to beware of statements from some quarters, especially from overseas, that make the mistake of referring to 'weak' or 'strong' El Niño events. The implication is often made that the impacts, in terms of rainfall, from a 'weak' El Niño (in terms of SST anomaly) would be only a small reduction in rainfall. This is not necessarily the case. Remember that some so-called 'weak' El Niños produce some of our worst droughts, while some so-called 'major' or 'strong' El Niños (such as the 1997-98 event) do not necessarily produce widespread drought.

Alternatively, the American perspective on the potential for an El Niño can be gained by looking at the Climate Prediction Centre Internet site at

For more details on this article or on any other climaterelated topic, phone Dave McRae on 07 4688 1459 or email

Region North