Seasonal Climate Outlook Message for May to August 2011

Commentary on rainfall probabilities based on 'phases' of the SOI

The Southern Oscillation Index (SOI) remains extremely positive, with monthly values of the index rising from +17.5 in March to +23.9 in April. These values currently place the SOI in a Consistently Positive Phase, now the ninth consecutive month in this phase.

However, the Queensland Climate Change Centre of Excellence considers that, at this time of year, the Southern Oscillation Index (SOI) or other measures of the El Niño-Southern Oscillation (ENSO) are less reliable as indicators of rainfall for the season ahead. Based on previous years when the SOI has been in a 'Consistently Positive' phase at the end of April, there is a 40 to 60% probability that rainfall during May to July will be above the long-term median throughout much of Queensland.  A 40 to 60% probability of above- or below-median rainfall means that a ‘Consistently Positive’ phase of the SOI in April, has no bearing on rainfall from May to July.

In assessing this information, it is worth considering other years with a ‘Consistently Positive’ SOI phase at the end of April. Since 1950, these include: 1950, 1956, 1959, 1960, 1963, 1971, 1974, 1975, 2000, 2006 and 2008.

When using a climate forecast it should be remembered that the probability, or percent chance, of something occurring is just that – a probability. For example, if there is a 70% probability of above-median rainfall, then there is also a 30% chance of below-median rainfall. It does not mean that rainfall will be 70% more than the median.

Users are advised not to rely on a single climate outlook alone and should consider the range of climate risk assessment information that is available.

Further seasonal climate outlook information is available at www.longpaddock.qld.gov.au and www.bom.gov.au