Values of the Southern Oscillation Index (SOI) have decreased over the last two months (March +2.3, April -6.2). According to the SOI Phase system, this places the SOI in a ‘Rapidly Falling’ phase.
Based on previous years when the SOI has been in a ‘Rapidly Falling’ phase at the end of April, the probability of above-median rainfall for the next three-month period (May to July) is between 40 and 60 per cent throughout most of Queensland. A 40 to 60 per cent probability of above- or below-median rainfall means that a ‘Rapidly Falling’ phase of the SOI, in April, has little bearing on rainfall from May to July.
In assessing this information, it is worth considering other years with a ‘Rapidly Falling’ SOI phase at the end of April. Since 1950, these include: 1952, 1965, 1967, 1969, 1976, 1993, 1994, 1995, 1997, 2004 and 2005.
When using a climate outlook 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 per cent probability of above-median rainfall, then there is also a 30 per cent chance of below-median rainfall. It does not mean that rainfall will be 70 per cent more than the median.
As the SOI is less reliable as an indicator of rainfall for the autumn season, 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. In the first instance users in Queensland may wish to consult the monthly climate statement produced by the Queensland Climate Change Centre of Excellence.
Further seasonal climate outlook information is available at www.bom.gov.au