Seasonal Climate Outlook Message for November 2014 to February 2015

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

The monthly value of the Southern Oscillation Index (SOI) was -6.6 in September and -8.2 in October.  While the SOI Phase system has marginally placed the SOI in a ‘Consistently Near-Zero’ phase, DSITIA considers that ‘Consistently Negative’ is a more appropriate classification of the system at this time.  DSITIA notes that the classification actually hinged on an atypical positive daily SOI value on the last day of October.  Extreme daily fluctuations of the SOI can occur as part of the ‘noise’ associated with random fluctuations in the climate system (i.e. not associated with the more persistent ENSO ‘signal’).  DSITIA therefore urges users to consider rainfall probabilities associated with a ‘Consistently Negative’ phase of the Southern Oscillation as being more relevant at this time.

Based on previous years when the SOI has been in a ‘Consistently Near-Zero’ phase at the end of October, the probability of above-median rainfall for the next three-month period (November to January) is 40 to 60 per cent for much of Queensland.

However, based on previous years when the SOI has been in a ‘Consistently Negative’ phase at the end of October, the probability of above-median rainfall for the next three-month period (November to January) is less than 50 per cent for most of Queensland.  Indeed, for Cape York and many eastern coastal areas, the probability of above-median November to January rainfall is even less than 30 per cent.

Furthermore, a range of analyses (based on different approaches) currently support a low probability of widespread wet conditions across Queensland this summer (although this can't be totally ruled out).

When using a climate outlook it should be remembered that the probability, or per cent 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.

Users should note that while climate outlook schemes cannot provide outlooks with absolute certainty, users who follow a skilful scheme should benefit from doing so in the long-term. Thus, users should consider the historical track record of any scheme, and such information is becoming increasingly available.

Further seasonal climate outlook information for Queensland is available in the monthly climate statement produced by the Department of Science, Information Technology, Innovation and the Arts and at www.bom.gov.au.