Commentary on rainfall probabilities based on 'phases' of the SOI
The monthly value of the Southern Oscillation Index (SOI)* fell from +16.3 in July to +4.4 for August. According to the SOI Phase system, the SOI is in a ‘Consistently Positive’ phase.
A map showing the probability of exceeding median rainfall for the next three-month period (September to November) based on a 'Consistently Positive’ phase of the SOI over July and August is available. This map is based on previous years from 1889 to 2015 which, like 2021, had a ‘Consistently Positive’ phase of the SOI for August (i.e. 1892, 1893, 1900, 1909, 1910, 1915, 1916, 1917, 1920, 1924, 1938, 1947, 1950, 1955, 1956, 1958, 1960, 1973, 1974, 1975, 1981, 1988, 1996, 1998, 2010 and 2011). This map indicates a 60 to 80 per cent probability of exceeding median September to November rainfall across most of Queensland.
Readers should note that seasonal outlooks are stated in terms of probabilities. For example, an outlook may be stated as ‘a 60 to 80 per cent probability of exceeding median rainfall’. Such a statement should be interpreted as also meaning a 20 to 40 per cent probability of below-median rainfall. In cases where outcomes with a high probability may be more likely, this does not mean that less probable events will not occur in any given year.
Furthermore, 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.
The Science and Technology Division of the Department of Environment and Science (DES) provides outlooks for the summer period (November to March). The outlooks for summer rainfall are based on conditions leading up to summer, including the state of the El Niño–Southern Oscillation (ENSO) phenomenon and factors which alter the impact of ENSO on Queensland rainfall (i.e. the more slowly changing extra-tropical sea-surface temperature pattern in the Pacific Ocean). The DES Monthly Climate Statement for September 2021 is now available.
* www.longpaddock.qld.gov.au/soi/soi-data-files (monthly SOI 1887-1989 base period)