Commentary on rainfall probabilities based on 'phases' of the SOI
The monthly value of the Southern Oscillation Index (SOI) was -6.7 for August and -8.5 for September. According to the SOI Phase system, the SOI is in a ‘Consistently Near-Zero’ phase.
A map showing the probability of exceeding median rainfall for the next three-month period (October to December) based on a ‘Consistently Near-Zero’ phase of the SOI over August and September is available. This map is based on previous years from 1889 to 2015 which, like 2018, had a ‘Consistently Near-Zero’ phase of the SOI for September (i.e. 1895, 1897, 1898, 1904, 1905, 1907, 1912, 1913, 1915, 1918, 1919, 1927, 1929, 1930, 1931, 1933, 1937, 1944, 1948, 1952, 1959, 1961, 1963, 1966, 1968, 1978, 1980, 1984, 1985, 1986, 1992,1995, 1999, 2003, 2004, 2007 and 2013). This map indicates a 40 to 60 per cent probability of above-median rainfall for most of Queensland.
The current classification of the SOI phase (i.e. ‘Consistently Near-Zero’) is quite borderline. A fractionally more negative value of the SOI, in either August or September, would have resulted in a different classification (i.e. ‘Consistently Negative’) and a lower probability of above-median rainfall.
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 exceeding median rainfall, then there is also a 30 per cent probability of below-median rainfall. It does not mean that rainfall will be 70 per cent more than the median.
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 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 October 2018 is now available.