Commentary on rainfall probabilities based on phases of the SOI
Seasonal climate outlook message for January to March 2016
The monthly value of the Southern Oscillation Index (SOI) fell from -3.2 in November to -10.1 in December. According to the SOI Phase system, the SOI is in a ‘Rapidly Falling’ phase.
A map showing the probability of above-median rainfall for the next three-month period (January to March) is available. This map is based on previous years from 1900 to 1998 which, like 2015, had a rapidly falling SOI over November and December (i.e. 1908, 1912, 1918, 1934, 1935, 1940, 1943, 1948, 1952, 1972, 1976 and 1995). This map indicates a 50 to 70 per cent probability of above-median rainfall for much of Queensland with a lower than normal probability of above-median rainfall in parts of south-eastern Queensland.
However it is difficult to draw meaningful statistics from a selection of only 12 years. It should be noted that a further five years in the longer term record (1890 to 2014) had a rapidly falling SOI during November and December (i.e. 1899, 2001, 2002, 2012 and 2013). If these years are also taken into account, the probability of exceeding median rainfall would be close to normal for most of Queensland.
Rather than track the SOI over summer, the Department of Science, Information Technology and Innovation (DSITI) bases its monthly climate statements, over the summer period, on conditions leading up to summer, including the state of the El Niño–Southern Oscillation (ENSO) phenomenon prior to summer, and on factors which alter the impact of ENSO on Queensland rainfall (i.e. the more slowly changing extra-tropical sea surface temperature (SST) pattern in the Pacific Ocean). The DSITI Monthly Climate Statement for January 2016 is available.
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.
Last updated 18 March 2011