The Department of Environment and Science (DES) monitors sea-surface temperature (SST) anomalies in key regions of the Pacific Ocean over autumn, winter and spring, and provides objective outlooks for summer (November to March) rainfall on this basis. Based on the evolving SST pattern in the Pacific Ocean, the Science and Technology Division of DES considers that the probability of exceeding median summer (November to March) rainfall is higher than normal across Queensland.
The most closely monitored driver of Queensland rainfall is the El Niño-Southern Oscillation (ENSO) phenomenon. Climate scientists monitor several ENSO indices, including the atmospheric Southern Oscillation Index (SOI) and SST anomalies in the Niño 3.4 region of the central equatorial Pacific Ocean. Over the last three months (September to November) the SOI* has remained quite positive (+9.5) and SSTs in the Niño 3.4 region** have been much cooler (-0.7°C) than average. The sign and magnitude of these anomalies are indicative of a ‘La Niña’ climate pattern and the Bureau of Meteorology and other international climate agencies now officially classify current ENSO conditions as being in a La Niña state.
The DES outlook for summer rainfall in Queensland, issued at the beginning of November (see map in PDF), is based on an objective analysis of SST gradients across key regions of the Pacific Ocean, including the ‘South Pacific Convergence Zone’ (SPCZ). La Niña conditions tend to weaken the SST gradient between the central Pacific and the south-western Pacific, which is a key leading indicator of summer rainfall in Queensland. Whilst the DES approach is based on historical SST and rainfall records, the current outlook for summer is also supported by climate modelling conducted by the Bureau of Meteorology and international climate agencies. Given the extremely wet start to summer (see PDF), the probabilities of exceeding median summer rainfall across Queensland are now likely to be even higher than indicated at the beginning of November.
Readers are reminded that seasonal outlooks are expressed in terms of probabilities. The DES approach bases these probabilities on summer rainfall in previous years with similar SST conditions to the current year. This analysis may, for example, show that above-median summer rainfall occurred in 60 per cent of those years. However, this also means that summer rainfall was at, or below, the long-term median in 40 per cent of those years. Therefore, an outlook which states that there is ‘a 60 per cent probability of above-median rainfall’ should also be interpreted as there being ‘a 40 per cent probability of median or below-median rainfall’.
For more information, please contact Ken Day: firstname.lastname@example.org
* www.longpaddock.qld.gov.au/soi/soi-data-files (monthly SOI 1887-1989 base period)
** www.cpc.ncep.noaa.gov/data/indices (monthly OISST.v.2 1991-2020 base period)