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 leading up to summer, the Science and Technology Division of DES considered that the probability of exceeding median summer (November to March) rainfall was 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. As anticipated, the current ‘La Niña’ phase of ENSO has persisted through summer (December to February), with the SOI* remaining quite positive (+7.7) and the SST anomaly in the Niño 3.4 region** remaining cooler than average (-0.9°C). March to April is generally a time when El Niño and La Niña events break down and new events form. This period, which is known as the ‘autumn predictability gap’, might therefore see SOI and SST anomalies moderate.
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.
So far this summer (November to February), much of Queensland has received at least near-average rainfall (see map in PDF). Parts of south-eastern Queensland have been extremely wet, recording more than twice the average rainfall for this period. In contrast, parts of central and northern Queensland have experienced drier than normal conditions.
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 at: 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)