Monthly Climate Statement for August 2023
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 Division of DES considers that the probability of exceeding median summer (November to March) rainfall is currently in the near-normal range (30 to 70 per cent) across much of 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 central equatorial Pacific Ocean. The most recent three-month average (May to July) value of the SOI* is quite negative (-7.3) and the average SST anomaly in the Niño 3.4 region** is warmer than average (+0.8°C). During this three-month period the Niño3.4 SST anomaly has warmed in each month – the monthly anomalies being +0.5°C in May, +0.9°C in June and +1.1°C in July. Most international climate models currently project further warming of SSTs in the central equatorial Pacific over the remainder of winter and into spring.
The Australian Bureau of Meteorology currently maintain an ‘El Niño Alert’. ‘El Niño Alert’ means there is about a 70 per cent probability of El Niño this year. However, international agencies have different criteria for classifying ENSO events. The World Meteorological Organization and the USA National Oceanic and Atmospheric Administration have stated that current ENSO conditions meet their criteria for declaring an El Niño event in the central equatorial Pacific.
Sea-surface temperature anomalies in the central equatorial Pacific tend to ‘lock in’ over the winter, spring and summer seasons, and this persistence provides a basis for seasonal forecasting. The current DES outlook for summer rainfall in Queensland is based on an objective analysis of SST anomalies measured in key regions of the Pacific Ocean, including those regions related to ENSO. On this basis, the Science Division of DES considers that the probability of exceeding median summer (November to March) rainfall is currently in the near-normal range (between 30 and 70 per cent) across much of Queensland (see map in PDF). DES will provide an updated outlook for summer rainfall in September, to factor in the evolving ENSO-related SST pattern.
Readers are reminded that seasonal outlooks are expressed in terms of probabilities. The probabilities shown in the above map are based on an objective analysis of historical data and show the summer rainfall outcome in years when SST conditions were closest 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 Stuart Burgess at: stuart.burgess@des.qld.gov.au
* www.longpaddock.qld.gov.au/soi/soi-data-files (monthly SOI 1887-1989 base period)
** www.cpc.ncep.noaa.gov/data/indices (monthly OISST.v2.1 1991-2020 base period)
