Monthly Climate Statement for December 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 leading up to summer, the Science Division of DES considered that the probability of below-median summer (November to March) rainfall was higher than normal for 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 average SST* anomaly in the central equatorial Pacific over the last three-month period (September to November) is +1.7°C, which falls well within the ‘El Niño’ range (>+0.5°C as recognised by DES). The average value of the SOI** over the last three-month period is -9.6, which also falls well within the ‘El Niño’ range (<-5.0 as recognised by DES).
On 19 September, the Australian Bureau of Meteorology officially declared that an ‘El Niño’ is underway in the tropical Pacific. This announcement followed earlier ‘El Niño’ declarations from both the World Meteorological Organization and the USA National Oceanic and Atmospheric Administration. Most international climate models indicate that the current El Niño pattern is likely to persist through summer.
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 DES outlook for summer rainfall in Queensland, issued at the beginning of November, 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 less than 30 percent for much of Queensland (see map in PDF). This analysis also indicates a low (4 per cent) probability of much wetter than average conditions (>decile 7 rainfall) across Queensland’s main grazing district over the coming summer (November to March).
Tropical cyclone Jasper formed near the Solomon Islands on 5 December and made landfall between Cairns and Cooktown on 13 December. Associated high rainfall occurred over several days along the tropical east coast between Cape Melville and Ingham (see map in PDF). A landfalling tropical cyclone in Queensland, combined with such high rainfall, is a rare occurrence under ‘El Niño’ conditions this early during the tropical cyclone season. The tropical cyclone season lasts from 1 November to 30 April.
So far this summer, the long-term median summer (November to March) rainfall (decile 6 or above) has already been exceeded in parts of tropical north Queensland, due to tropical cyclone Jasper (see map in PDF). Other parts of the State which have now almost exceeded their median summer rainfall include parts of central and southern Queensland.
Readers are reminded that seasonal outlooks are expressed in terms of probabilities. The probabilities shown on the rainfall outlook 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. An analysis may, for example, indicate that below-median summer rainfall occurred in most of those years. However, this also means that summer rainfall was at, or above, the long-term median in some of those years. Therefore, an outlook which states that there is ‘an 80 per cent probability of below-median rainfall’ should also be interpreted as there being ‘a 20 per cent probability of median or above-median rainfall’.
For more information, please contact Stuart Burgess at: stuart.burgess@des.qld.gov.au
* www.cpc.ncep.noaa.gov/data/indices (monthly OISST.v2.1 NINO3.4 1991-2020 base period)
** www.longpaddock.qld.gov.au/soi/soi-data-files (monthly SOI 1887-1989 base period)
