Monthly Climate Statement for May 2024
The Department of Environment, Science and Innovation (DESI) 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 March SST pattern in the Pacific Ocean, the Science Division of DESI considers that the probability of exceeding median summer (November to March) rainfall is currently above-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 most recent three-month average value of the SOI* is quite negative (-7.5 for February to April) and the corresponding SST** anomaly in the Niño 3.4 region of the central equatorial Pacific is strongly positive (+1.2°C) - the value of both indices being consistent with the El Niño conditions which prevailed over summer. However, in mid-April, the Bureau of Meteorology announced that the El Niño had ended, this assessment being based on a cooling trend of SSTs in the central Pacific, together with colder than average sub-surface temperatures in this region. Most international climate models currently indicate further cooling of SSTs in the central equatorial Pacific into winter and the BoM has now (on 14 May) revised its ENSO Outlook from ‘Inactive’ to ‘La Niña Watch’.
The current DESI outlook for summer rainfall in Queensland is not based on ENSO indices, which are subject to the autumn predictability gap. Rather, the DESI outlook is based on an objective analysis of historical conditions using an index based on SST anomalies measured in March in key regions of the extra-tropical Pacific Ocean, which have been related to rainfall over much of Queensland in the following summer. On this basis, the Science Division of DESI considers that the probability of exceeding median summer (November to March) rainfall is currently above-normal for much of Queensland (see map in PDF).
ENSO-related SST anomalies, whether cool or warm, tend to build in the central Pacific over winter and spring and then persist over summer, providing a basis for seasonal forecasting. Therefore, DESI will provide an updated outlook for summer rainfall in June, at which time the outlook will begin to factor in the evolving ENSO-related SST pattern. Further updates will follow each month from July to November.
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)
