Monthly Climate Statement for May 2021
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 March 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 currently slightly 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. Having been at more extreme (La Niña) levels over spring and summer, key ENSO indices are now returning to more average values. The most recent three-month SOI value* (+3.8 at the end of April) is much lower than the peak value reached at the end of February (+14.7). Likewise, the most recent three-month Niño 3.4 anomaly** (-0.6°C) is much weaker than the recent peak value of -1.3°C at the end of December. The Bureau of Meteorology classifies current conditions as ‘ENSO-neutral’.
At this time of year, the ENSO phenomenon tends to be in a state of transition. As such, the relationship between ENSO indices and subsequent winter, spring and summer rainfall is very weak. However, SST anomalies in the extra-tropical Pacific tend to be more persistent and are strongly related to rainfall in Queensland over the following summer. The current DES outlook for summer rainfall in Queensland is based on an objective analysis of this extra-tropical SST pattern. On this basis, the Science and Technology Division of DES considers that the probability of exceeding median summer (November to March) rainfall is currently slightly higher than normal for much of Queensland (see map in PDF).
SST anomalies in the central equatorial Pacific tend to ‘lock in’ over the winter, spring and summer seasons. This persistence provides a basis for seasonal forecasting. DES will provide an updated outlook for summer rainfall in June, at which time this outlook will begin to factor in the evolving ENSO-related SST pattern.
Readers are cautioned that seasonal outlooks are expressed in terms of probabilities. Even though an outcome has a high probability of occurring based on historical records, a less likely outcome may still occur in any given year. For more information, please contact Ken Day: ken.a.day@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 1991-2020 base period)
