Monthly Climate Statement for September 2020
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. The Science and Technology Division of DES considers that the probability of exceeding median summer (November to March) rainfall is currently 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 central equatorial Pacific Ocean. The SOI rose from a quite negative value in June (-9.3) to a quite positive value in August (+8.4). However, the most recent three-month average SOI value (+1.2 for June to August) is well-within the ENSO-neutral range. Sea-surface temperature anomalies in the central equatorial Pacific have been cooler than average over the last three-month period (-0.4°C for June to August). The Bureau of Meteorology’s ENSO Outlook has been revised from ‘La Niña Watch’ to ‘La Niña Alert’, as many climate models project further cooling of SSTs in the central equatorial Pacific during the lead up to summer.
The relationship between SST in the central equatorial Pacific and summer rainfall in Queensland strengthens through spring. The SST gradient from the central equatorial Pacific to the southwestern Pacific is a particularly useful long-lead indicator of summer rainfall in Queensland. The current DES outlook for summer rainfall in Queensland is based on an objective analysis of SST gradients across key regions of the Pacific Ocean. On this basis, the Science and Technology Division of DES considers that the probability of exceeding median summer (November to March) rainfall is currently higher than normal across Queensland (see map in PDF).
Sea-surface temperature 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. The outlook for summer rainfall will be updated in October and November, factoring in the evolving ENSO-related SST pattern.
Readers should note that seasonal outlooks are expressed in terms of probabilities. For example, an outlook may be stated as ‘a 60 to 70 per cent probability of above-median rainfall’. Such a statement should be interpreted as also meaning a 30 to 40 per cent probability of below-median rainfall. In cases where outcomes with a high probability may be more likely, this does not mean that less probable events will not occur in any given year.
For further information, please contact Ken Day: ken.a.day@des.qld.gov.au
