Monthly Climate Statement for September 2019
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 for all of Queensland.
The most closely monitored driver of Queensland rainfall is the El Niño-Southern Oscillation (ENSO) phenomenon. At this time of year, the relationship between ENSO and rainfall tends to strengthen. Climate scientists monitor several ENSO indices, including the atmospheric Southern Oscillation Index (SOI) and SST anomalies in the central equatorial Pacific Ocean. The SST anomaly in the Niño 3.4 region of the central Pacific has cooled over the last three-month period, from a value which was close to El Niño thresholds in June (+0.6°C), to a value which is now well-within ENSO-Neutral thresholds (+0.2°C for August). Although ENSO is in a neutral state, the SOI has remained quite negative, averaging -6.3 from June to August.
DES also monitors an index based on the SST pattern across the south-western Pacific which, at this time of year, is strongly correlated with Queensland summer rainfall. SST anomalies in the south-western Pacific are currently much warmer than normal, and such anomalies tend to be very persistent, favouring above average rainfall in Queensland over the following summer.
The Bureau of Meteorology’s outlook currently (as at 12 September) indicates a higher than normal probability of drier than average conditions for September to November. Looking further forward, DES considers that the probability of exceeding median rainfall for the summer season (November to March) is currently higher than normal for all of Queensland (see map in PDF). This outlook is based on an objective analysis of the evolving Pacific Ocean SST pattern. DES will update the outlook for summer rainfall in October and November, factoring in any change to the Pacific Ocean SST pattern over this period.
Readers should note that seasonal outlooks are stated 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 more information please contact Ken Day at: ken.a.day@des.qld.gov.au.
