Monthly Climate Statement for September 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 evolving 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 near-normal across Queensland.
The most closely monitored driver of Queensland rainfall is the El Niño-Southern Oscillation (ENSO) phenomenon. The relationship between ENSO indices and subsequent Queensland summer rainfall begins to strengthen over winter and peaks through spring. The Bureau of Meteorology and international agencies classify the current atmospheric and oceanic conditions as ‘ENSO-neutral’.
Climate scientists monitor several ENSO indices, including the atmospheric Southern Oscillation Index (SOI) and SST anomalies in the central equatorial Pacific Ocean. DES also monitors the SST gradient across the central and south-western Pacific Ocean (i.e. the South Pacific Convergence Zone), which, at this time of year tends to persist and is strongly related to upcoming summer rainfall in Queensland. Key ENSO indices returned to near-average values in autumn, having been at more extreme (La Niña) levels last spring and over summer. The three-month average Niño 3.4 SST anomaly* remains near-average (-0.3°C for June to August). However, the most recent three-month average SOI value** is quite positive (+6.9 for June to August). It remains to be seen whether the SOI remains positive, or returns to more neutral values during spring. Most climate models currently project some cooling of SSTs in the central equatorial Pacific over spring, which would also support a positive SOI.
ENSO-related 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. The current DES outlook for summer rainfall in Queensland (see map in PDF) is based on an objective analysis of SST gradients across key regions of the Pacific Ocean, including those regions related to ENSO. DES will continue to update the outlook for summer rainfall during spring, factoring in any change to the ENSO-related SST pattern during this period.
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.cpc.ncep.noaa.gov/data/indices (monthly OISST.v.2 1991-2020 base period)
** www.longpaddock.qld.gov.au/soi/soi-data-files (monthly SOI 1887-1989 base period)
