The Bureau of Meteorology (BoM) has confirmed that a La Niña climate pattern is influencing Australia's weather this summer. However, in a significant departure from historical norms, the cooler temperatures and increased rainfall typically associated with this phenomenon may be largely absent.
What is La Niña and Why is it Different This Time?
La Niña is a phase of the El Niño Southern Oscillation (ENSO) cycle, characterised by cooler-than-average sea surface temperatures in the central equatorial Pacific and warmer waters near Australia's east coast. This usually strengthens trade winds, boosting the potential for rainfall across the continent.
The BoM declared in November that La Niña conditions had been in place since early October. Yet, its long-range outlook for December to February suggests a summer that defies expectations. Dr Zhi-Weng Chua, a senior climatologist at the bureau, cites three key reasons: the current La Niña's relative weakness, its short duration, and the overarching influence of global heating.
A Forecast of Heat and Uncertainty
Contrary to the classic La Niña signature, the BoM's models predict that most of the country will experience above-average maximum and minimum temperatures this summer. Rainfall projections for major capitals like Sydney, Melbourne, and Brisbane are around average, with some regions, including parts of central Queensland, facing drier conditions.
"La Niña leans towards cooler temperatures, but global warming is influencing this – there is a push and pull, and maybe La Niña is not the dominant influence," explained Dr Chua. This week's heatwave warnings for parts of New South Wales, Western Australia, and the Northern Territory underscore this new reality.
Rainfall Impact and Underlying Risks
Associate Professor Andrea Taschetto, an ENSO expert at UNSW, noted that La Niña's rainfall influence is often strongest during spring. The system's continued weakness as summer begins suggests its impact may now be limited. The bureau's outlook indicates no elevated chance of above-average rainfall for most populated areas.
However, a significant risk factor remains. Sea surface temperatures around Australia were the second-warmest on record for October. The BoM warns that these persistently warm waters, particularly off the east coast, can provide increased moisture and energy, potentially enhancing the severity of storms, cyclones, and rain systems even if overall seasonal totals are near average.
In essence, while the La Niña label applies, its effects are being fundamentally reshaped by a warmer climate, leading to a summer forecast marked more by heat and variable weather than by consistent, widespread cool and wet conditions.
