Despite a heatwave warning this week for large parts of the country, Australia is under the influence of a La Niña – a climate phenomenon which in the past has meant wetter and cooler weather.
The Bureau of Meteorology declared in November the system was in place but there are signs this year’s La Niña may not bring what many Australians might expect.
So what is going on?
What is La Niña?
La Niña relates to the conditions in the Pacific Ocean, to Australia’s east, that can have a strong influence on the country’s weather.
Climatologists monitor a cycle in the Pacific known as the El Niño southern oscillation – or Enso for short.
In this cycle’s La Niña phase, there are cooler-than-usual waters in the central equatorial Pacific and warmer waters are pushed towards Australia’s eastern coastline.
The trade winds that blow from the east strengthen, bringing the potential for rainfall.
The bureau confirmed last week that La Niña conditions had been in place since early October.
What does La Niña mean for weather?
While no two La Niñas are the same, broadly these events have delivered cooler weather and more rainfall in the past.
The “triple dip” La Niñas that ran from 2020 to 2023 delivered record rainfall.
But the bureau’s current long-range weather outlook suggests “wetter and cooler” might not happen this time around.
The bureau’s models suggest rainfall this summer could be roughly average for Brisbane, Sydney, Canberra, Melbourne, Adelaide and Perth. Central Queensland, northern parts of central New South Wales and north-western Western Australia could be drier than usual.
For temperatures, most of the country is predicted to have above-average maximum and minimum temperatures between December and February.
Why would this La Niña not bring cooler weather?
Three reasons, Dr Zhi-Weng Chua, a senior climatologist at the bureau, said: La Niña’s strength and duration, and global heating.
“This La Niña is relatively weak and relatively short-lived so that suggests the impacts from it will be weaker too.”
Enso is not the only influence on Australia’s climate. Others include the position of the westerly winds that blow south of the continent and the location of warmer waters in the Indian Ocean off Australia’s north-west.
But the other influence is global heating.
The bureau’s forecast for summer shows most of the country has an increased chance of experiencing above-average maximum and minimum temperatures.
“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 and global warming is having more influence,” Chua said.
Parts of NSW, WA and the Northern Territory were facing heatwave conditions this week – another reminder that this La Niña may not be very cool at all.
What about the rainfall during this La Niña?
As well as being a relatively weak La Niña where the cooler temperatures in the central Pacific only just meet the required threshold, the bureau also thinks the system could break down early next year.
Dr Andrea Taschetto, an associate professor at UNSW and an Enso expert, said when it comes to rainfall La Niña’s influence tends to be stronger in spring. She said the fact that Australia’s summer has begun and the system remains weak likely shows its influence is probably small.
The bureau’s forecast for summer – from December to February – suggests the most populated areas and capital cities don’t have an elevated chance of getting above-average rainfall.
Chua says the fact the outlook is not showing a clear rainfall signal one way or the other for many places suggests there’s some uncertainty in what could happen.
“We see the outlook is unclear, so day-to-day variability can become a bigger factor,” he said.
Sea surface temperatures for the Australian region were the second warmest on record for an October month, and these warmer-than-average conditions are expected to continue through summer – especially off the east coast.
This, the bureau has warned, can provide “increased moisture and energy, that can enhance the severity of storms, cyclones and rain systems”.
