The Australian Bureau of Meteorology has declared a La Niña weather event is under way, with modelling predicting it “will persist until the late southern hemisphere summer or early autumn 2022”.
So what is it, and what will this mean for our summer?
What is La Niña and how often does it occur?
Across most of Australia, El Niño or La Niña conditions have the strongest influence on how the climate varies year to year. They belong to a cycle known as the El Niño-Southern Oscillation, which has an irregular timescale of several years.
“El Niño and La Niña arise due to atmosphere and ocean interactions,” said Dr Agus Santoso, a senior research associate at UNSW’s Climate Change Research Centre.
During a La Niña event, strong trade winds blow west across the Pacific Ocean, pushing warm surface water towards Asia and the seas north of Australia. The warmer waters lead to increased rainfall across northern and eastern Australia.
The second consecutive La Niña announced on Tuesday is the first time back-to-back events have occurred in a decade – since 2010-11 and 2011-12.
But it isn’t that uncommon for multi-year La Niña events to occur, according to BoM. For example, La Niña affected three consecutive years from autumn 1998 to autumn 2001.
What does it mean for this summer?
According to BoM, the six wettest winter-to-spring periods recorded in eastern Australia have all occurred during La Niña years.
The weather event is also linked to cooler daytime temperatures. Australia experienced a weak La Niña event last summer, resulting in the coolest summer in nine years and wettest in four years, with 29% more rain than average.
Despite this, 2020 was Australia’s fourth-warmest year on record.
The current La Niña is “probably not as strong” as the event of 2011-12, Santoso said.
He expects “wetter-than-normal conditions this time around but hopefully not as impactful [an event]” as 10 years ago.
The current La Niña may also be weaker than the one last summer. The strength of La Niña is determined by how much the sea surface temperature deviates from the norm. Typically, the stronger an event, the more rainfall.
However, another climate driver, called the southern annular mode (Sam), is now in a positive phase, which is associated with increased precipitation over southeastern Australia.
“If you only have a La Niña then there will be less of an impact compared to if you have a La Niña plus positive phase of the Sam,” Santoso said.
La Niña is also associated with greater cyclone activity. BoM estimated in October that there was a two-in-three chance Australia’s region would have more than the seasonal average of 11 cyclones.
The weather bureau confirmed on Monday the first tropical cyclone of the season had formed in the Australian region, which is expected to weaken without making landfall.
BoM upgraded the El Niño-Southern Oscillation alert system from La Niña watch to La Niña alert in early October, meaning there was a 70% chance of a La Niña forming.
The US National Oceanic and Atmospheric Administration had already announced the return of La Niña on 14 October.
The climate pattern results in more dry days across the southern third of the US, increasing the likelihood of drought in the south-west.
“When you have a La Niña … moisture in the eastern side of the Pacific becomes reduced,” Santoso said.
La Niña can also generate higher monsoon rainfall over the Indian subcontinent, Santoso said, raising the risk of flooding.
What about the effect of climate change?
Climate change has an impact on La Niña events, Santoso said. “Particularly if you get warmer sea surface temperatures surrounding Australia, that can enhance convective activities that can produce storms.”
A recent review Santoso co-authored concluded that the frequency of El Niño and La Niña events would increase under business-as-usual scenarios of greenhouse gas emissions.
Unless emissions are reduced, El Niño and La Niña events are projected to increase from 5.6 events per century in the present to 8.9 and 8.3 events per century in future, respectively.