There are growing signs Australia's most impactful climate driver, La Niña, will develop during the coming months.
La Niña refers to an altered state of the tropical Pacific, lasting anywhere from months to years, when cooler-than-average waters combine with changes in the overlying atmosphere to disrupt global weather patterns.
For Australia, La Niña would increase the prospects of a wet spring and summer, and to a lesser degree, potentially subdue extreme summer heat.
Another La Niña this year would be the fourth in just five years — well above the average of one every four years, and a frequency only previously seen twice since 1900.
The signature La Niña cool tongue of water along the equator is slowly expanding to the west from South America. (Supplied: National Oceanic and Atmospheric Administration)
La Niña gaining slow traction
The return of La Niña has been predicted by weather models all year, but after months where the Pacific has remained firmly in a neutral state, both the ocean and atmosphere are now showing signs of a transition.
Firstly, the signature tongue of cool water extending west from South America is becoming more prominent, although for now water temperatures remain in neutral territory.
But water temperatures alone can't be used to judge the presence of La Niña.
The critical requirement is a response or enhancement by the atmosphere, a situation where the ocean and overlying weather re-enforce each other and can therefore maintain an anomalous state for an extended period.
When the Pacific enters a La Niña state stronger trade winds pile up warmer water near Australia which increase cloud development and rain. (Supplied: BOM)
During La Niña episodes there are numerous interconnected changes to the weather, and several have already materialised this September, including
- Easterly trade winds across the tropical Pacific have strengthened.
- Cloud cover near the dateline has decreased while becoming more slightly more widespread near Indonesia.
- The SOI (a measure of the pressure difference between Tahiti and Darwin) has shifted to around +10, above the La Niña threshold of +7.
- Water temperatures below the surface of the Pacific are below average, supplying a source for further surface cooling in the months ahead.
Forecasters also use modelling to predict climate drivers and a September survey shows 60 per cent of global models tip La Niña by November, up from 41 per cent during the August survey.
Sixty per cent of models now forecast La Niña to develop during the coming months while none predict a return of El Niño. (Supplied: CFS)
When combining model forecasts and observations the US weather agency the National Oceanic and Atmospheric Administration (NOAA) is even more confident, as outlined in their latest monthly diagnostic
"La Niña's arrival is favored (sic) to emerge in September-November (71% chance) and is expected to persist through January-March 2025," the NOAA said.
What La Niña would mean for Australia's weather
If the Pacific does indeed continue a transition to La Niña during the coming months, the shift in convection from the central to the western Pacific will become prolonged, or in other words, a greater chance of cloud and rain will emerge over Australian longitudes.
Australia's 12 wettest years on record were all associated with La Niña, however most occurred during the stronger events, and often in combination with other wet climate drivers like a negative Indian Ocean Dipole.
This year, a delayed spring formation will only tip the odds moderately to favour an increase in rain.
If past events are analysed, the boost in rainfall through spring is most pronounced across the Murray Basin and tropical north, although a smaller swing to wetter conditions is also observed across the remaining interior and east.
It's therefore no surprise the Bureau of Meteorology's (BOM) latest seasonal outlook also favours a wetter finish to the year across most of central and eastern Australia – including up to a 75 per cent chance of above median rain in pockets of New South Wales and Queensland.
The BOM's seasonal outlook favours a wetter than normal finish to the year across eastern Australia. (ABC News)
By year's end the impacts on rainfall during La Niña start to ease, although most summers are still wetter than normal across northern and eastern Australia.
While La Niña's impact is well documented, every event is unique, as was evident during the last two La Niñas.
The 2021-22 event produced record rain and flooding in parts of south-east Queensland and NSW, but failed to deliver any increase in rain over the Top End.
Conversely the 2022-23 edition brought a near record wet season to the tropics, while rain was below normal in small parts of south-east Queensland.
La Niña's additional cloud and stronger easterly trade winds can also reduce summer daytime temperatures, although considering a weaker La Niña is tipped, climate change could counterbalance any cooling effect this year.
Other direct and indirect impacts of La Niña through the warmer months include:
- An earlier monsoon onset in tropical Australia, arriving generally two weeks before El Niño years.
- The first tropical cyclone is earlier, and season frequency making landfall is double El Niño years.
- A decreased frequency of extreme daily high temperatures.
- Southern coastal locations, such as Adelaide and Melbourne, see an increased frequency of prolonged warm spells (but less exceptional individual hot days).
Climate change masking La Niña's emergence
Agencies are unlikely to officially declare La Niña's arrival until oceanic and atmospheric changes become well established over a sufficient period (normally months) to ensure the signals aren't just typical short-term fluctuations in weather.
As a result, the BOM's alert status is set firmly at La Niña watch, meaning a roughly 50 per cent chance of development.
However there is one factor which is perhaps masking how close the Pacific actually is to a fully-fledged La Niña, and that's climate change.
The Bureau's latest climate driver update has 'La Niña Watch' in place, which indicates a 50 per cent chance of a La Niña this year. (Supplied: BOM)
The current Sea Surface Temperature (SST) in the key monitoring region, called the NINO3.4 index, is only 0.1C below average, but in a warming world it has become more difficult to meet the La Niña threshold of 0.5C to 0.8C below average.
This has become a major headache for meteorological agencies since the change in weather associated with El Niño and La Niña is not dependant on the absolute water temperature, but rather relies on a difference in the water temperature between the central and west Pacific.
If the NINO3.4 index is adjusted to factor in climate change, the latest measurement falls to around 0.6C below average, already knocking on La Niña's door.