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By ABC meteorologist Tom Saunders
Posted Sat 16 Mar 2024 at 5:50amSaturday 16 Mar 2024 at 5:50am
The magnitude of La Niña's impacts on Australia's weather depends on duration, intensity, and how it interacts with other climate drivers.(Supplied: Taryn Seccombe)
El Niño's demise and La Niña's emergence is gaining traction, increasing the odds Australia will face another lengthy period of above average rain and cooler temperatures by the back half of 2024. The growing confidence is being led by the National Oceanic and Atmospheric Administration (NOAA) who estimate the probability of La Niña returning exceeds 80 per cent by spring.
The forecast is well above La Niña's average occurrence of once every four years, and if it comes to fruition would represent the fourth La Niña in five years, a frequency only seen twice previously since 1900.
While the US weather agency already flagged La Niña as early as December, Australia's Bureau takes a far more conservative approach to long range predictions, only looking five months ahead, although a La Niña Watch declaration is possibly only weeks away.
How will La Niña impact our weather?
La Niña refers to a prolonged spell of six months to years when a cool tongue of water along the tropical Pacific basin leads to a change in weather patterns across the globe, including an increase in easterly winds and cloud development across Australia.
The magnitude of impacts on Australia's weather depends on La Niña's duration, intensity, and how it interacts with other climate drivers like the Indian and Southern Ocean, along with the natural variability of weather which ensures no two events are ever the same.
If La Niña does form in 2024 the first critical question is when?
La Niña has a peak influence on Australia rainfall during spring, and a moderate impact through the following summer and autumn. (ABC News)
At the earliest La Niña could be established by June, and according to NOAA's latest monthly diagnostic report for the Pacific is now the most likely outcome by mid winter.
"A transition from El Niño to ENSO neutral is likely by April-June 2024 (83 per cent chance), with the odds of La Niña developing by June-August 2024 (62 per cent chance)," stated NOAA's latest diagnostic. Once La Niña has formed, the typical response is Australian rainfall is enhanced. Looking back through all La Niña events since 1900 reveals the greatest impact is through spring, when Australian rainfall on average increases by nearly 50 per cent, well beyond the 20 per cent reduction seen in El Niño springs.
Why La Niña is so highly favoured in 2024
So why is NOAA so confident? The bullish prediction is from a combination of model forecasts, observations, and historical tendencies — all three currently indicating a rapid shift is likely to La Niña during the coming months.
While model accuracy is lowest early in the year what has caught meteorologist's eyes during the past fortnight is the first clear evidence of a transition — a blob of cold water appearing off west of the Galapagos Islands. A cold pool of water surfaced this week west of the Galapagos Islands, a sure sign El Niño is on the way out.
The cold pool was caused by upwelling, breaking through the surface warm water associated with El Niño, then spreading laterally to the west along the equator.
Further upwelling of cold water below the surface should ensure El Niño’s decay accelerates during the coming months by eroding the warm equatorial waters responsible for changes in the overlying weather patterns.
Even a decaying El Niño can promote rain
Analysing the graph above of El Niño and La Niña impacts on Australia reveals a few unexpected results, including a boost to autumn rainfall in the year after El Niño develops.
So if history tells us to expect above average rainfall during a decaying El Nino, which we are in right now, why do the BOM's seasonal forecasts favour drier weather this autumn?
The tail end of El Niño can produce a wet autumn and the next week has the potential to bring widespread rain.(ABC News)
The BOM's ACCESS model appears to be struggling to predict El Niño's demise as quickly or as accurately as other leading models, and it's one of the only models which doesn't tip La Niña by August.
As a result, we could see rapid shifts in the BOM's outlooks to favour wetter than average weather during the coming weeks and months.
Even during the next week modelling is showing heavy rain from tropical lows and cyclones across Australia's north and moderate falls across large parts of NSW.
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