Extract from ABC News
Autumn's susceptibility to unpredictable weather has the likelihood of an El Niño hanging in the balance, according to climate scientists.
Key points:
- Autumn is the toughest season for predicting El Niño and La Niña
- The Bureau of Meteorology has issued an El Niño Watch for 2023
- A small, unexpected weather event during the season could stop it in its tracks
The country is currently on El Niño watch, which means there is a 50 per cent chance of it developing later this year, or about twice the normal likelihood.
The latest Australia Bureau of Agricultural and Resources Economics and Science report is already foreshadowing what might happen to global crop production if it comes to fruition.
The El Niño climate driver is the opposite of the wet-weather associated with La Niña. It instead has a reputation for bringing warmer, drier weather to Australia, and drought.
But the Bureau of Meteorology's climate updates have been accompanied with a clause.
"Long range forecasts made during early autumn typically have a lower accuracy than those made at other times of the year," BOM states.
CSIRO climate scientist Nandini Ramesh said it was because one unexpected weather event, such as a cyclone, could set off a chain reaction of events that could stop the development of an El Niño in its tracks.
The butterfly effect
The uncertainty is known as the "autumn predictability barrier".
It is related to chaotic systems, popularly described by the butterfly effect metaphor.
"[The idea is] the flap of a butterfly's wings is enough to set off a tornado somewhere on the other side of the world," Dr Ramesh said.
"It's used to describe our atmosphere.
"And what this means is that the system can be very sensitive, at times, to small perturbations."
Dr Ramesh said that during autumn a cyclone, or even a sudden burst of wind, could change conditions in the Pacific Ocean and atmosphere to the point that an El Niño event was kicked off or stopped.
"A sudden burst of wind or a sudden storm system passing through that changes the pattern of winds over the ocean can then change currents or temperatures over the ocean," she said.
She said these sorts of unpredictable weather systems could occur outside their typical seasons, but during autumn the Pacific Ocean was more sensitive to small changes.
Dr Ramesh likened the effect to a ball perched on top of a hill.
"A very tiny push is enough to send the ball rolling down either one side of the hill or another," she said.
"That's why it's so difficult to predict what's going to happen, even though we understand the physics behind these events fairly well.
"The Pacific Ocean and the air overhead are extremely sensitive to pushes in any direction from March to May.
"But once the ball rolls down one side rather than another, it's much easier to predict which way it will keep rolling."
This occurred during 2014 when a westerly wind burst caused cooler waters to flow into the south-eastern Pacific, eventuating in only a weak El Niño rather than the strong event predicted.
When will we know if it's El Niño?
The Bureau of Meteorology issues an alert once there is a 70 per cent chance of an El Niño developing.
But Dr Ramesh said it was very unlikely that kind of certainly would show until early winter — unless it was a very strong signal.
"One you've passed into June or July, the system has started to develop in whichever direction it's going to go," she said.
Dr Ramesh said "pre-conditioning" signs of El Niño could be observed at this time of year, which is how climate scientists were able to say there was a 50 per cent chance.
"Right now we've had an accumulation of warm water under the surface of the Pacific Ocean and that is necessary for an El Niño event to occur," she said.
Climate change brings uncertainty
La Niña and El Niño were natural phenomena that occurred before there were large-scale greenhouse emissions from human activity.
CSIRO research director Jaci Brown, from the Climate Intelligence program, said climate change was making the outcome of future El Niño's harder to predict.
The expectation of warmer, drier conditions in Australia during al El Niño is based on past observations but Dr Brown said this was not as useful anymore.
"The earth on which it's operating in is one that is warmer and holds more moisture in the atmosphere," she said.
"It's not clear exactly what effect this will have on El Niño in the future.
"We expect that a warmer world means that El Niño events will appear warmer and drier, and that the extra moisture holding capacity of atmosphere means that La Niña's might have heavier rainfall than they might otherwise."
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