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'Autumn predictability barrier' means April is the toughest month to make climate forecasts

Friday April 2, 2021 - 21:24 AEDT

After the recent record rain and the previous summer's record dry, what does this winter's climate have in store? 


The Bureau of Meteorology is predicting April to June to be likely wetter than average across the north of Australia and drier than average for the southern half of Queensland, inland parts of NSW and Victoria, and far eastern SA.


But making seasonal forecasts at this time of year is difficult, due to what forecasters call the "autumn predictability barrier". 


"At this time of year, we're probably at the lowest point at the moment in terms of skill in predicting seasonal drivers such as El Niño, La Niña, or the Indian Ocean Dipole," said Felicity Gamble, a climatologist from the Bureau of Meteorology.


Climate reset switch


Dr Gamble said that in March and April, the world's climate often underwent a global "reset", where El Niño/La Niña (ENSO) and the Indian Ocean Dipole returned to neutral, making it harder to predict what the upcoming season would bring. 


"This is what we've just seen with this most recent La Niña, which just returned to neutral values," Dr Gamble said.


"It's also the time when we start to see early signs of the next climate-driver event developing. Often it starts to show a few little signs at this time of year before becoming more established during the winter months," she said.


In other words, at this time of year, the crucial ocean temperatures that drive the Australian climate could go either way, and small weather events in the tropics can lead to big climate impacts later in the year.


"It's like a reset switch. And we're starting again with the next cycle," Dr Gamble said.


Cyclone knocks last winter's outlook


Last autumn, the Bureau of Meteorology forecast a wetter than average winter after most climate models predicted the Indian Ocean Dipole (IOD) would move into its rain-bringing negative phase.


But then in May, a late-season tropical cyclone off the north-west of Australia cooled sea temperatures enough to switch the IOD into its drying, positive phase. 


"We went from predicting a particularly wet period during May, June, July to suddenly seeing that outlook change to more of a dry outlook. And that's ultimately what we ended up seeing," Dr Gamble said.


On the other hand, once La Niña was established last spring, the Bureau of Meteorology was able to make confident predictions of a wetter than average summer, which much of the country experienced.


Climate in neutral gear


Right now, Australia's two big climate drivers, ENSO and the IOD, are in neutral.


At times like this, Australian weather becomes more influenced by shorter-term drivers like the Madden-Julian Oscillation, or MJO, which mainly affects northern Australia.


"The MJO is going to be the strongest driver of our rainfall patterns over the next few weeks as it comes into the Australian region," Dr Gamble said. 


In fact, weather models are predicting the development of a low off the north-west of Australia over the coming days, as the MJO influences northern Australia.


Beyond this, the Bureau of Meteorology said May showed a mostly neutral climate signal, with some areas of the east likely to be drier than average.


Climate secrets deep in the ocean


Researchers in Australia are working to break through the autumn predictability barrier, in part by better understanding ocean temperatures below the sea surface.


El Niño and La Niña events can be first detected in changes in sub-surface water temperatures, according to Harun Rashid, a principal research scientist with the CSIRO Climate Science Centre.


"If we include the sub-surface temperature in climate models, for example the ocean temperatures at 300 metres' depth, as well as the surface temperature, then you have less of a predictability barrier," he said.


At the Bureau of Meteorology, senior research scientist Andrew Marshall said it was hard to make long-term predictions at this time of year because the oceans had not yet coupled with the atmosphere.


"By coupled, I mean the atmosphere and the ocean working together," he said.


"For example, when the ocean starts to warm, then the atmosphere above that ocean will respond. And then that can reinforce the warming in the ocean.


"It's really once the atmosphere and ocean couple, that we then we can better understand how these events will evolve. And we then have better prediction skill, and typically that occurs from about May onwards."







- ABC

© ABC 2021

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