Black summer bushfires may have caused rare ‘triple dip’ La Niña, study suggests
Smoke from Australia’s 2019-20 black summer fires may have resulted in the rare “triple dip” La Niña that lasted from 2020 to 2022, research suggests.
Modelling from scientists at the US National Center for Atmospheric Research has found that smoke aerosols from the bushfires interacted with clouds to cool surface waters over the south-eastern subtropical Pacific Ocean.
This created favourable conditions for a La Niña to form, the researchers believe.
The research, published in the journal Science Advances, could be used to improve climate cycle predictions in future.
The study’s first author, Dr John Fasullo of the National Center for Atmospheric Research, said that although the bushfire smoke only persisted in the atmosphere for several months, it triggered a long-lasting feedback loop. “We did not expect such a strong planetary-scale signature with these fires,” he said.
Smoke particles resulted in smaller cloud droplets over the southern hemisphere, Fasullo said. “That makes them brighter, and it also makes them live longer [because they are less likely to rain out of the atmosphere]. The net effect of that is more sunlight gets reflected back to space.”
The result was a phenomenon akin to what’s known as June gloom in the US, “where you have very cold ocean conditions but very warm atmospheric conditions above this cloud layer – that’s referred to as an inversion,” Fasullo said.
“Winds carried these anomalous conditions into the deep tropics, and that instigated the feedbacks that are commonly associated with La Niña,” he said. “The winds became stronger as a consequence of the surface cooling off, and then the flow going into the tropics was also drier, because a cold surface means you have less humidity in the atmosphere.”
“Because of that, the whole band of precipitation that usually exists in the tropics move northwards, and that’s a critical component for getting a La Niña.”
East-to-west equatorial trade winds strengthen in the Pacific during a La Niña, increasing the chances of above-average rainfall in the spring and summer over much of Australia and south-east Asia. It has the inverse effect in other regions, such as in the southern US where the chance of drought is increased.
Cooler surface temperatures over the south-eastern Pacific persisted until 2022.
“We were surprised by how long the cooling lasted even with knowledge of these feedback loops,” Fasullo said.
He said the research identified an opportunity to improve El Niño-Southern Oscillation (ENSO) forecasts, which predicts seasonal La Niñas and El Niños. “Currently, there’s no forecast system that actually considers wildfire emissions, but there is the potential to include these … and improve our seasonal predictions,” he said.
“With climate change, these fires are going to become bigger, more intense and longer lasting,” he said. “Obviously it’s a huge negative to have such a strong, impactful fire, but it does provide a source of predictability perhaps.”
Dr Agus Santoso of the University of New South Wales, who was not involved in the study, said that triple La Niñas were “quite a rare event” and that the research provided a possible explanation for the most recent phenomenon which began in 2020.
Santoso, a senior research associate at the UNSW Climate Change Research Centre, added: “Sometimes the La Niña can be prolonged if the El Niño [preceding it] was very strong. But the recent [triple] La Niña … was preceded by weak El Niños in 2018 and 2019.”
He added: “In 2020, the forecast for La Niña was not very reliable. It was not clear whether we were going to have a La Niña or not.”
Incorporating wildfires into ENSO prediction systems could be beneficial, Santoso said, adding: “it seems like it is only when we have really strong bushfires that the mechanism can work”.
Though the 2019-20 bushfires ejected smoke aerosols high into the stratosphere – the second layer in Earth’s atmosphere – the 2020 La Niña resulted from interactions between smoke and clouds in the troposphere, the lowest layer of the atmosphere, Fasullo said.