GEOGRAPHY

Wildfires can trigger a rare “triple dip” La Niña phenomenon


The soot from Australia’s deadly bushfires in 2019-2020 surprised the world.

IMAGE CREDIT: RICK RYCROFT/AP

In 2019, Australia experienced the deadliest wildfires on record in the country. The fires destroyed 190,000 square kilometers of land, killing dozens of people and about 1 billion animals, and destroying thousands of buildings. The smoke from the forest fires can be seen even in space.

Surprisingly, according to a new modelling study published May 10 in Science Advances, this smog may have triggered an important climate phenomenon in the other hemisphere: the rare 3-year La Niña in the tropical Pacific.

Samantha Stevenson, a climate scientist at the University of California, Santa Barbara, said the findings underscore the idea that fires and other events “can have a very strong impact on the climate that we don’t fully understand” and that predictions for La Niña and other seasonal events could improve if scientists can explain the causes of wildfires.

The La Niña phenomenon is when cold water deep off the coast of South America causes periodic cooling of the surface waters of the tropical Pacific Ocean. This shift affects weather around the world. The recent “three dips” La Niña phenomenon has not only brought flooding to Pakistan and eastern Australia, but has also exacerbated drought in Africa, where tens of millions of people are battling severe hunger and water scarcity.

Using NCAR’s climate model, John Fasullo, a climate scientist at the National Center for Atmospheric Research, who led the study, and colleagues simulated global atmospheric conditions from 2019 to 2021. In the first group, the model calculated smoke by estimating emissions from wildfires. In the second group, the model combined specific satellite data on wildfire smoke, much of it from Australia.

Past studies have shown that gas plumes from volcanic eruptions can create La Niña-like conditions by reflecting sunlight and cooling surfaces. But volcanic aerosols tend to stay in the atmosphere longer and float higher than wildfire smoke, prolonging their effects. As a result, the team believes that the effects of smoke on La Niña will pale in comparison. “But we were wrong.” Fasullo said.

Next, this cooler air changes the position of the Tropical Convergence Zone (ITCZ), a rain belt normally located on the equator where easterly winds from the northern and southern hemispheres collide. Cold air pushed ITCZ northward, strengthening easterly winds over the tropics. These winds push warm surface water westward, helping cold water rise from depths, further cementing La Niña.

Robert Jnglin Wills, a climate scientist at ETH Zurich who was not involved in the study, said that while wildfires appeared to significantly affect La Niña in simulations, none of the models were perfect.

Michelle L’Heureux, a climate scientist at the National Oceanic and Atmospheric Administration’s Center for Climate Prediction, said testing multiple models would help validate the findings, especially those used for real-time predictions. It’s hard to separate the impact of wildfires from the impact of another unexplained trend that has been pushing the tropical Pacific towards La Niña since the ’70s, she said. “We may just be seeing two factors. But we still need to understand how much impact each factor has. L’Heureux said.

Fasullo’s team is testing the model with other La Niña phenomena that may have been affected by wildfires. He said it may not be a coincidence that one of the other 3 recorded La Niña events since 1950 coincided with another massive bushfire in Australia in 1974-1975.

Fasullo said the study also highlights the need for seasonal forecasting and climate models to properly interpret wildfires. A 2022 UN report found that the likelihood of catastrophic fires increased by 57% by the end of the century due to climate change and land-use change. Fasullo said unraveling the global impact of these regional events will be even more important in the future. “Wildfires can be a very important source of predictability, at least in some cases.” (Source: Li Huiyu, China Science News)

Related paper information:https://doi.org/10.1126/sciadv.adg1213



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