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Tropical Climate Change Is a Puzzle—Could Aerosols Be a Piece?

The climate in the tropical Pacific can be fickle. Alexey Fedorov can attest; when he began his career in climate dynamics in 1997, it was the strongest El Niño year on record. The climatic changes contributed to both massive flooding and droughts worldwide. The expectation at the time was that future El Niño events would become even stronger, but El Niño has been relatively quiet since then.

The eastern tropical Pacific (ETP) stretches from the southern tip of the Baja California Peninsula to northern Peru. Credit: Gabriela Agurto, Karla Belén Jaramillo, Jenny Antonia Rodríguez, and Elizabeth Andrade, CC BY-SA 4.0

Because El Niño is one temporary extreme of the natural oscillation in tropical Pacific water temperatures and atmospheric circulation, scientists have tried to predict how climate change will affect the average conditions in the tropics and, in turn, the extremes. Climate models have projected long-term warming of the eastern tropical Pacific Ocean—a region that stretches from the southern tip of the Baja California Peninsula to northern Peru—and weakening of the atmospheric circulation above it, both of which are characteristics of El Niño. But throughout the satellite era, researchers have observed an opposite trend.

“It’s one thing if the models get the trend right but the magnitude off. But when they are the opposite, it’s not great,” said Ulla Heede, a Ph.D. student at Yale University.

In a paper published in July in Nature Climate Change, Heede and Fedorov, now a professor of ocean and atmospheric sciences at Yale, showed that a combination of atmospheric aerosols and a thermostat-like mechanism might be keeping the eastern tropical Pacific cooler than expected. They also said the effect is temporary.

Investigating an “Ocean Thermostat”

An important feature of the tropical Pacific is the zonal atmospheric airflow called the Walker circulation. “In simple terms, it’s the trade winds,” said Fedorov. The trade winds blow warm surface water west, causing an upwelling of cold water in the eastern tropical Pacific and an east-west temperature gradient. The temperature gradient and Walker circulation are tightly linked.

“Over the past 30 years or so, if you look at the trends, you can see a very dramatic strengthening of the Walker circulation,” Fedorov said. “And this is not what the models predict.”

Researchers have thought the discrepancy between models and the observed strengthening might be due to natural climate variability or a built-in ocean thermostat that regulates the eastern tropical Pacific temperature. The latter could occur because the upwelled cold water in the eastern Pacific takes longer to warm relative to the warm surface waters in the western Pacific, which would strengthen the temperature gradient and corresponding Walker circulation.

Heede and Fedorov used 40 models from the Coupled Model Intercomparison Project Phase 6 to see whether the described ocean thermostat regulates eastern tropical Pacific temperature. When they simulated an abrupt carbon dioxide (CO2) increase in the atmosphere, they found that several models exhibited the ocean thermostat. “If [the models] had an ocean thermostat, they tended to have less eastern Pacific warming and less slowdown of the tropical circulation,” Heede said.

But when they ran simulations using the realistic historical emissions, the projections differed from the abrupt-CO2 simulation. The ocean thermostat might be contributing to the Pacific’s response to CO2, but something else in the emissions was having an impact.

Are Aerosols Responsible?

Because emissions consist of both greenhouse gases and aerosols, Heede thought aerosols could be to blame. Anthropogenic aerosols are harmful pollutants emitted as by-products of combustion that can have a cooling effect because they scatter solar radiation away from Earth’s surface. Aerosols dissipate from the atmosphere faster than greenhouse gases, which can last for centuries, so they tend to have the most potent effects close to where they’re produced. The far-reaching impacts of aerosols, likely acting through ocean and atmospheric circulation, are not as well understood. “I have to be honest,” Fedorov said. “Before Ulla started looking at aerosols, I didn’t think about it, because in the tropical Pacific you rarely think about aerosols’ dynamical role.”

With 12 models, the researchers could isolate the effects of greenhouse gases and aerosols. Greenhouse gas–only simulations projected warming in the Pacific similar to the abrupt-CO2 simulation, whereas aerosol-only simulations projected cooling. When mixed, “the aerosols, on average, tend to cancel out the warming that would otherwise have happened in the equatorial region,” Heede said.

Like Fedorov, Aaron Levine, a research scientist at the Cooperative Institute for Climate, Ocean, and Ecosystem Studies at the University of Washington who was not involved in the study, was surprised to see aerosols affecting the tropical Pacific. “I don’t see a lot of them in the Pacific,” said Levine, “but they’re strong in the Atlantic.”

Levine expects that the impact in the Pacific might be related to its connections to other oceans, particularly the Atlantic, and future studies should expand on this research by including global data from the models. “The paper really focused on the tropics,” he said.

“It’s Not Going to Stay Forever”

Although the models responded to aerosols differently, most of them projected eventual eastern tropical Pacific warming over several decades. “Where I think our paper really has something interesting to say is looking into the future,” Heede said. “No matter whether it was aerosols or the ocean thermostat that’s previously canceled out the warming, it’s not going to stay forever.”

As countries worldwide enact clean-air policies, the aerosols in the atmosphere will diminish, and so will their cooling effect. Without aerosols, warming could lead to more extreme weather events and warming of the planet.

“I think [aerosols are] another piece of the puzzle in terms of understanding future projections of El Niño and how El Niño is going to change,” said Levine.

—Andrew Chapman (@Andrew7Chapman), Science Writer



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