Aerosols add a new wrinkle to climate change in the tropical Pacific Ocean

A new study from Yale suggests that aerosols in the atmosphere could temporarily maintain ocean temperatures in the eastern equatorial Pacific.

The results, published in the journal Nature Climate Change, indicate that the surprisingly modest warming observed in the tropical Pacific in recent decades may be short-lived, with more dramatic changes to come. The results can also help climatologists make better predictions of how global warming will affect weather conditions, ecosystems and the impacts of storms all around the Pacific Rim.

“Understanding how the tropics change due to global warming is an important task for scientists, as this region is a key driver of weather and climate events around the world,” said first author Ulla Heede, graduate student at the Department of Earth. & planetary science at Yale.

The main author of the study is Yale climatologist Alexey Fedorov, professor of oceanic and atmospheric sciences in the Faculty of Arts and Sciences. Fedorov’s work studies global weather systems and phenomena such as the Atlantic overturning meridian circulation, one of the largest water circulation systems on the planet, and El Niño events.

In the new study, Heede and Fedorov focus on the eastern equatorial Pacific Ocean, a region characterized by cold water rising to the surface, driven by surface winds called trade winds. The periodic release of these winds causes El Niño, the warm phase of the El Niño Southern Oscillation, characterized by unusually warm waters in the Pacific.

If the eastern Pacific is warming faster than the surrounding oceans, this signals a slowing of the atmospheric tropical circulation known as the Walker circulation, which manifests itself in a reduction in both the trade winds and the amount of cold water. rising from the ocean depths.

“A slowdown in tropical circulation would lead to changes in El Niño events and the tropical rain belt,” Fedorov said. “These changes would affect societies across the tropics and beyond.”

But this slowdown has not yet happened, despite numerous climate model projections. Indeed, the region has warmed less than the surrounding areas and even cooled over the past decades, while the tropical atmospheric circulation has strengthened. This begs the question: is it just natural climate variability or are other factors to blame?

According to the new study from Yale, these were atmospheric aerosols, tiny particles emitted by multiple sources, including human activities.

“It is known that aerosols resulting from air pollution and combustion have a cooling effect on the Earth’s climate, and that aerosols have partially offset some of the warming effects of greenhouse gases since l ‘pre-industrial era,’ Heede said. “We show in our study that aerosols probably contribute to the delay of Eastern Equatorial warming and the slowing of tropical circulation, which would otherwise have occurred.”

Heede added: “It is important to keep in mind that this delay is temporary. In the future, as greenhouse gas emissions increase further, they will become the dominant factor in the tropical climate of the Pacific. likely leading to further warming of the eastern Pacific. “

The delay in warming east of the equator is likely to continue for several decades, the researchers said.

Heede and Fedorov said there may also be a thermostat-like mechanism operating in the tropical Pacific that is helping delay warming. Upwelling of cold water in the eastern equatorial Pacific due to the trade winds. With global warming, the deeper ocean warms more slowly than surface waters, helping to delay the warming of the eastern equatorial Pacific, while the temperature in the western equatorial Pacific continues to rise. These two factors reinforce the trade winds – as these winds are critically dependent on the east-west ocean temperature contrast – and keep relatively cooler waters rising until the deeper ocean warms enough to overcome this effect. .

For the study, the researchers worked with datasets from the Coupled Model Intercomparison Project (CMIP6), an international long-term climate research program, using a compilation of 40 different climate models. The researchers performed climate simulations of historical periods in the past and also projected future climate scenarios.