A Looming El Niño Could Dry the Amazon

On paper that The Amazon rainforest is a static expanse: constantly wet, impenetrable, constantly teeming with biology. But in reality, the region endures periodic droughts when rains die down, trees become overstressed and wetlands dry up. Up and down. As with forests around the world, this is part of the natural order.

One of the reasons for droughts in the Amazon could be starting soon, potentially causing even more stress on an ecosystem already devastated by deforestation and fires caused by human activity. The El Niño-Southern Oscillation is a Pacific Ocean phenomenon in which a band of water that transitions from neutral to exceptionally cold or warm develops off the coast of South America. The cold “La Niña” conditions of recent years are weakening and may give way to warm “El Niño” conditions later this year, according to modeling by the National Oceanic and Atmospheric Administration. And for the Amazon, that can cause drought.

It’s still too early to tell when El Niño will hit and how severe it will end up being. But scientists remember how bad it got during the El Niño eight years ago. “In 2015-2016 we observed that the air temperature over Amazonia was perhaps the highest in the last century,” says Juan Carlos Jiménez-Muñoz, physicist and remote sensing specialist at the University of Valencia. “Especially about Amazonia [El Niño] suppresses rain, and widespread drought is generally expected.” But, Jiménez-Muñoz warns, “Every El Niño is different — they may have different regional or local impacts.”

That’s because El Niño changes the atmospheric circulation significantly. As this blob of warm water forms in the Pacific, it creates more evaporation and sends moist air up into the sky. This water eventually falls over the ocean as rain. This upsets the Walker Circulation and sends sinking, relatively dry air over the South American landmass, resulting in less rain over the Amazon. “In general, more rain falls on the ocean,” says Earth system scientist James Randerson of the University of California, Irvine. “It just doesn’t rain that much on the global mainland. The continents are losing water, especially South America.”

When El Niño is not active and conditions are normal, moisture evaporates from the Amazon and rises to the sky before falling on the forest as rain. The Amazon can recycle up to half of its rainfall this way. “The Amazon is an atmospheric moisture factory,” says Paola A. Arias, a climate scientist at the University of Antioquia in Colombia. “Typically when you have these drought events, you also have a reduction in this precipitation recycling.”

Because El Niños vary in strength, they differ in how much they suppress rain over the Amazon. They also differ in exactly where they cause droughts and for how long. If the development of an El Niño is more centered in the central Pacific, it tends to result in a drought centered in the northeastern part of the Amazon. If more concentrated in the eastern Pacific, the drought may be more widespread and last a little longer. But for 2023, it’s too early to say how all of this will pan out — Randerson says scientists should have a better idea this spring. “The fact that we’ve been in this sustained La Niña for so long,” says Randerson, “I think makes it more likely that you’re going to move into a stronger El Niño state.”

https://www.wired.com/story/a-looming-el-nino-could-dry-the-amazon/ A Looming El Niño Could Dry the Amazon

Zack Zwiezen

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