Scientists Uncover Origin of Southwest Monsoon – Mother Jones

Gene Lower/ZUMA Press

This story was originally published by High Country News and is reproduced here as part of it Climate office cooperation.

One evening at the end of June, Alex Jimenez, artist at Tucson Water, hosted an outdoor art installation designed to “invite rain through sound.” The Santa Cruz Sound Experience, held under one of the bridges spanning the dry Santa Cruz River, featured a three-hour sensory collection of the region’s seasonal summer rains. Towards the end of the event, the heavens answered the call and attendees celebrated as raindrops fell.

Monsoon season has come again in the southwest. But this season is different from previous monsoons: It’s the first since scientists demonstrated that the North American monsoon—which drenches Sonora, northern Sinaloa and northeastern Chihuahua in Mexico, and the southern fringes of Arizona and New Mexico—is different from seasonal rains in the rest of the world. And, unfortunately for southwesters – who welcome the rain and need a break from the summer heat – the effect is likely to wane as the climate warms.

Monsoons, present on every continent except Antarctica, are continental-scale wind patterns that transport water vapor and cause seasonal rain. Generally, they occur when intense sunlight during summer causes the earth to heat up. Warm air rises and draws water vapor from the ocean, creating a “thermal contrast between the land and the nearby ocean and a circulation of air between the two,” explained William Boos, a climate scientist at the University of California, Berkeley.

Scientists and lay observers have long believed that the North American monsoon was also caused by this “thermal forcing,” with cooler water vapor drawn from the Pacific off the west coast of Mexico. For Boos, however, something about the North American monsoon, which is smaller and more oddly shaped than its peers, has “always been a little odd.”

In 2021, Boos and Salvatore Pascale, who researches climate dynamics at the University of Bologna in Italy, published an article in the journal Nature which showed that the Southwest’s summer storms were not caused by typical heat stress. Rather, they were caused by what scientists call “mechanical forcing,” which has to do with the soil. When the mid-latitude jet stream – the belt of easterly winds that circles the entire planet – collides with the Rocky Mountains, the region deflects the winds south into Mexico. As the winds move east, they push over Mexico’s Sierra Madre after picking up water vapor from the eastern Pacific and tropical Mexico. Then, when the jet rises, forcing moisture-laden air over the mountain terrain, the vapor condenses into “orographic rain” that falls on the western side of the mountains, creating the monsoon.

“The orographic effect is extremely important, especially in terms of what’s going to happen with climate change,” said scientist Agustin Robles of the Sonora Institute of Technology’s Environmental Modeling and Sustainability Laboratory. “We’ll see most of the changes there.”

There’s a simple reason why scientists didn’t already understand the role of geology in creating the monsoon: The technology to do it didn’t exist. While the Tibetan Plateau is so large that it could be modeled for its effect on climate starting in the 1980s, the Sierra Madre was too small and too thin for computers to render accurately until recently. Boos and Pascale used a state-of-the-art supercomputer to compare a model of the region’s topography with a version in which they set all landscape elevations to zero. Since this version essentially flattened Mexico, they called it “FlatMex.” In FlatMex, the monsoon disappeared entirely, leading to the conclusion that the North American monsoon is generated by wind passing over the Sierra Madre.