One evening in late June, Tucson Water-based artist Alex Jimenez hosted an outdoor art show designed to “call the rain through sound.” Held under one of the bridges that cross the dry Santa Cruz River, the Santa Cruz Sound Experience included a three-hour sensory compilation of the area’s summer monsoon rains. Toward the end of the event, the sky answered the call, and those present celebrated as raindrops fell.
The monsoon season is back in the southwest again. But this season is different from past monsoons: It’s the first since scientists have demonstrated that the North American monsoon — which sinks Sonora, northern Sinaloa, northeastern Chihuahua in Mexico, the southern side of Arizona and New Mexico — differs from monsoons in the rest of the world. Unfortunately for Southwesters—who welcome rainfall and need a break from the summer heat—this phenomenon is likely to weaken as the climate warms.
Monsoons, which are found on every continent except Antarctica, are continental-scale wind patterns that transport water vapor and cause seasonal rains. Generally, they occur when intense sunlight during the summer heats up the earth. William Boss, a climatologist at the University of California, Berkeley, explained that warm air rises and pulls water vapor from the ocean, creating a “thermal contrast between the land and the nearby ocean, and air circulation between the two.”
Scientists and casual observers have long believed that the North American monsoons are also caused by this “thermal effect,” with cold water vapor drawn from the Pacific Ocean off the west coast of Mexico. But for Boss, something about the North American monsoon, which is smaller and more exotic than its counterparts, was “always a little weird.”
In 2021, Boos and Salvatore Pascale, who research climate dynamics at the University of Bologna in Italy, Publish an article in the magazine temper nature which showed that summer storms in the southwest were not caused by the typical thermal forcing. Instead, they were caused by something scientists call a “mechanical effect,” which has something to do with the terrain. When the mid-latitudes jet stream – the set of eastward-flowing winds that circles the entire planet – collides with the Rocky Mountains, the range deflects winds southward, into Mexico. As the winds move east, they push over the Mexican Sierra Madre, after collecting water vapor from the tropics in the eastern Pacific and Mexico. Then, as the jet stream rises, causing moisture-laden air to rise over the mountainous terrain, the steam condenses into “orographic rain” that falls on the western side of the mountains, triggering the monsoons.
“The orographic impact is very important, especially in terms of what will happen with climate change,” said scientist Agustin Robles of the Sonora Environmental Modeling and Sustainability Laboratory Technological Institute. “We’ll see the bulk of the changes there.”
There’s a simple reason scientists don’t already know geology’s role in creating the monsoons: The technology to do so didn’t exist. While the Tibetan Plateau is too large to model according to its impact on climate starting in the 1980s, the Sierra Madre was too small and too magnificent for computers to accurately display until recently. Boos and Pascale used a sophisticated supercomputer to compare their terrain model to a version in which they set all landscape heights to zero. Since this copy actually flattened Mexico, they called it “FlatMex.” At FlatMex, the monsoons have almost disappeared, which leads them to conclude that the North American monsoons are caused by winds passing over the Sierra Madre.