One night in late June, Tucson Water’s artist-in-residence Alex Jimenez hosts an outdoor art installation designed to “call rain through sound.” Held under a bridge across the dry Santa Cruz River, the Santa Cruz Sound Experience features a three-hour compilation of the senses, the region’s seasonal summer rainfall. Towards the end of the event, the sky answered the call and attendees celebrated as the raindrops fell.
The southwest monsoon season is here again. But this season is different from past monsoons: It’s the season since scientists have shown that the North American monsoon (covering Sonora, northern Sinaloa and northeastern Chihuahua in Mexico, as well as the southern fringes of Arizona and New Mexico) differs from other regions For the first time since sex rains a different world. And, unfortunately for Southwesterners who love precipitation and need to escape the summer heat, this phenomenon may wane as the climate warms.
Present on every continent except Antarctica, monsoons are continental-scale wind patterns that transport water vapor and cause seasonal rainfall. Generally, they occur in summer when intense sunlight causes the land to heat up. Warm air rises and absorbs water vapor from the ocean, creating “a thermal contrast between the land and the nearby ocean, and the air circulation between the two,” explains William Booth, a climate scientist at the University of California, Berkeley.
Scientists and lay observers alike have long believed that the North American monsoon is also caused by this “thermal forcing,” in which cooler water vapor is drawn in from the Pacific Ocean off Mexico’s west coast. For Booth, however, the North American monsoon, which is smaller and more oddly shaped than other regions, is “always a little odd.”
In 2021, Boos and Salvatore Pascale, who study climate dynamics at the University of Bologna, Italy, published an article in the magazine nature This suggests that summer storms in the Southwest are not caused by typical thermal forcing. Instead, they’re caused by what scientists call “mechanical forcing,” which is terrain-related. When the mid-latitude jet stream (the belt of easterly winds that surrounds the planet) collides with the Rocky Mountains, that range deflects winds southward, reaching Mexico. As the winds move east, they push through Mexico’s Sierra Madre after collecting water vapor from the eastern Pacific and Mexico’s tropics. Then, as the jet stream rises, pushing moisture-rich air up the mountains, the vapor condenses into “terrain rain” that falls on the west side of the mountains, creating the monsoon.
“Topographic influences are critical, especially in terms of what’s going to happen with climate change,” said Agustin Robles, a scientist in the Environmental Modelling and Sustainability Laboratory at the Sonora Institute of Technology. “We’re going to see most of the changes there.”
There’s a simple reason scientists haven’t figured out the role of geology in creating monsoons: The technology to do so doesn’t exist. While the Tibetan Plateau is so large that its impact on climate has been modeled since the 1980s, until recently the Sierra Madre was too small and detailed for computers to render accurately. Boos and Pascale used an advanced supercomputer to compare the terrain model of the area with their model that set the elevation of all landscapes to zero. Since this version literally razed Mexico to the ground, they called it “FlatMex”. In FlatMex, the monsoon was nearly gone, leading them to conclude that the North American monsoon was caused by winds passing through the Sierra Madre.