Jean Lore/Zuma Press
This story was originally written by highland news and here as climate desk Cooperation.
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.
“There’s a classic idea that as the air gets warmer, it can hold more water vapor, so it will send more water to the continents,” Booth said. While this may be true for other monsoons — including the Southeast Asian monsoon, which has become wetter — it’s different in regions like the Southwest, where most of the rain comes from thunderstorms and their associated cumulus clouds. Thunderstorms are caused by differences in temperature and humidity between the air close to the ground and the air higher in the atmosphere. Once the difference between the two air temperatures reaches a certain level, they flip over, swapping places. Due to gravity, hotter, less dense air rises and cooler, denser air sinks. But as the upper atmosphere warms, the difference between the two temperatures gets smaller — meaning fewer thunderstorms and weaker monsoons.
Communities in the Southwest, already facing increasing drought and extreme heat, need to improve air quality and the infrastructure to ensure their access to water, and they must also figure out how to cope with more days of heat. Unfortunately, Dan Greg, natural resource services manager for Bernalillo County, N.M., said “there aren’t many options” to address the reduced summer rainfall. His agency primarily encourages water users to conserve water, maintain wells and collect rainwater.
In the Southwest, these impacts will disproportionately affect those who depend directly on rainfall. Sheryl Joy, acting seed bank manager for Native Seeds/SEARCH in Tucson, said that for Arizona’s indigenous communities that have developed agricultural systems organized around summer rainfall, “the continued decline in monsoon rainfall could be devastating for these communities. Sexual Influence”. Continue to use these practices.
In Sonora, Mexico, where most of the monsoon rain falls, there is less infrastructure to address water shortages than in the American Southwest. “Unlike Arizona or California, which have long-term planning and responses, such as Tier 1 shortage announcements, our agency did not anticipate the impact of a weakened monsoon,” Robles said. “They tend to blame drought because it’s really a change in the monsoon over the past 30 or 40 years.”
Jonah Ivy of Tucson Watershed Management Group focuses on helping residents use falling water instead of wasting it as runoff. “If now we’re pushing all the water out of our landscape, what does a weaker monsoon matter?” he said. “Even though the monsoons are weaker, we still live in the wettest desert in the world. We still live abundantly.”
