Scientists Unravel Origin of Southwest Monsoon – Mother Jones

Gene Lager/ZUMA Press

This story was originally published by Highland news and is shown here as part of the Climate counter collaboration.

One evening at the end of June, Tucson Water’s artist-in-residence Alex Jimenez hosted an outdoor art installation designed to “call the rain through sound.” Held under one of the bridges crossing the dry Santa Cruz River, the Santa Cruz Sound Experience featured a three-hour sensory compilation of the region’s seasonal summer rains. Towards the end of the event, the sky answered the call and those in attendance celebrated as the raindrops fell.

The monsoon season has arrived again in the southwest. But this season is unlike previous monsoons: It’s the first since scientists showed that the North American monsoon — which drenches Sonora, northern Sinaloa and northeastern Mexico’s Chihuahua, and the southern fringes of Arizona and New Mexico — differs from seasonal rains in the rest of the world. And unfortunately for southwesterners — who welcome the precipitation and need a break from the summer heat — the phenomenon is likely to weaken as the climate warms.

Monsoons, which occur on every continent except Antarctica, are continental-scale wind patterns that transport water vapor and cause seasonal rains. They generally occur when intense sunlight in summer causes the land to warm up. Warm air rises and draws in water vapor from the ocean, creating a “thermal contrast between the land and the nearby ocean, and an air circulation between the two,” explains William Boos, a climate scientist at the University of California, Berkeley.

Scientists and lay observers alike have long believed that the North American monsoon was also caused by this “thermal forcing,” drawing in cooler water vapor from the Pacific Ocean off Mexico’s western coast. For Boos, however, there was something about the North American monsoon, which is smaller and odder-shaped than its ilk, “always a little quirky.”

In 2021, Boos and Salvatore Pascale, who researches climate dynamics at the University of Bologna in Italy, will published an article in the magazine Nature which showed that the summer storms in the southwest were not caused by the typical thermal forcing. Rather, they were caused by something scientists call “mechanical forcing,” which has to do with terrain. When the mid-latitude jet stream — the band of easterly winds that circles the entire planet — collides with the Rocky Mountains, the range deflects the wind south, toward Mexico. As winds move east, they push over Mexico’s Sierra Madre, after collecting water vapor from the tropics of the eastern Pacific and Mexico. Then, as the jet stream lifts and forces moisture-laden air over mountainous terrain, the vapor condenses into “orographic rain” that falls on the western side of the mountains, creating the monsoon.

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

There’s a simple reason why scientists hadn’t yet discovered the role of geology in creating the monsoon: the technology to do this didn’t exist. While the Tibetan Plateau is so large that it could be modeled from the 1980s for its effect on climate, until recently the Sierra Madre was too small and fine for computers to represent accurately. Boos and Pascale used a state-of-the-art supercomputer to compare a model of the region’s topography with one in which they zeroed out all the elevations of the landscape. Since that version basically flattened Mexico, they called it “FlatMex.” In FlatMex, the monsoon virtually disappeared, leading to their conclusion that the North American monsoon is created by winds passing over the Sierra Madre.

The recent research built on previous studies of the North American monsoon. A few years ago, Pascale, Boos and six other collaborators published an investigation that challenged the idea that climate change will increase precipitation in North America.

“There’s a classic idea that as the air gets warmer, it can hold more water vapor, so it will supply more water to the continent,” Boos said. While that may be true for other monsoons — including the Southeast Asian monsoon, which has already gotten wetter — it’s different in regions like the Southwest, where most of the rainfall comes from thunderstorms and the cumulus clouds associated with them. Thunderstorms are caused by a difference in temperature and humidity of the air near ground level and the air higher up in the atmosphere. Once the difference between the two air temperatures reaches a certain level, they turn around and switch places. Gravity causes the warmer, less dense air to rise and the colder, denser air to sink. But as the upper atmosphere warms, there’s less difference between the two temperatures — meaning fewer thunderstorms and a weaker monsoon.

Communities in the Southwest, already dealing with increasing drought and extreme heat, will need to improve both air quality and the infrastructure that guarantees their access to water, as well as find ways to cope with more days of high temperatures. . Unfortunately, there are “not many options” for dealing with dwindling summer rains, said Dan McGregor, natural resources manager for Bernalillo County, New Mexico. His office mainly encourages water users to conserve water, maintain their wells and collect rainwater.

In the Southwest, these effects will disproportionately affect those directly dependent on the rain. Sheryl Joy, Acting Seed Bank Manager at the Native Seeds/SEARCH organization in Tucson, said that for Native Arizona communities that have developed farming systems organized around the summer rains, “a sustained decline in monsoon rains could have devastating effects” on the communities that receive them. continue to use practices.

In Sonora, Mexico, where the most monsoon precipitation falls, there is less infrastructure to deal with water shortages than in the southwestern US. “Unlike Arizona or California, which have long-term planning and responses like the Tier 1 deficit announcements, our institutions here have not anticipated the effects of the weaker monsoon,” Robles said. “They tend to blame the drought when it’s really an alteration of the monsoon of the past 30 or 40 years.”

Jonah Ivy of Tucson’s Watershed Management Group focuses on helping residents use the falling water instead of wasting it as a drain. “What does a weaker monsoon matter if we’re pushing all the water off our landscapes right now?” he said. “Even with a weaker monsoon, we still live in the wettest desert in the world. We still live in abundance.”

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