Scientists Are Unraveling the Origins of the Southwest's Monsoon – Mother Jones
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This story was originally published by High Country News It is reproduced here as part of Climate office cooperation.
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.
The latest research builds on previous studies of the North American monsoon. A few years ago, Pascal, Boss, and six other collaborators published study that challenged the idea that climate change would increase precipitation across North America.
"There is a classic idea that as the air gets warmer, it can absorb more water vapor, so it will carry more water to the continent," Boss said. While this may be true for other monsoons—including the Southeast Asian monsoon, which is already getting wetter—it varies in regions like the Southwest, where most of the precipitation comes from thunderstorms and the associated cumulus clouds. Thunderstorms are caused by the difference in the temperature and humidity of the air near ground level and the air at the top of the atmosphere. As soon as the difference between the two air temperatures reaches a certain level, they flip, switching places. The hotter and less dense air rises and the cooler and denser air sinks due to gravity. But as the upper levels of the atmosphere warm, the difference between the two temperatures decreases – meaning fewer thunderstorms and weak monsoons.
Communities in the southwest, already facing increasing drought and extreme heat, will need to improve air quality and infrastructure that ensures they have access to water, and they will also have to find ways to handle more days in higher temperatures. Unfortunately, "there aren't a lot of options" for addressing lower summer precipitation, said Dan McGregor, director of Natural Resources Services in Bernalillo County, New Mexico. His agency mainly encourages water users to conserve water, maintain wells, and harvest rainwater.
In the southwest, these effects will disproportionately affect those who depend directly on rainfall. For indigenous communities in Arizona who have developed organized farming systems around summer precipitation, a "continuous decrease in seasonal rainfall can have devastating effects," said Cheryl Joy, acting seed bank manager for Native Seeds/SEARCH in Tucson. Communities that continue to use these practices.
In Sonora, Mexico, where most of the monsoon rains fall, there is less infrastructure to address water shortages than in the southwestern United States. "Unlike Arizona or California, which have long-term planning and responses such as level 1 shortage announcements, our institutions here did not anticipate the effects of weaker monsoons," Robles said. "They tend to blame the drought, when in fact it is an adjustment to the monsoons over the past 30 or 40 years."
Jonah Ivy of the Watershed Management Group in Tucson focuses on helping residents use the falling water, rather than wasting it as running water. "What is the significance of a weak monsoon if we are currently keeping all the water off our landscapes?" He said. "Even with the weak monsoon, we still live in the world's wettest desert. We still live in abundance."
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