October 5, 2022

Gene Lower/ZUMA Press

This story was originally published by High Country News and is reproduced here as part of it Climate office cooperation.

One evening at the end of June, Alex Jimenez, artist at Tucson Water, hosted an outdoor art installation designed to “invite rain through sound.” The Santa Cruz Sound Experience, held under one of the bridges spanning the dry Santa Cruz River, featured a three-hour sensory collection of the region’s seasonal summer rains. Towards the end of the event, the heavens answered the call and attendees celebrated as raindrops fell.

Monsoon season has come again in the southwest. But this season is different from previous monsoons: It’s the first since scientists demonstrated that the North American monsoon—which drenches Sonora, northern Sinaloa and northeastern Chihuahua in Mexico, and the southern fringes of Arizona and New Mexico—is different from seasonal rains in the rest of the world. And, unfortunately for southwesters – who welcome the rain and need a break from the summer heat – the effect is likely to wane as the climate warms.

Monsoons, present on every continent except Antarctica, are continental-scale wind patterns that transport water vapor and cause seasonal rain. Generally, they occur when intense sunlight during summer causes the earth to heat up. Warm air rises and draws water vapor from the ocean, creating a “thermal contrast between the land and the nearby ocean and a circulation of air between the two,” explained William Boos, a climate scientist at the University of California, Berkeley.

Scientists and lay observers have long believed that the North American monsoon was also caused by this “thermal forcing,” with cooler water vapor drawn from the Pacific off the west coast of Mexico. For Boos, however, something about the North American monsoon, which is smaller and more oddly shaped than its peers, has “always been a little odd.”

In 2021, Boos and Salvatore Pascale, who researches climate dynamics at the University of Bologna in Italy, published an article in the journal Nature which showed that the Southwest’s summer storms were not caused by typical heat stress. Rather, they were caused by what scientists call “mechanical forcing,” which has to do with the soil. When the mid-latitude jet stream – the belt of easterly winds that circles the entire planet – collides with the Rocky Mountains, the region deflects the winds south into Mexico. As the winds move east, they push over Mexico’s Sierra Madre after picking up water vapor from the eastern Pacific and tropical Mexico. Then, when the jet rises, forcing moisture-laden air over the mountain terrain, the vapor condenses into “orographic rain” that falls on the western side of the mountains, creating the monsoon.

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

There’s a simple reason why scientists didn’t already understand the role of geology in creating the monsoon: The technology to do it didn’t exist. While the Tibetan Plateau is so large that it could be modeled for its effect on climate starting in the 1980s, the Sierra Madre was too small and too thin for computers to render accurately until recently. Boos and Pascale used a state-of-the-art supercomputer to compare a model of the region’s topography with a version in which they set all landscape elevations to zero. Since this version essentially flattened Mexico, they called it “FlatMex.” In FlatMex, the monsoon disappeared entirely, leading to the conclusion that the North American monsoon is generated by wind 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 a study which challenged the idea that climate change will increase rainfall across North America.

“There’s a classic idea that as the air warms, it can hold more water vapor, so it will carry more water to the continent,” Boos said. While this may be true for other monsoons — including the Southeast Asian monsoon, which has already become wetter — it’s different in areas like the Southwest, where most of the rain comes from thunderstorms and the clouds associated with them. Thunderstorms are caused by a difference in temperature and humidity of air near ground level and air higher in the atmosphere. As soon as the difference between the two air temperatures reaches a certain level, they turn over, change positions. Warmer, less dense air rises and cooler, denser air sinks, due to gravity. But as the upper atmosphere warms, there is less difference between the two temperatures – and that means fewer storms 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 ensures their access to water, and will also need to find ways to cope with more hot days. Unfortunately, “there aren’t many options” to deal with declining summer precipitation, said Dan McGregor, director of natural resource services for Bernalillo County, New Mexico. His service mainly encourages water users to conserve water, maintain their wells and collect rainwater.

In the Southwest, these effects will disproportionately affect those who rely directly on the rains. Sheryl Joy, Associate Seed Bank Director at Native Seeds/SEARCH in Tucson, said that for Native communities in Arizona that have developed agricultural systems organized around summer rainfall, “the continued decline in monsoon rainfall could have devastating impacts’ on communities that continue to use these practices.

In Sonora, Mexico, where most of the monsoon rain falls, there is less infrastructure to deal with water shortages than in the US Southwest. “Unlike Arizona or California, which have long-term planning and responses, such as Tier 1 shortage announcements, here our institutions have not anticipated the effects of the weaker monsoon,” Robles said. “They tend to blame the drought, when it’s really a modification of the monsoon over the last 30 or 40 years.”

Jonah Ivy of Tucson’s Watershed Management Group focuses on helping residents use the water that falls, rather than wasting it as runoff. “What does a weaker monsoon matter if we’re currently pushing all the water off our landscapes?” 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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