UC Berkeley study reveals single cause of North American monsoon

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A UC Berkeley study revealed unique geographic features that differentiate the North American monsoon, which inundates Mexico and the southwestern United States with heavy precipitation during the summer, from other monsoons.

The campus associate professor of earth and planetary sciences, William Boos, authored the study alongside Salvatore Pascale, a junior assistant professor studying climate dynamics at the University of Bologna in Italy. According to Boos, the study, published in the scientific journal Nature on November 24, overturned the “long-held idea” that the North American monsoon is caused by the land-ocean heat contrast.

“The North American monsoon has a peculiar structure compared to all the others, and we were curious as to why,” Boos said in an email. “We have shown that the phenomenon… fundamentally does not work as we thought. “

Instead, the study explains how the monsoon precipitation in North America is caused when the western Sierra Madre mountains in Mexico deflect strong bands of wind, or jet currents, toward the equator. This causes the air to rise in the mountains, which lifts the warm, humid air to create precipitation.

This finding classifies the North American monsoon as a mechanically forced monsoon, rather than thermally forced like other tropical monsoons.

“The North American monsoon, in particular, is critical to the hydrology of western Mexico and the southwestern United States,” Pascale said in an email.

Pascale noted that the lack of understanding of the North American monsoon, in particular, is in part due to its size: the resolution of global climate models, like those used in the study, makes it difficult to read details on a small monsoon.

In addition to the simulation, the researchers also used a nonlinear standing wave model in the study to isolate the Sierra Madre mountains from influencing the simulated atmosphere.

“We used a lot of computing time – the equivalent of a laptop running for millions of hours – on one of the biggest supercomputers in the world at the Lawrence Berkeley National Lab,” Boos said in the e -mail.

However, Boos noted that despite the supercomputer’s capabilities, it took a lot of time and effort to perform the calculations and simulations and for the researchers to manage the data.

As the study reverses earlier scientific understanding of the North American monsoon, the researchers also aim to put a new emphasis on the jet stream’s interactions with mountain ranges. Additionally, Boos and Pascale hope to expand research on monsoons and rainfall patterns now that its “basic science” has been refined.

“The future of the North American monsoon under man-made global warming is very difficult and still quite uncertain,” Pascale said in the email. “We hope our study will help us better understand this. “

Contact Riley Cooke at [email protected], and follow her on Twitter at @rrileycooke.

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