UC Berkeley study reveals unique cause of North American monsoon

photo of a monsoon storm
John Fowler/Creative Commons
A UC Berkeley study found that, unlike other monsoons, unique geographical features create the North American monsoon.
John FowlerCreative Commons Attribution 2.0 Generic License.

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A UC Berkeley study found unique geographical features that differentiate the North American monsoon, which drenches Mexico and the southwestern United States with heavy rainfall during the summer, from other monsoons.

Campus earth and planetary sciences associate professor 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 science journal Nature on Nov. 24, overturned the “longstanding idea” that the North American monsoon is caused by land-ocean heat contrast.

“The North American monsoon has a structure that is peculiar compared to all of the others, and we were curious about why,” Boos said in an email. “We’ve shown that the phenomenon … fundamentally doesn’t work the way we thought it did.”

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

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

“The North American monsoon, in particular, is crucial for the hydrology of western Mexico and the southwestern US,” Pascale said in an email.

Pascale noted the lack of understanding about the North American monsoon, in particular, is partially due to its size: The resolution of global climate models, like those used in the study, make it difficult to see detail on a small monsoon.

In addition to the simulation, the researchers also used a nonlinear stationary 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 email.

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

While the study overturns the prior scientific understanding of the North American monsoon, the researchers also aim to place new emphasis on jet stream interactions with mountain ranges. In addition, Boos and Pascale hope to expand research on monsoons and rainfall patterns, now that its “fundamental science” is refined.

“The future of the North American monsoon under human-caused global warming is very challenging and still quite uncertain,” Pascale said in the email. “We hope our study will help us to understand that better.”

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