Lightning strikes the desert in Florence, Arizona, during the North American monsoon in September 2021

John Sirlin / Alamy

The North American monsoon is caused by the flow of air over mountains, according to computer modelling. It may be one of the only monsoons to be triggered this way.

Monsoons typically occur in the summer when land rapidly warms, which transfers energy to the atmosphere above, creating air circulation patterns that can produce heavy rainfall.

William Boos at the University of California, Berkeley, wondered how much of a role the Sierra Madre Occidental mountain range in western Mexico might play in the North American monsoon, because mountains can deflect the path of winds. The North American monsoon hits Mexico and the south-western US every summer, bringing rain and thunderstorms between July and September.

To investigate, Boos and his colleagues ran global simulations of the atmosphere under two scenarios: one with normal geographical conditions and one with the Sierra Madre Occidental mountains flattened to 0 kilometres above sea level. The team also modelled the impact of mountain ranges on winds.

The group’s model suggests that the Sierra Madre Occidental mountains deflect the jet stream – a band of eastern moving air – southwards over the mountains, where it cools and condenses into clouds.

“The physics that governs the water supply to a vast region of North America, including much of Mexico and the US Southwest, is very different from what we thought it was,” says Boos. His team hopes the findings can be used to improve monsoon forecasts.

Journal reference: Nature, DOI: 10.1038/s41586-021-03978-2

More on these topics:

By admin

Leave a Reply

Your email address will not be published. Required fields are marked *