January 18, 2022 10:16 pm

The orography and not the climate is conducive to the North American Monsoon

Deviation of the jet stream eastward (orange outlines) from mid-latitudes toward the equator, where it produces precipitation (blue shading, units of mm / day) as it ascends over the Sierra Madre mountain range (magenta). – BOOS, W. R., AND S. PASCALE


A study has shown that the North American monsoon is not caused by seasonal variation in climate like a typical tropical monsoon, but because of the Mexican orography that strongly influences it.

The new findings, published in the journal ‘Nature’, could have important consequences for evaluating the possible impact of climate change in the region.

Monsoons are not only found in South Asia, but are part of a global-scale circulation that affects almost all tropical regions (eg the Australian monsoon, the African monsoon, etc.). North America also has one, the North American monsoon, It affects western Mexico and the southwestern United States, in particular Arizona and New Mexico.

To date, this monsoon was considered similar to other monsoons, although smaller in size. However, new research by two scientists from the University of California at Berkeley (United States) and the University of Bologna (Italy) now provides a new perspective on the processes that drive their formation.

“The results of our study demonstrate that the North American monsoon does not originate in the seasonal oscillation of the ITCZ ​​(Intertropical Convergence Zone) over the continental masses like a typical monsoon. In fact, its origin represents a unique case, since it is strongly influenced by the Mexican orography, which plays a key role in the generation of a standing wave in the extratropical atmospheric circulation and in the diversion of the jet stream towards the western coast of Mexico. “

This is explained by Salvatore Pascale, one of the two authors of the study, a researcher at the Atmospheric Physics Group of the Augusto Righi Department of Physics and Astronomy of the University of Bologna and of the Center for Sustainability and Climate Change of the Business School of the University of Bologna.

“This new vision of the North American monsoon is relevant to understand how climate change can affect this monsoon and how rising temperatures can change the extent of these meteorological phenomena “, he adds.

Monsoons indirectly influence global atmospheric circulation. They also play a critical role in regulating the climate in many tropical areas, which often have dry winters and wet summers. They are essential to bring water to regions inhabited by billions of people. The North American monsoon is especially associated with heavy summer rainfall in an area that spans more than 1,000 kilometers and it plays a crucial role in the hydrology of western Mexico and the southwestern United States.

In the summer, monsoons are generally caused by the rapid warming of tropical land masses and the consequent transfer of energy to the upper atmosphere. In turn, this generates a circulation capable of producing heavy rains.

Until now, the North American monsoon was thought to originate in the same way. Thanks to a set of numerical simulations, the scientists analyzed the origin of this monsoon, demonstrating that the mountain ranges of the region and their interaction with the extratropical circulation are responsible for their formation.

These results have implications for the models and types of analysis used to dynamically predict the rains that the North American monsoon brings in the region, especially taking into account the consequences that climate change could have. in terms of droughts or extreme weather events.


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