The formation process of the North American monsoon is unique in the world

25 November 2021

A new study published in Nature shows that this weather phenomenon does not originate as a common tropical monsoon but is strongly influenced by the presence of the Mexican orography. The news could have important consequences for assessing the possible effects of climate change in the region.

Monsoons are not exclusive to South Asia: they are part of a global-scale circulation that encompasses almost all regions in the Tropics (e.g., Australian monsoon, African monsoon, etc.), and there is also one in North America, the North American monsoon, which affects western Mexico and the southwestern United States (particularly Arizona and New Mexico).

Until recently, this monsoon was considered similar to other monsoons, albeit smaller in size. New research published in Nature and carried out by two scientists from the University of California, Berkeley (United States) and the University of Bologna now offers a new perspective on the processes that drive its formation.

“The results of our study show that the North American monsoon does not start as a common monsoon from the seasonal oscillation of the ITCZ (Intertropical Convergence Zone) on the continental masses, but its origin is strongly influenced by the Mexican orography, which plays a key role in generating a standing wave in the extratropical atmospheric circulation and diverting air currents in the direction of the west coast of Mexico: this is a unique case in the world,” explains Salvatore Pascale, one of the two authors of the study, a researcher in the Atmospheric Physics group of the “Augusto Righi” Department of Physics and Astronomy at the University of Bologna and the Centre for Sustainability and Climate Change of Bologna Business School. “This new insight into the nature of the North American monsoon is also important for understanding the possible effects of climate change on this monsoon, which are still quite uncertain at the moment, and how rising temperatures may alter the scale of these weather phenomena.”

Monsoons indirectly influence global atmospheric circulation and play a fundamental role in regulating the climate of most of the Tropics, characterized by dry winters and rainy summers. Their presence contributes to bringing water to regions inhabited by billions of people. The North American monsoon, in particular, is associated with intense summer rainfall over an area that extends more than a thousand kilometers from north to south and therefore plays a central role in the hydrology of western Mexico and the southwestern United States.

Generally, monsoons originate, in the summer season, from the rapid warming of tropical landmasses and the consequent transfer of this energy to the overlying atmosphere, generating a circulation capable of producing abundant rainfall: so far, the North American monsoon seemed to fall into this category. However, by analyzing the dynamics of its formation through a series of numerical simulations, the scientists have now shown that decisive for its formation are instead the mountain ranges in the region and their interaction with the extratropical circulation.

These results indicate the need to revisit the models and type of analysis used to predict rainfall brought by the North American monsoon to the region, especially with respect to the consequences that the effects of climate change could have in terms of drought or extreme weather events.

The study was published in Nature under the title “Mechanical forcing of the North American monsoon by orography”. The authors are William R. Boos of the University of California, Berkeley (USA) and Salvatore Pascale of the Department of Physics and Astronomy “Augusto Righi” (Atmospheric Physics Group) of the University of Bologna and the Centre for Sustainability and Climate Change of Bologna Business School.