This new knowledge of the North American monsoon is important for determining how climate change will effect the monsoon and how rising temperatures would influence the scope of these weather events.

Monsoons are not only seen in South Asia; they are also a part of a global circulation that impacts practically all tropical regions (e.g. Australian monsoon, African monsoon, etc.). The North American monsoon, which impacts western Mexico and the southwestern United States, particularly Arizona and New Mexico, is another.

Until now, this monsoon was thought to be similar to other monsoons, albeit smaller. However, recent research published in Nature by two scientists from the University of California, Berkeley (USA) and the University of Bologna sheds new light on the processes that cause it to form.

“The results of our study show that the North American monsoon does not originate from the seasonal oscillation of the ITCZ (Intertropical Convergence Zone) over continental masses like a typical monsoon. Indeed, its origin represents a unique case as it is strongly influenced by Mexican orography which plays a key role in generating a stationary wave in the extratropical atmospheric circulation and deflecting the jet stream towards the Mexican west coast,” says Salvatore Pascale, one of the two authors of the study.

“This new insight into the North American monsoon is relevant for understanding how climate change may affect this monsoon and how rising temperatures may change the extent of these weather phenomena.”

Monsoons have a negligible effect on global atmospheric circulation. They also serve a critical role in climate regulation in a number of tropical regions, which often experience dry winters and wet summers.

They play a critical role in supplying water to places populated by billions of people. The North American monsoon is most closely connected with widespread summer rainfall across an area of more than 1,000 kilometers, and it is critical to the hydrology of western Mexico and the southwestern United States.

Monsoons in the summer season are mainly caused by the rapid warming of tropical landmasses and the subsequent transfer of energy to the upper atmosphere. As a result, a circulation capable of producing torrential rains is created. Until recently, it was believed that the North American monsoon originated in a similar way. Scientists examined the origins of this monsoon using computer simulations, concluding that its creation is due to the region’s mountain ranges and their interaction with the extratropical circulation.

These findings have implications for models and analysis used to dynamically anticipate rainfall brought by the North American monsoon in the region, particularly in light of the potential repercussions of climate change in terms of droughts and 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) at the University of Bologna and the Centre for Sustainability and Climate Change at the Bologna University Business School.

Source: 10.1038/s41586-021-03978-2

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