Is the North American monsoon a true monsoon or just a seasonal weather pattern?
In short: Every summer, a dramatic shift in wind and moisture transforms the arid landscapes of the American Southwest and northwestern Mexico. While often called the North American monsoon, climatologists still debate whether this complex phenomenon qualifies as a true monsoon, given its unique characteristics compared to its massive Indian counterpart.
The North American monsoon is a seasonal weather cycle that brings intense thunderstorms to the southwestern United States and northwestern Mexico, typically occurring between June and mid-September. The process begins as intense solar heating warms the land, creating a thermal low-pressure trough over the Mexican Plateau and the Desert Southwest. This shift draws moist air from the Gulf of California and the eastern Pacific, while easterly winds aloft transport additional moisture from the Gulf of Mexico. These conditions, combined with the rugged topography of the Sierra Madre Occidental and other mountain ranges, trigger daily cycles of afternoon thunderstorms.
Despite its name, the classification of this event remains a subject of scientific controversy. Traditional definitions of a monsoon often require a complete 180-degree reversal of prevailing winds. In North America, the winds shift from westerly to southerly, which some climatologists argue is an incomplete reversal. Furthermore, the system is less persistent than the Indian monsoon, largely because the Mexican Plateau lacks the immense height and scale of the Tibetan Plateau. However, research culminating in 1993 led NOAA to formally recognize the existence of a summer monsoon over much of Mexico and the U.S. intermountain region.
The monsoon is highly variable, with precipitation levels often fluctuating more from year to year than the average seasonal rainfall itself. In Tucson, Arizona, for instance, recorded monsoon rainfall has ranged from as little as 1.59 inches to as much as 13.84 inches. This variability is driven by complex interactions between atmospheric circulation, sea surface temperatures, and the specific positioning of the subtropical ridge. When this ridge sits too close to a region, its sinking air can suppress storm development, leading to a 'break' in the monsoon. Conversely, when the ridge is positioned optimally, it facilitates the transport of tropical moisture, fueling the diverse ecosystems of the Sonoran, Mojave, and Chihuahuan deserts.
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Sources
- Summary by Intellect · source: North American monsoon
- Also: Wikidata — North American Monsoon · CC0
- Also: Wikidata — North American monsoon precipitation reconstructed from tree-ring latewood · CC0
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