A monsoon is a term given to a specific weather pattern, typified by high levels of rainfall occurring during the summer months, and significantly drier weather during the rest of the year. Monsoons occur mainly in subtropical regions, with particularly large monsoons observed in India, southeast Asia and Australia.
The North American Monsoon is one example of this weather pattern in action, and refers to a pattern of increased rainfall that occurs across much of the southwestern USA and northern Mexico, often commencing in July and ending in September.
How Does the North American Monsoon Form?
A specific set of conditions is required for the formation of the North American Monsoon pattern. The spring months across much of the southwest USA and northern Mexico are incredibly dry. During this time, a zonal westerly airflow prevails over the region, which does not transport much moisture from the Pacific Ocean through to the majority of the southwestern USA interior. However, during May and June, the North American Monsoon pattern begins to develop, as a high-pressure ridge (a monsoon high) develops over northern Mexico.
The ridge then migrates northward and increases its pressure even further. During July and August, the center of the ridge is typically anchored over the state of New Mexico; at the same time, a thermal low develops, usually over Arizona and the north-western states of Mexico.
The prevailing wind direction also changes due to the migration of the high-pressure ridge, from a westerly to a southerly and a south-easterly, transporting its moisture from the Pacific Ocean just to the west of Baja California, the Gulf of California and the Gulf of Mexico. As a result, moist air is often available over the southwest USA and north Mexico region, an essential ingredient for the development of thunderstorms.
North American Monsoon Rainfall
The rainfall typically consists of brief but torrential thunderstorms, which are often small-sized, isolated and occur on an irregular basis. They build and develop during the afternoon, fueled by the heating of the Earth’s surface, and tend to last for a few hours into the night before dissipating.
The sporadic occurrence of rainfall in the North American Monsoon contrasts with many other monsoons – for example, the Indian and southeast Asian monsoons tend to see regular occurrences of rain, occurring almost every day and covering a very wide area. On the contrary, in the southwestern USA and northern Mexico, a specific place or area may see a storm occurring on one day, only to then have a number of days without any rain at all.
The North American Monsoon has a large amount of variance in the distribution and intensity of the rainfall, which can vary year on year, meaning that some monsoon seasons are incredibly active, whereas others see little activity.
This is often due to the position of the monsoon high, which occasionally fails to move northward. During the exceptionally dry 2020 monsoon season, the high-pressure ridge only made it as far north as the USA-Mexico border, bringing moist easterly winds to northern Mexico only.
Why is the North American Monsoon Important for the Interior Southwest?
Since the interior of the US southwest is rather arid for much of the year, and is incredibly dry during the spring and early summer months, the area depends largely on rainfall from the North American Monsoon.
Data from a report by the Climate Assessment for the Southwest shows that around a half of all annual rainfall in New Mexico and southern Arizona derives from the summer monsoon. Since annual rainfall levels in the region are generally rather low, with an average of 12 and 13 inches of rainfall recorded per year across Arizona and New Mexico respectively, summer rainfall contributes a significant portion of rainfall in the area.
An active monsoon season is therefore crucial in breaking up a long dry spell and preventing drought.
2020 North American Monsoon Season
This can be evidenced by the 2020 North American Monsoon season, which was incredibly inactive. During the summer months, portions of the Great Basin and Four Corners regions received less than 70% of their normal precipitation, and for some areas rainfall amounts were less than half.
Many cities in the southwestern US recorded their driest or near-driest monsoon on record, with some areas (such as Las Vegas) not recording any summer rainfall at all. The inactive monsoon accelerated large parts of the interior southwest into a severe or exceptional drought.
A summary published in late 2020 by the Center for Western Weather and Water Extremes describes that the percentage of the western US (including the southwest) receiving drought, graded as ‘severe’ or above, accelerated from 23% to 55% between 30th June and 29th September. This shows that much of the southwestern US depends on an active monsoon season, as this rainfall is difficult to replace during other times of the year.
Rain in the Southwest
However, the summer is not the only rainfall season in the southwest. The late fall and winter months also see proportionally large amounts of precipitation falling here.
Large areas of the region, in particular eastern California, Nevada, Utah and Arizona, on average record little over half of their annual rainfall between October and March. Unlike the summer monsoon, this rainfall is typically frontal in nature, consisting of light rain or drizzle that covers large areas, and has a long time to soak into the ground and therefore the water table.
On the other hand, the local and sporadic nature of summer thunderstorms means that some areas are completely drenched while many others remain dry, and any water that does not rapidly infiltrate into the soil is quickly evaporated. Thus, although a lack of summer monsoon contributes to drought conditions, it is likely not the only driver.
During the winter of 2019-2020, parts of the interior southwest saw a drier than average season, particularly across eastern California, Nevada and Utah. Therefore, although the 2020 drought was partly due to the lack of summer rain, lower than average winter rainfall may also have contributed to this.
Although the North American Monsoon is not as active as the monsoons of India and southeast Asia, it is nonetheless a well-marked weather pattern that occurs on an annual basis. It contributes a significant proportion of the annual rainfall seen in the U.S. southwest. Although the lack of a summer monsoon significantly increases the risk of droughts in the region, winter rainfall is also rather important in preventing droughts.
References
2020 North American monsoon recap – Center for western weather and water extremes. (2020, October 22). Center for Western Weather and Water Extremes – Center For Western Weather and Water Extremes. https://cw3e.ucsd.edu/2020-north-american-monsoon-recap/
Di Liberto, T. (2020, March 26). Grading the 2019-2020 winter outlook. NOAA Climate.gov | science & information for a climate-smart nation. https://www.climate.gov/news-features/blogs/enso/grading-2019-2020-winter-outlook
NASA looks at the North American monsoon. (2017, August 4). YouTube. https://www.youtube.com/watch?v=baqLxRbIM0s&t=45s
Nauslar, N. J., Hatchett, B. J., Brown, T. J., Kaplan, M. L., & Mejia, J. F. (2019). Impact of the North American monsoon on wildfire activity in the southwest United States. International Journal of Climatology, 39(3), 1539-1554. https://doi.org/10.1002/joc.5899
North American monsoon highlights. (n.d.). National Weather Service. https://www.weather.gov/abq/northamericanmonsoon-intro
The North American Monsoon. (2004, August). Climate Prediction Center – National Weather Service. https://www.cpc.ncep.noaa.gov/products/outreach/Report-to-the-Nation-Monsoon_aug04.pdf
Temperature and precipitation. (n.d.). CLIMAS. https://climas.arizona.edu/sw-climate/temperature-and-precipitation
What is a Monsoon? (n.d.). NWS Tucson. https://www.wrh.noaa.gov/twc/monsoon/monsoon_whatis.php
What is a thermal low/Heat low? (2019, May 22). The Alabama Weather Blog. https://www.alabamawx.com/?p=190669
