North American monsoon

Last updated January 04, 2025

The North American monsoon, variously known as the Southwest monsoon, the Mexican monsoon, the New Mexican monsoon, or the Arizona monsoon^[1] is a term for a pattern of pronounced increase in thunderstorms and rainfall over large areas of the southwestern United States and northwestern Mexico.

Geographically, the weather pattern is centered over the Sierra Madre Occidental in the Mexican states of Sinaloa, Durango, Sonora and Chihuahua,^[2] and typically occurs between June and mid-September. Thunderstorms are fueled by daytime heating and build up during the late afternoon and early evening. Typically, these storms dissipate by late night, and the next day starts out fair, with the cycle repeating daily. The cycle typically loses its energy by mid-September when much drier conditions are reestablished over the region.

Whether the North American monsoon is a true monsoon remains controversial.

Mechanism

The North American monsoon is a complex weather process that brings moisture from the Gulf of California (and to lesser extent the eastern Pacific and Gulf of Mexico) over northwestern Mexico and southwestern US resulting in summer thunderstorms, especially at higher elevations. The North American monsoon is not as strong or persistent as its Indian counterpart, mainly because the Mexican Plateau is not as high or as large as the Tibetan Plateau in Asia. However, the North American monsoon shares most of the basic characteristics of its Indian counterpart.^[3]

In the monsoon area, the late spring period is very hot and dry, because the dry western side subtropical high pressure ridge and dry continental air have not begun to 'shift' northwards yet. During this period, inland areas have extremely low relative humidity, and characteristically very low dew points, which are frequently well below the freezing mark. In some years, this delayed effect is more substantial if the dryline which separates the hot and dry airmass to the Northwest from the humid monsoon airmass to the Southeast, fails to migrate. This can prevent tropical moisture from reaching farther Northwest towards Death Valley until later in the summer. If this pattern prevails, the Nevada deserts may receive almost no monsoon. ^{[ citation needed ]}

In early summer, the monsoon starts with a shift in wind patterns as Mexico and the southwest U.S. warm under intense solar heating.^[4] The prevailing winds start to flow from somewhat cooler moist ocean areas into hotter, dry land areas.^[3] Precipitation increases in late May to early June in southern Mexico and spreads along the western slopes of the Sierra Madre Occidental, reaching New Mexico and southeast Arizona in early July. It extends into the southwest United States as it matures in mid-July, when an area of high pressure, called the monsoon or subtropical ridge, develops in the upper atmosphere over the Four Corners region, creating wind flow aloft from the east or southeast.^[5] At the same time, a thermal low (a trough of low pressure from intense surface heating) develops over the Mexican Plateau and the Desert Southwest of the United States.^[6]

The thermal low sets up circulation that brings pulses of low level moisture from the Gulf of California and eastern Pacific. The Gulf of California, a narrow body of water surrounded by mountains, is particularly important for low-level moisture transport into Arizona and Sonora. Upper level moisture is also transported into the region, mainly from the Gulf of Mexico by easterly winds aloft. Once the forests of the Sierra Madre Occidental green up from the initial monsoon rains, evaporation and plant transpiration can add additional moisture to the atmosphere which will then flow into Arizona and New Mexico. Finally, if the southern Plains of the U.S. are unusually wet and green during the early summer months, that area can also serve as a moisture source.^[3]

As precipitable water values rise in early summer, brief but often torrential thunderstorms can occur, especially over mountainous terrain.^[7] This activity is occasionally enhanced by the passage of tropical waves and the entrainment of the remnants of tropical cyclones.^[8]^[9]

Effects

Monsoon precipitation accounts for a substantial portion of annual precipitation in northwest Mexico and the Southwestern United States. Most of these areas receive over half their annual precipitation from the monsoon.^[3] Many desert plants are adapted to take advantage of this brief wet season. Because of the monsoons, the Sonoran and Mojave are considered relatively "wet" when ranked among other deserts such as the Sahara, and helps fuel the Chihuahuan Desert's extreme diversity.

Monsoons often play a role in reducing wildfire threats by providing moisture at higher elevations during the wildfire season.^[10] Heavy monsoon rain can lead to excess winter plant growth, in turn a summer wildfire risk. A lack of monsoon rain can hamper summer seeding, reducing excess winter plant growth but worsening drought.

Flash flooding is a serious danger during the monsoon. Dry washes can become raging rivers in an instant, even when no storms are visible as a storm can cause a flash flood tens of miles away.^[11] Lightning strikes are also a significant danger. Because it is dangerous to be caught in the open when these storms suddenly appear, many golf courses in Arizona have thunderstorm warning systems. In Albuquerque, flash flooding from storms funneled into the Rio Grande Valley by the Sandia-Manzano mountain range has prompted the city to develop an extensive system of concrete-lined arroyos and retention structures, similar to the flood control channels in the Los Angeles River basin.^[12]

Once the monsoon is underway, mountain ranges, including the Sierra Madre Occidental, the Mogollon Rim, and the Rio Grande Rift ranges provide a focusing mechanism for the daily development of thunderstorms. Thus much of the monsoon rainfall occurs in mountainous terrain. For example, monsoon rainfall in the Sierra Madre Occidental typically ranges from 10 to 15 inches. Since the southwest U.S. is at the northern fringe of the monsoon, precipitation is less and tends to be more variable. Areas farther west of the core monsoon region, namely California and Baja California, typically receive only scattered monsoon-related rainfall. In those areas, the intense solar heating is not strong enough to overcome a continual supply of cold water from the North Pacific Ocean moving down the west coast of North America. Winds do turn toward the land in these areas, but the cool moist air actually stabilizes the atmosphere.^[3] The monsoon pushes as far west as the Southeastern Deserts of California, which include the Imperial and Coachella Valley's, as well as Peninsular Ranges and Transverse Ranges of Southern California. These storms rarely reach the major coastal areas of San Diego or Los Angeles. As shown in the panorama below, a wall of thunderstorms, only a half-hour's drive away, is a common sight from the sunny skies along the coast during the monsoon.

Monsoonal thunderstorms as seen from El Cajon, California

Variability

Rainfall during the monsoon is not continuous. It varies considerably, depending on a variety of factors. There are usually distinct "burst" periods of heavy rain during the monsoon, and "break" periods with little or no rain.^[3] The variability is difficult to understand and predict, because it results from the complex interactions between atmospheric circulation features at both the synoptic (100 to 1000 km spatially, 1 day to 1 week, temporally) and mesoscale (several km to 100 km, hours to one day temporally) and the extremely varied topography. The larger-scale atmospheric motions may control the distribution of water vapor and the general stability or instability (that is, the tendency to form storms) in the atmosphere; nevertheless, local topographic effects are critical to the geographic and even temporal distribution of convective activity.^[2]

The monsoon ridge is almost as strong as the one which develops over Asia during the summer. However, since the lower level moisture flow is not as persistent as in the Indian monsoon, the upper level steering pattern and disturbances around the ridge are critical for influencing where thunderstorms develop on any given day. The exact strength and position of the subtropical ridge also governs how far north the tropical easterly winds aloft can spread. If the ridge is too close to a particular area, the sinking air at its center suppresses thunderstorms and can result in a significant monsoon "break." If the ridge is too far away or too weak, the east winds around the high are inadequate to bring tropical moisture into the mountains of Mexico and southwest U.S. However, if the ridge sets up in a few key locations, widespread and potentially severe thunderstorms can develop.^[3]

Monsoon variability from one summer to the next is substantial, and exceeds the normal monsoon seasonal precipitation at most locations. For example, the normal monsoon precipitation at Tucson, Arizona is 6.06 inches (154 mm). The driest monsoon season measured 1.59 inches (40 mm), and the wettest measured 13.84 inches (352 mm).^[13]

Research since 2010 has investigated the possible causes behind North American monsoon variability. The following factors affect the North American monsoon:

Sea surface temperature anomalies
Large-scale circulation patterns
Previous year's precipitation
Location of the Intertropical Convergence Zone
Variability in the Gulf of California moisture surges

None of these factors can perfectly predict the variability. These factors are related to each other and are not independent. For example, sea surface temperatures affect all the other factors to some extent.^[13]

In some years, the Nevada deserts may receive almost no monsoon influence if the western subtropical high shifts less than in typical years. For instance in 2020, the subtropical high remained further south than usual due to the influence of a strong ridge of high pressure over the northern Pacific, displacing the jet stream further inland.^[5] The combination of these factors blocked moist tropical air from reaching the American Southwest, leading to below average rainfall.

Definition

Whether the North American Monsoon (NAM) is a "true monsoon" has been controversial. Until the late 1970s, there was serious debate about whether a monsoon truly existed in North America. However, according to NOAA, considerable research culminating in 1993 established the fact that a summer monsoon develops over much of Mexico and the intermountain region of the U.S.^[3]^[14]^[15]

The controversy continues in part because of the incomplete reversal of the winds during the NAM. The prevailing winds shift from westerly before to southerly during the NAM.^[4]^[16] Because this is not a complete 180-degree reversal, some climatologists claim the weather pattern is not a true monsoon.^[17] Other climatologists disagree.^[18]

Related Research Articles

A monsoon is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal oscillation of the Intertropical Convergence Zone (ITCZ) between its limits to the north and south of the equator. Usually, the term monsoon is used to refer to the rainy phase of a seasonally changing pattern, although technically there is also a dry phase. The term is also sometimes used to describe locally heavy but short-term rains.

The horse latitudes are the latitudes about 30 degrees north and south of the Equator. They are characterized by sunny skies, calm winds, and very little precipitation. They are also known as subtropical ridges or highs. It is a high-pressure area at the divergence of trade winds and the westerlies.

An anticyclone is a weather phenomenon defined as a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere as viewed from above. Effects of surface-based anticyclones include clearing skies as well as cooler, drier air. Fog can also form overnight within a region of higher pressure.

<span class="mw-page-title-main">Intertropical Convergence Zone</span> Meteorological phenomenon

The Intertropical Convergence Zone, known by sailors as the doldrums or the calms because of its monotonous windless weather, is the area where the northeast and the southeast trade winds converge. It encircles Earth near the thermal equator though its specific position varies seasonally. When it lies near the geographic equator, it is called the near-equatorial trough. Where the ITCZ is drawn into and merges with a monsoonal circulation, it is sometimes referred to as a monsoon trough.

A tropical wave, in and around the Atlantic Ocean, is a type of atmospheric trough, an elongated area of relatively low air pressure, oriented north to south, which moves from east to west across the tropics, causing areas of cloudiness and thunderstorms. Tropical waves form in the easterly flow along the equatorial side of the subtropical ridge or belt of high air pressure which lies north and south of the Intertropical Convergence Zone (ITCZ). Tropical waves are generally carried westward by the prevailing easterly winds along the tropics and subtropics near the equator. They can lead to the formation of tropical cyclones in the north Atlantic and northeastern Pacific basins. A tropical wave study is aided by Hovmöller diagrams, a graph of meteorological data.

The Sierra Madre Occidental is a major mountain range system of the North American Cordillera, that runs northwest–southeast through northwestern and western Mexico, and along the Gulf of California. The Sierra Madre is part of the American Cordillera, a chain of mountain ranges (cordillera) that consist of an almost continuous sequence that form the western "sounds" of North America, Central America, South America, and West Antarctica.

The Madden–Julian oscillation (MJO) is the largest element of the intraseasonal variability in the tropical atmosphere. It was discovered in 1971 by Roland Madden and Paul Julian of the American National Center for Atmospheric Research (NCAR). It is a large-scale coupling between atmospheric circulation and tropical deep atmospheric convection. Unlike a standing pattern like the El Niño–Southern Oscillation (ENSO), the Madden–Julian oscillation is a traveling pattern that propagates eastward, at approximately 4 to 8 m/s, through the atmosphere above the warm parts of the Indian and Pacific oceans. This overall circulation pattern manifests itself most clearly as anomalous rainfall.

<span class="mw-page-title-main">Monsoon trough</span> Weather phenomenon

The monsoon trough is a portion of the Intertropical Convergence Zone in the Western Pacific, as depicted by a line on a weather map showing the locations of minimum sea level pressure, and as such, is a convergence zone between the wind patterns of the southern and northern hemispheres.

Tropical Storm Emilia was a rare tropical cyclone that affected the Baja California Peninsula in July 2006. The sixth tropical depression and fifth tropical storm of the 2006 Pacific hurricane season, it developed on July 21 about 400 miles (640 km) off the coast of Mexico. It moved northward toward the coast, reaching peak winds of 65 mph (105 km/h) before turning westward and encountering unfavorable conditions. Emilia later turned to the north, passing near Baja California as a strong tropical storm. Subsequently, the storm moved further away from the coast, and on July 27 it dissipated.

The characteristics of United States rainfall climatology differ significantly across the United States and those under United States sovereignty. Summer and early fall bring brief, but frequent thundershowers and tropical cyclones which create a wet summer and drier winter in the eastern Gulf and lower East Coast. During the winter, and spring, Pacific storm systems bring Hawaii and the western United States most of their precipitation. Low pressure systems moving up the East Coast and through the Great Lakes, bring cold season precipitation to from the Midwest to New England, as well as Great Salt Lake. The snow to liquid ratio across the contiguous United States averages 13:1, meaning 13 inches (330 mm) of snow melts down to 1 inch (25 mm) of water.

Hurricane Isis was the only hurricane to make landfall during the 1998 Pacific hurricane season. The ninth tropical storm and sixth hurricane of the season, Isis developed on September 1 from an interaction between a tropical wave and a large surface circulation to the southwest of Mexico. It moved northward, striking the extreme southeastern portion of the Baja California peninsula before attaining hurricane status in the Gulf of California. Isis made landfall at Topolobampo in the Mexican state of Sinaloa on September 3, and quickly lost its low-level circulation. The remnants persisted for several days before dissipating over the U.S. state of Idaho on September 8.

Mexico tropical cyclone rainfall climatology discusses precipitation characteristics of tropical cyclones that have struck Mexico over the years. One-third of the annual rainfall received along the Mexican Riviera and up to half of the rainfall received in Baja California Sur is directly attributable to tropical cyclones moving up the west coast of Mexico. The central plateau is shielded from the high rainfall amounts seen on the oceanward slopes of the Sierra Madre Oriental and Occidental mountain chains.

The climate of the United States varies due to changes in latitude, and a range of geographic features, including mountains and deserts. Generally, on the mainland, the climate of the U.S. becomes warmer the farther south one travels, and drier the farther west, until one reaches the West Coast.

Rain is a atmospheric phenomenon of water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water for hydroelectric power plants, crop irrigation, and suitable conditions for many types of ecosystems.

A humid subtropical climate is a temperate climate type characterized by long and hot summers, and cool to mild winters. These climates normally lie on the southeast side of all continents, generally between latitudes 25° and 40° and are located poleward from adjacent tropical climates, and equatorward from either humid continental or oceanic climates. It is also known as warm temperate climate in some climate classifications.

<span class="mw-page-title-main">Earth rainfall climatology</span> Study of rainfall

Earth rainfall climatology Is the study of rainfall, a sub-field of meteorology. Formally, a wider study includes water falling as ice crystals, i.e. hail, sleet, snow. The aim of rainfall climatology is to measure, understand and predict rain distribution across different regions of planet Earth, a factor of air pressure, humidity, topography, cloud type and raindrop size, via direct measurement and remote sensing data acquisition. Current technologies accurately predict rainfall 3–4 days in advance using numerical weather prediction. Geostationary orbiting satellites gather IR and visual wavelength data to measure realtime localised rainfall by estimating cloud albedo, water content, and the corresponding probability of rain.

A Gulf of California moisture surge, or simply gulf surge, is a meteorological event where a pulse of high humidity air is pushed up the Gulf of California. Gulf surges bring moisture to southern Arizona during the North American Monsoon. Prior to the 1970s, the consensus of meteorologists was the moisture that fueled the central and southern Arizona monsoon resulted from the movement of the Bermuda High to a more south and west position, which in turn transported water vapor to the region from the Gulf of Mexico. However, operational meteorologists in the 1970s described episodic surges of moisture that infiltrated the area that was thought to originate in the Gulf of California. It was noted that these episodes were likely to be associated with a convective system near the tip of the Baja peninsula such as a tropical cyclone or an easterly wave.

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References

↑ Adams, David; Comrie, Andrew (1997). "The North American Monsoon". Bulletin of the American Meteorological Society. 78 (10): 2197–2213. Bibcode:1997BAMS...78.2197A. doi: 10.1175/1520-0477(1997)078<2197:TNAM>2.0.CO;2 .
1 2 This article incorporates public domain material from Adams, David K. (1997). "Review of Variability in the North American Monsoon". Impact of Climate Change and Land Use in the Southwestern United States. United States Geological Survey. Archived from the original on July 18, 1997.
1 2 3 4 5 6 7 8 This article incorporates public domain material from North American Monsoon. National Weather Service. Archived from the original on June 12, 2008.
1 2 Grantz, K; Rajagopalan, B; Clark, M; Zagona, E (2007). "Seasonal Shifts in the North American Monsoon". Journal of Climate. 20 (9): 1923–1935. Bibcode:2007JCli...20.1923G. doi: 10.1175/JCLI4091.1 . S2CID 55111148.
1 2 Duginski, Paul (August 9, 2020). "Drought continues to expand as the monsoon in the Southwest has been largely a no-show". Los Angeles Times. Retrieved June 16, 2021.
↑ "The Monsoon". National Weather Service Forecast Office Flagstaff, Arizona. Archived from the original on February 28, 2008. Retrieved February 28, 2008.
↑ Junker, Norman W. "Maddox Type IV Event" . Retrieved February 29, 2008.
↑ "Reports to the Nation: The North American Monsoon" (PDF). Climate Prediction Center . Retrieved February 29, 2008.
↑ Roth, David M. "Tropical Cyclone Rainfall for the West" . Retrieved February 29, 2008.
↑ US Crop Reporting Board; Bureau of Agricultural Economics; Agricultural Marketing Service; Agricultural Statistics Board (2006). Crop Production. Crop Reporting Board, Statistical Reporting Service, U.S. Department of Agriculture. pp. 30, 36.
↑ "North American Monsoon Flash Floods". NOAA. Retrieved November 9, 2022.
↑ "Arroyos - Albuquerque's Environmental Story". albuqhistsoc.org. Retrieved May 7, 2024.
1 2 This article incorporates public domain material from Monsoon Inter-annual Variability (PDF). National Weather Service . Retrieved September 14, 2022.
↑ Reyes, S; Douglas, MW; Maddox, RA (1994). "El Monzon del suroeste de Norteamérica (TRAVASON/SWAMP)". Atmósfera. 7: 117–137.
↑ Douglas, MW; Li, S (1996). "Diurnal variation of the lower tropospheric flow over the Arizona low desert from SWAMP-1993 observations". Monthly Weather Review. 124 (6): 1211–1224. Bibcode:1996MWRv..124.1211D. doi: 10.1175/1520-0493(1996)124<1211:DVOTLT>2.0.CO;2 .
↑ "North American Monsoon Highlights". Albuquerque Weather Office, NOAA. Retrieved September 14, 2022.
↑ Rohli, Robert V.; Vega, Anthony J. (2011). Climatology. Jones & Bartlett Learning. p. 187. ISBN 978-0763791018. Archived from the original on June 19, 2013. Retrieved July 23, 2011. Although the North American monsoon region experiences pronounced precipitation seasonally, it differs from a true monsoon, which is characterized by a distinct seasonal reversal of prevailing surface winds. No such situation occurs in [North America]
↑ Cook, Ben; Seager, Richard (September 2013). "The Future of the North American Monsoon (Global Decadal Climate Predictability)". Lamont-Doherty Earth Observatory of Columbia University.

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[Adams-1] Adams, David; Comrie, Andrew (1997). "The North American Monsoon". Bulletin of the American Meteorological Society. 78 (10): 2197–2213. Bibcode:1997BAMS...78.2197A. doi: 10.1175/1520-0477(1997)078<2197:TNAM>2.0.CO;2 .

[usgs-2] 1 2 This article incorporates public domain material from Adams, David K. (1997). "Review of Variability in the North American Monsoon". Impact of Climate Change and Land Use in the Southwestern United States. United States Geological Survey. Archived from the original on July 18, 1997.

[noaa-3] 1 2 3 4 5 6 7 8 This article incorporates public domain material from North American Monsoon. National Weather Service. Archived from the original on June 12, 2008.

[Grantz-4] 1 2 Grantz, K; Rajagopalan, B; Clark, M; Zagona, E (2007). "Seasonal Shifts in the North American Monsoon". Journal of Climate. 20 (9): 1923–1935. Bibcode:2007JCli...20.1923G. doi: 10.1175/JCLI4091.1 . S2CID 55111148.

[LAT_2020-08-09-5] 1 2 Duginski, Paul (August 9, 2020). "Drought continues to expand as the monsoon in the Southwest has been largely a no-show". Los Angeles Times. Retrieved June 16, 2021.

[6] "The Monsoon". National Weather Service Forecast Office Flagstaff, Arizona. Archived from the original on February 28, 2008. Retrieved February 28, 2008.

[7] Junker, Norman W. "Maddox Type IV Event" . Retrieved February 29, 2008.

[8] "Reports to the Nation: The North American Monsoon" (PDF). Climate Prediction Center . Retrieved February 29, 2008.

[9] Roth, David M. "Tropical Cyclone Rainfall for the West" . Retrieved February 29, 2008.

[10] US Crop Reporting Board; Bureau of Agricultural Economics; Agricultural Marketing Service; Agricultural Statistics Board (2006). Crop Production. Crop Reporting Board, Statistical Reporting Service, U.S. Department of Agriculture. pp. 30, 36.

[11] "North American Monsoon Flash Floods". NOAA. Retrieved November 9, 2022.

[12] "Arroyos - Albuquerque's Environmental Story". albuqhistsoc.org. Retrieved May 7, 2024.

[variability-13] 1 2 This article incorporates public domain material from Monsoon Inter-annual Variability (PDF). National Weather Service . Retrieved September 14, 2022.

[14] Reyes, S; Douglas, MW; Maddox, RA (1994). "El Monzon del suroeste de Norteamérica (TRAVASON/SWAMP)". Atmósfera. 7: 117–137.

[15] Douglas, MW; Li, S (1996). "Diurnal variation of the lower tropospheric flow over the Arizona low desert from SWAMP-1993 observations". Monthly Weather Review. 124 (6): 1211–1224. Bibcode:1996MWRv..124.1211D. doi: 10.1175/1520-0493(1996)124<1211:DVOTLT>2.0.CO;2 .

[16] "North American Monsoon Highlights". Albuquerque Weather Office, NOAA. Retrieved September 14, 2022.

[Rohli-17] Rohli, Robert V.; Vega, Anthony J. (2011). Climatology. Jones & Bartlett Learning. p. 187. ISBN 978-0763791018. Archived from the original on June 19, 2013. Retrieved July 23, 2011. Although the North American monsoon region experiences pronounced precipitation seasonally, it differs from a true monsoon, which is characterized by a distinct seasonal reversal of prevailing surface winds. No such situation occurs in [North America]

[18] Cook, Ben; Seager, Richard (September 2013). "The Future of the North American Monsoon (Global Decadal Climate Predictability)". Lamont-Doherty Earth Observatory of Columbia University.

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