North American Monsoon

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Summer monsoon rain over eastern New Mexico New Mexico Monsoon 2011.jpg
Summer monsoon rain over eastern New Mexico

The North American monsoon, variously known as the Southwest monsoon , the Mexican monsoon, the New Mexican monsoon, or the Arizona monsoon, [1] is a pattern of pronounced increase in thunderstorms and rainfall over large areas of the southwestern United States and northwestern Mexico, typically occurring between July and mid September. During the monsoon, thunderstorms are fueled by daytime heating and build up during the late afternoon-early evening. Typically, these storms dissipate by late night, and the next day starts out fair, with the cycle repeating daily. The monsoon typically loses its energy by mid-September when drier and cooler conditions are reestablished over the region. Geographically, the North American monsoon precipitation region is centered over the Sierra Madre Occidental in the Mexican states of Sinaloa, Durango, Sonora and Chihuahua. [2]

Monsoon seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea

Monsoon is traditionally defined as 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 the asymmetric heating of land and sea. 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 sometimes incorrectly used for locally heavy but short-term rains, although these rains meet the dictionary definition of monsoon.

Southwestern United States Geographical region of the USA

The Southwestern United States, also known as the American Southwest, is the informal name for a region of the western United States. Definitions of the region's boundaries vary a great deal and have never been standardized, though many boundaries have been proposed. For example, one definition includes the stretch from the Mojave Desert in California to Carlsbad, New Mexico, and from the Mexico–United States border to the southern areas of Colorado, Utah, and Nevada. The largest metropolitan areas are centered around Phoenix, Las Vegas, Tucson, Albuquerque, and El Paso. Those five metropolitan areas have an estimated total population of more than 9.6 million as of 2017, with nearly 60 percent of them living in the two Arizona cities—Phoenix and Tucson.

Mexico Country in the southern portion of North America

Mexico, officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatemala, Belize, and the Caribbean Sea; and to the east by the Gulf of Mexico. Covering almost 2,000,000 square kilometres (770,000 sq mi), the nation is the fifth largest country in the Americas by total area and the 13th largest independent state in the world. With an estimated population of over 120 million people, the country is the eleventh most populous state and the most populous Spanish-speaking state in the world, while being the second most populous nation in Latin America after Brazil. Mexico is a federation comprising 31 states and Mexico City, a special federal entity that is also the capital city and its most populous city. Other metropolises in the state include Guadalajara, Monterrey, Puebla, Toluca, Tijuana and León.


Causes and effects

Weather pattern of the North American Monsoon Monsoonmapb.JPG
Weather pattern of the North American Monsoon
3-second video of a lightning strike during a monsoon over Island in the Sky, Canyonlands National Park

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. There is a shift in wind patterns in summer which occurs as Mexico and the southwest U.S. warm under intense solar heating. As this happens, the flow reverses. The prevailing winds start to flow from moist ocean areas into dry land areas.

Monsoon of South Asia

The monsoon of South Asia is among several geographically distributed global monsoons. It affects the Indian subcontinent, where it is one of the oldest and most anticipated weather phenomena and an economically important pattern every year from June through September, but it is only partly understood and notoriously difficult to predict. Several theories have been proposed to explain the origin, process, strength, variability, distribution, and general vagaries of the monsoon, but understanding and predictability are still evolving.

Mexican Plateau Region in Chihuahua, Mexico

The Central Mexican Plateau, also known as the Mexican Altiplano, is a large arid-to-semiarid plateau that occupies much of northern and central Mexico. Averaging 1,825 m (5,988 ft) above sea level, it extends from the United States border in the north to the Trans-Mexican Volcanic Belt in the south, and is bounded by the Sierra Madre Occidental and Sierra Madre Oriental to the west and east, respectively.

Tibetan Plateau a plateau in Central Asia

The Tibetan Plateau, also known in China as the Qinghai–Tibet Plateau or the Qing–Zang Plateau or Himalayan Plateau, is a vast elevated plateau in Central Asia and East Asia, covering most of the Tibet Autonomous Region and Qinghai in western China, as well as Ladakh and Lahaul & Spiti in India. It stretches approximately 1,000 kilometres (620 mi) north to south and 2,500 kilometres (1,600 mi) east to west. With an average elevation exceeding 4,500 metres (14,800 ft), the Tibetan Plateau is sometimes called "the Roof of the World" because it stands over 3 miles (4.8 km) above sea level and is surrounded by imposing mountain ranges that harbor the world's two highest summits, Mount Everest and K2, and is the world's highest and largest plateau, with an area of 2,500,000 square kilometres (970,000 sq mi). Sometimes termed the Third Pole, the Tibetan Plateau contains the headwaters of the drainage basins of most of the streams in surrounding regions. Its tens of thousands of glaciers and other geographical and ecological features serve as a "water tower" storing water and maintaining flow. The impact of global warming on the Tibetan Plateau is of intense scientific interest.

The monsoon begins in late May to early June in southern Mexico and quickly spreads along the western slopes of the Sierra Madre Occidental, reaching Arizona and New Mexico 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. 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. [3]

Arizona state of the United States of America

Arizona is a state in the southwestern region of the United States. It is also part of the Western and the Mountain states. It is the sixth largest and the 14th most populous of the 50 states. Its capital and largest city is Phoenix. Arizona shares the Four Corners region with Utah, Colorado, and New Mexico; its other neighboring states are Nevada and California to the west and the Mexican states of Sonora and Baja California to the south and southwest.

New Mexico State of the United States of America

New Mexico is a state in the Southwestern region of the United States of America; its capital and cultural center is Santa Fe, which was founded in 1610 as capital of Nuevo México, while its largest city is Albuquerque with its accompanying metropolitan area. It is one of the Mountain States and shares the Four Corners region with Utah, Colorado, and Arizona; its other neighboring states are Oklahoma to the northeast, Texas to the east-southeast, and the Mexican states of Chihuahua to the south and Sonora to the southwest. With a population around two million, New Mexico is the 36th state by population. With a total area of 121,592 sq mi (314,920 km2), it is the fifth-largest and sixth-least densely populated of the 50 states. Due to their geographic locations, northern and eastern New Mexico exhibit a colder, alpine climate, while western and southern New Mexico exhibit a warmer, arid climate.

Four Corners region of the United States consisting of the southwestern corner of Colorado, northwestern corner of New Mexico, northeastern corner of Arizona and southeastern corner of Utah

The Four Corners is a region of the United States consisting of the southwestern corner of Colorado, southeastern corner of Utah, northeastern corner of Arizona, and northwestern corner of New Mexico. The Four Corners area is named after the quadripoint at the intersection of approximately 37° north latitude with 109° 03' west longitude, where the boundaries of the four states meet, and are marked by the Four Corners Monument. It is the only location in the United States where four states meet. Most of the Four Corners region belongs to semi-autonomous Native American nations, the largest of which is the Navajo Nation, followed by Hopi, Ute, and Zuni tribal reserves and nations. The Four Corners region is part of a larger region known as the Colorado Plateau and is mostly rural, rugged, and arid. In addition to the monument, commonly visited areas within Four Corners include Monument Valley, Mesa Verde National Park, Chaco Canyon, Canyons of the Ancients National Monument and Canyon de Chelly National Monument. The most populous city in the Four Corners region is Farmington, New Mexico, followed by Durango, Colorado.

Pulses of low level moisture are transported primarily 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. 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. [4]

Gulf of California A gulf of the Pacific Ocean between the Baja peninsula and the Mexican mainland

The Gulf of California is a marginal sea of the Pacific Ocean that separates the Baja California Peninsula from the Mexican mainland. It is bordered by the states of Baja California, Baja California Sur, Sonora, and Sinaloa with a coastline of approximately 4,000 km (2,500 mi). Rivers which flow into the Gulf of California include the Colorado, Fuerte, Mayo, Sinaloa, Sonora, and the Yaqui. The gulf's surface area is about 160,000 km2 (62,000 sq mi). Depths range from fording at the estuary near Yuma, Arizona, to in excess of 3,000 meters (9,800 ft) in the deepest parts.

Pacific Ocean Ocean between Asia and Australia in the west, the Americas in the east and Antarctica or the Southern Ocean in the south.

The Pacific Ocean is the largest and deepest of Earth's oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean in the south and is bounded by Asia and Australia in the west and the Americas in the east.

Gulf of Mexico An Atlantic Ocean basin extending into southern North America

The Gulf of Mexico is an ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North American continent. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. The U.S. states of Texas, Louisiana, Mississippi, Alabama, and Florida border the Gulf on the north, which are often referred to as the "Third Coast", in comparison with the U.S. Atlantic and Pacific coasts.

As precipitable water values rise in early summer, brief but often torrential thunderstorms can occur, especially over mountainous terrain. [5] This activity is occasionally enhanced by the passage of tropical waves and the entrainment of the remnants of tropical cyclones. [6] [7]

Entrainment (meteorology)

Entrainment is a phenomenon of the atmosphere which occurs when a turbulent flow captures a non-turbulent flow. It is typically used to refer to the capture of a wind flow of high moisture content, or in the case of tropical cyclones, the capture of drier air.

Monsoons play a vital role in managing wildfire threat by providing moisture at higher elevations and feeding desert streams. 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.

Wildfire uncontrolled fire in an area of combustible vegetation that occurs in the countryside or a wilderness area

A wildfire or wildland fire is a fire in an area of combustible vegetation occurring in rural areas. Depending on the type of vegetation present, a wildfire can also be classified more specifically as a brush fire, bushfire, desert fire, forest fire, grass fire, hill fire, peat fire, vegetation fire, or veld fire.

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; it is therefore wise to avoid camping in a dry wash during the monsoon. 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.

Patterns of rainfall

An isolated thunderstorm rolls through Wah Wah Valley, Utah. This type of monsoonal pattern is very common in the late summer of the southwest US. Rain in the Wah Wah Valley.jpg
An isolated thunderstorm rolls through Wah Wah Valley, Utah. This type of monsoonal pattern is very common in the late summer of the southwest US.

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. Monsoon precipitation, however, accounts for a substantial portion of annual precipitation in northwest Mexico and the Southwest U.S. Most of these areas receive over half their annual precipitation from the monsoon. [4]

The North American Monsoon circulation pattern typically develops in late May or early June over southwest Mexico. By mid to late summer, thunderstorms increase over the "core" region of the southwest U.S. and northwest Mexico, including the U.S. and Mexican states of Arizona, New Mexico, Sonora, Chihuahua, Sinaloa and Durango. [4] The monsoon typically arrives in mid to late June over northwest Mexico, and early July over the southwest U.S. Once the monsoon is underway, mountain ranges, including the Sierra Madre Occidental and the Mogollon Rim 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 spotty 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. [4] The monsoon pushes as far west as the Peninsular Ranges and Transverse Ranges of southern California, but rarely reaches the coastal strip. 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.

This 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. [4]


Unlike the Pacific region of the United States and the west coast of Baja California, precipitation in the North American monsoon is not associated with large-scale mid-latitude cyclones, but with thunderstorms which have very different spatial/temporal distribution characteristics. The difficulty in understanding the variability of summertime convective activity in the southwestern U.S. and northwestern Mexico 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]

See also

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Anticyclone opposite to a cyclone

An anticyclone is a weather phenomenon defined by the United States National Weather Service's glossary as "a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere". 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. Mid-tropospheric systems, such as the subtropical ridge, deflect tropical cyclones around their periphery and cause a temperature inversion inhibiting free convection near their center, building up surface-based haze under their base. Anticyclones aloft can form within warm core lows such as tropical cyclones, due to descending cool air from the backside of upper troughs such as polar highs, or from large scale sinking such as the subtropical ridge. The evolution of an anticyclone depends on a few variables such as its size, intensity, moist-convection, Coriolis force etc.

Low-pressure area region where the atmospheric pressure is lower than that of surrounding locations

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Sierra Madre Occidental mountain range

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 consists of an almost continuous sequence of mountain ranges that form the western 'backbone' of North America, Central America, South America and West Antarctica.

Madden–Julian oscillation

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.

Monsoon trough

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 (2006) Pacific tropical storm in 2006

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United States rainfall climatology

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Hurricane Isis (1998) Category 1 Pacific hurricane in 1998

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Mexico tropical cyclone rainfall climatology

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.

Climate of the United States

The climate of the United States varies due to differences 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 further south one travels, and drier the further west, until one reaches the West Coast.

Humid subtropical climate category in the Köppen climate classification system

A humid subtropical climate is a zone of climate characterized by hot and humid summers, and mild winters. These climates normally lie on the southeast side of all continents, generally between latitudes 25° and 35° and are located poleward from adjacent tropical climates.

Climate of Los Angeles

The Climate of Los Angeles is a year-round mild-to-hot and mostly dry climate for the LA metropolitan area in California. The climate is classified as a Mediterranean climate, which is a type of dry subtropical climate. It is characterized by seasonal changes in rainfall—with a dry summer and a winter rainy season. Under the modified Köppen climate classification, the coastal areas are classified as Csb, and the inland areas as Csa.

Earth rainfall climatology

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. Geographic distribution of rain is largely governed by climate type, topography and habitat humidity. In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation. On the leeward side of mountains, desert climates can exist due to the dry air caused by compressional heating. The movement of the monsoon trough, or intertropical convergence zone, brings rainy seasons to savannah climes. The urban heat island effect leads to increased rainfall, both in amounts and intensity, downwind of cities. Global warming may also cause changes in the precipitation pattern globally, including wetter conditions at high latitudes and in some wet tropical areas, and drier conditions in parts of the subtropics and middle latitudes. Precipitation is a major component of the water cycle, and is responsible for depositing most of the fresh water on the planet. Approximately 505,000 cubic kilometres (121,000 cu mi) of water falls as precipitation each year; 398,000 cubic kilometres (95,000 cu mi) of it over the oceans. Given the Earth's surface area, that means the globally averaged annual precipitation is 990 millimetres (39 in). Climate classification systems such as the Köppen climate classification system use average annual rainfall to help differentiate between differing climate regimes.

Gulf of California moisture surge

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.

Climate of Asia

The Climate of Asia is wet across southeast sections, and dry across much of the interior. Some of the largest daily temperature ranges on Earth occur in western sections of Asia. The monsoon circulation dominates across southern and eastern sections, due to the presence of the Himalayas forcing the formation of a thermal low which draws in moisture during the summer. Southwestern sections of the continent experience low relief as a result of the subtropical high pressure belt; they are hot in the summer, warm to cool in winter, and may snow at higher altitudes. Siberia is one of the coldest places in the Northern Hemisphere, and can act as a source of arctic air masses for North America. The most active place on Earth for tropical cyclone activity lies northeast of the Philippines and south of Japan, and the phase of the El Niño-Southern Oscillation modulates where in Asia landfall is more likely to occur.


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  2. 1 2 PD-icon.svg This article incorporates public domain material  from the  United States Geological Survey document: Adams, David K. (1997). "Review of Variability in the North American Monsoon". Impact of Climate Change and Land Use in the Southwestern United States.
  3. "The Monsoon". National Weather Service Forecast Office Flagstaff, Arizona. Retrieved 2008-02-28.
  4. 1 2 3 4 5 PD-icon.svg This article incorporates  public domain material from the National Weather Service document "North American Monsoon" .
  5. Junker, Norman W. "Maddox Type IV Event" . Retrieved 2008-02-29.
  6. "Reports to the Nation: The North American Monsoon" (PDF). Climate Prediction Center . Retrieved 2008-02-29.
  7. Roth, David M. "Tropical Cyclone Rainfall for the West" . Retrieved 2008-02-29.