North American monsoon

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

on July 9, 2021 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. 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. 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. 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. The monsoon pushes as far west as the Southeastern Deserts of California, which include the Imperial and Coachella Valleys, 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. == Variability ==

, 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. 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. 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. 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. 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. Because this is not a complete 180-degree reversal, some climatologists claim the weather pattern is not a true monsoon. Other climatologists disagree. == See also ==