The North American monsoon, variously known as the Southwest monsoon, the Mexican monsoon, the New Mexican monsoon, or the Arizona monsoon is a pattern of pronounced seasonal 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, 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. 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. In early summer, the monsoon starts with a shift in wind patterns as Mexico and the southwest U.S. warm under intense solar heating. The prevailing winds start to flow from somewhat cooler moist ocean areas into hotter, dry land areas. 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. 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
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