Monsoon
A
monsoon is a wind pattern that reverses direction on a seasonal basis. The term was originally applied to monsoonal winds in the
Arabian Sea and
Indian Ocean. The word is also used to label the season in which this wind blows from the southwest in India and adjacent areas that is characterized by very heavy rainfall, and specifically the rainfall that is associated with this wind.
In terms of total precipitation, total area covered and the total number of people affected, the monsoon affecting the Indian Subcontinent dwarfs the North American monsoon (also called the "Mexican", "southwest", "desert", or "Arizona" monsoon).
The name comes from the Dutch word
monssoen, from Portuguese
monçao, from Arabic
mawsim meaning
season.
Monsoon is derived from the
Arabic word "موسم" (
mawsim'), which means season
. In English, the term was borrowed more directly from the Portuguese, monção
, and possibly via early modern Dutch, monson''. The term arose in the 16th century during the rise in navigation across the Indian Ocean, because the monsoonal winds were so critical to sailing:
In Goa they stayed till the Monson, or time of the windes came in to sayle for China. 1598 W. PHILIP tr. J. H. van Linschoten Disc. Voy. E. & W. Indies I. xcii. 143/1.
It is most often applied to the seasonal reversals of the wind direction along the shores of the Indian Ocean, especially in the
Arabian Sea, that blow from the southwest during one half of the year and from the northeast during the other.
The monsoon is very relevant to the people of South India, especially the people of Kerala state. But environmental degradation has weakened or changed the monsoon system prevalent for many centuries.
Monsoons are caused by the fact that land heats up and cools down more quickly than water, owing to the difference in their specific heat. Thus, in summer, land reaches a higher temperature than the ocean. The hot air over the land tends to rise, creating an area of
low pressure. This creates a constant wind blowing toward the land which
entrains the humid oceanic marine layer. Associated
rainfall is caused by the moist ocean air being lifted upward by mountains, surface heating, convergence at the surface, divergence aloft, or from storm-produced outflows at the surface. However the lifting occurs, it produces cooling which in turn produces condensation.
In winter, the land cools off quickly, but the ocean retains heat longer. The hot air over the ocean rises, creating a low pressure area and a breeze from land to ocean while a large area of high pressure is formed over the land, intensified by wintertime radiational cooling.
Monsoons are similar to
sea breezes, but they are much larger in scale, stronger, and are more constant.
As monsoons have become better understood, the term monsoon has been broadened to include almost all of the phenomena associated with the annual
weather cycle within the
tropical and
subtropical land regions of the earth.
Even more broadly, it is now understood that in the geological past, monsoon systems must have always accompanied the formation of
supercontinents such as
Pangea, with their extreme
continental climates.
Northeast Winter Monsoon (Asia)
|
Low lying clouds before rainfall in Singapore |
In
Asia, the northeastern winter monsoons take place from December to early March. The temperature over central Asia is lower, creating a zone of high pressure there. The
jet stream in this region splits into the southern subtropical jet and the polar jet. The subtropical flow directs northeasterly winds to blow across south Asia, creating dry
air streams which produce clear skies over India from the months of November to May.
Meanwhile, a low pressure system develops over northern Australia and winds are directed toward Australia.
During the Northeast Winter Monsoon, Australia and southeast Asia receive large amounts of rainfall.
Southwest Summer Monsoon
The Southwestern
Summer Monsoons occur from June to September, and are drawn towards the
Himalayas, creating winds blowing rain clouds towards India, some areas of which receive up to 10,000 mm of rain.
Indian Ocean Monsoon
The southwest monsoon is generally expected to begin around the middle of June and dies down by September. It begins first in the coastal state of
Kerala and moves upwards at a rate of roughly 1-2 weeks per state. The monsoon accounts for 80 percent of the rainfall in the country. Indian agriculture (which accounts for 25 percent of the GDP and employs 70 percent of the population) is heavily dependent on the rains, especially crops like cotton, rice, oilseeds and coarse grains. A delay of a few days in the arrival of the monsoon can, and does, badly affect the economy, as evidenced in the numerous droughts in India in the 90s.
The monsoon is widely welcomed and appreciated by citydwellers as well, for it provides relief from the climax of summer in June. However, because of the lack of adequate infrastructure in place, most major cities are often adversely affected as well. The roads, already shoddy, take a battering each year; houses and streets at the bottom of slopes and beside rivers are waterlogged,
slums are flooded, and the sewers and the rare hurricane drain start to back up and pour out toxic filth rather than drain it away. This translates into various minor casualties most of the time (although a large number of people in rural areas are struck dead by lightning while working in their fields); however, this lack of city infrastructure coupled with changing climate patterns also causes severe damage to and loss of property and life, as evidenced in the
Mumbai floods of 2005.
North American Monsoon
The North American Monsoon (NAM) occurs from late May or early June into September, originating over Mexico and spreading into the southwest United States by mid July. It affects Mexico along the
Sierra Madre Occidental as well as
Arizona,
New Mexico,
Nevada,
Utah,
Colorado,
West Texas, and
California. It pushes as far west as the
Peninsular Ranges and
Transverse Ranges of southern California but rarely reaches the coastal strip (a wall of desert thunderstorms only a half-hour's drive away is a common summer sight from the sunny skies along the coast during the monsoon). The North American Monsoon is known to many as the
Summer,
Southwest,
Mexican or
Arizona monsoon. It is also sometimes called the
Desert Monsoon as a large part of the affected area is desert.
The North American Monsoon is associated with an area of
high pressure called the
subtropical ridge that moves northward during the summer months and a
thermal low (a trough of low pressure which develops from intense surface heating) over the Mexican Plateau and the desert southwest of the United States. 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. The monsoon extends into the southwest United States as it matures in mid July when an area of high pressure, called the monsoon ridge, develops in the upper atmosphere over the
four corners region, creating an easterly to southeasterly wind flow aloft. This wind flow pattern directs moisture originating in the Gulf of Mexico, Gulf of California and the tropical Pacific by way of northern Mexico into the region, setting off brief, but often torrential thunderstorms, especially over mountainous terrain. This activity is occasionally enhanced by the passage of
easterly waves or the
entrainment of the remnants of
tropical storms.
As much as 70% of rainfall in the region occurs during the summer monsoon. Many desert plants are adapted to take advantage of this brief wet season in the usually-dry region.
Flash flooding is a serious danger during the monsoon season. 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 (never camp in a dry wash in the desert). 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.
The North American Monsoon affects much of the United States and Mexico. Major
drought episodes in the midwestern United States are associated with an amplification of the upper
tropospheric monsoon ridge, along with a weakening of the western edge of the "Bermuda high" and the low-level jet stream over the great plains[
1].
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Thunderstorms during the North American Monsoon as seen from El Cajon, California. The thunderstorms rarely push beyond the Peninsular Ranges to the clear skies of the coastal strip. |
*
Climate of India*
National Weather Service: The North American Monsoon* Initial text from the
Goddard Space Flight Center's public domain
Distributed Active Archive Center*
North American Monsoon Experiment*
Meet the Indian monsoons at PBS.org*
Arizona Central monsoon page
