(Redirected from West African Monsoon)
A 'monsoon' is a seasonal prevailing wind which lasts for several months and has lasting climatic effects. The term includes almost all of the phenomena associated with the annual weather cycle within the tropical and subtropical land regions of the earth. It most commonly refers to the wet monsoon, or
rainy season, in many parts of the world, but can also be used in reference to and the dry monsoon experienced periodically in the
South East Asian continent. The term was first used in English with this meaning in
India,
Bangladesh,
Pakistan and neighboring countries to mean the seasonal winds blowing from the
Indian Ocean and
Arabian Sea in the southwest bringing heavy
rainfall to the region. In
hydrology, monsoonal rainfall is considered to be that which occurs in any region that receives the majority of its rain during a particular season, and so monsoons are referred to in relation to other regions such as in
North America,
Sub-Saharan Africa,
Brazil and
East Asia.
The English name 'Monsoon', was derived from the
Arabic word ''mawsim'' meaning '
season', by way of
Portuguese and
Dutch.
In terms of total
precipitation, total area covered and the total number of people affected, the monsoons affecting the Indian Subcontinent dwarf the North American monsoon (also called the "Mexican", "southwest", "desert", or "Arizona" monsoon).
The definition includes major wind systems that change direction seasonally.
: "Most summer monsoons have a dominant westerly component and a strong tendency to ascend and produce copious amounts of rain (because of the condensation of water vapour in the rising air). The intensity and duration, however, are not uniform from year to year. Winter monsoons, by contrast, have a dominant easterly component and a strong tendency to diverge, subside, and cause drought."
[1]
History
Strengthening of the Asian monsoon has been linked to the uplift of the
Tibetan Plateau after the collision of India and Asia around 50 million years ago. Evidence for when this first happened remains controversial. Many geologists believe the monsoon first became strong around 8 million years ago based on records from the
Arabian Sea and the record of wind-blown dust in the
Loess Plateau of China. More recently plant fossils in China and new long-duration sediment records from the
South China Sea led
Peter Clift to propose a much older monsoon starting around 24 million years ago and linked to early Tibetan uplift. Testing of this hypothesis awaits deep ocean sampling by the
Integrated Ocean Drilling Program. The monsoon has varied significantly in strength since this time, largely linked to global climate change, especially the cycle of the
Pleistocene ice ages.
Processes
Monsoons are caused by the larger amplitude of the seasonal cycle of land temperature compared to that of nearby oceans. This differential warming happens because heat in the ocean is mixed vertically through a "mixed layer" that may be fifty metres deep, through the action of wind and buoyancy-generated turbulence, whereas the land surface conducts heat slowly, with the seasonal signal penetrating perhaps a metre or so. Additionally, the specific heat of liquid water is significantly higher than that of most materials that make up land. Together, these factors mean that the heat capacity of the layer participating in the seasonal cycle is much larger over the oceans than over land, with the consequence that land warms faster and 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 steady wind blowing toward the land, bringing the moist near-surface air over the oceans with it. Similar rainfall is caused by the moist ocean air being lifted upwards by mountains, surface heating, convergence at the surface, divergence aloft, or from storm-produced outflows at the surface. However the lifting occurs, the air cools due expansion in lower pressure, which in turn produces condensation.
In winter, the land cools off quickly, but the ocean keeps the 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 drying high pressure is formed over the land, increased by wintertime cooling.
Monsoons are similar to sea breezes, a term usually referring to the localized, diurnal (daily) cycle of circulation near coastlines everywhere, but they are much larger in scale, stronger and seasonal.
Monsoon systems
Low lying clouds before rainfall in
Singapore
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
Pangaea, with their extreme
continental climates.
Northeast Monsoon (Southern Asia and Australasia)
In Southern
Asia, the northeastern monsoons take place from December to early March. The temperature over central Asia is less than 25
oC as it is the northern hemisphere winter, therefore 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 western 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 South-East Asia and Australasia and winds are directed toward Australia known as a
monsoon trough.
During the northeast monsoon, and southeast
Asia receive large amounts of rainfall causing many floods near the coast.
Northern Indian Ocean Monsoon
Onset dates and prevailing wind currents of the southwest summer monsoon.
South-West Summer Monsoon
The southwestern
summer monsoons occur from June through September. The
Great Indian Desert (Thar Desert) and adjoining areas of the northern and central
Indian Subcontinent heats up too much during the hot seasons of summer. This causes a low pressure area over the northern and central Indian subcontinent. To fill up this void, the moisture-laden winds from the
Indian Ocean rush in to the subcontinent. These winds, rich in moisture, are drawn towards the
Himalayas, creating winds blowing storm clouds towards the subcontinent. However the Himalayas act like a high wall and do not allow the winds to pass into
Central Asia, forcing them to rise. With the gain in altitude of the clouds, the
temperature drops and precipitation occurs. Some areas of the subcontinent receive up to 10,000 mm of rain.
The
southwest monsoon is generally expected to begin around the start of June and dies down by September. The moisture-laden winds on reaching the southernmost point of the
Indian peninsula, due to its topology, become divided into two parts:
★ ''Arabian Sea Branch of the SW Monsoon''
★ ''Bay of Bengal Branch of the SW Monsoon''
The 'Arabian Sea Branch of the SW Monsoon' first hits the
Western Ghats of the coastal state of
Kerala,
India and hence
Kerala is the first state in India to receive rain from the South-West Monsoon. This branch of the monsoon moves northwards along the
Western Ghats giving rain to the coastal areas
west of the
Western Ghats. It is to be noted that the
eastern parts of the
Western Ghats do not receive much rain from this monsoon as the wind does not cross the
Western Ghats.
View of south-west monsoon rain in
Kerala.
The 'Bay of Bengal Branch of SW Monsoon' flows over the
Bay of Bengal heading towards
North-Eastern India and
Bengal, picking up more moisture from the
Bay of Bengal. Its hits the
Eastern Himalaya and provides a huge amount of rain to the regions of
North-East India,
Bangladesh and
West Bengal.
Cherrapunji, situated on the southern slopes of the
Eastern Himalaya in
Shillong,
India is one of the wettest places on Earth. After striking the
Eastern Himalaya it turns towards the
West, travels over the
Indo-Gangetic Plain, at a rate of roughly 1-2 weeks per state, pouring raining all along its way.
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 city-dwellers 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; lack of city infrastructure coupled with changing climate patterns also causes severe damage to and loss of property and life.
Bangladesh and some regions of
India like in
Assam and places of
West Bengal experiences heavy
flood, which claims huge number of lives and huge loss of property and causes severe damage to economy, as evidenced in the
Mumbai floods of 2005. Also in the recent past, areas in India that used to receive scanty rainfall throughout the year, like the
Thar Desert, have surprisingly ended up receiving floods due to the prolonged monsoon season.
June 1 is regarded as the date of onset of the monsoon in India, which is the average date on which the monsoon strikes Kerala over the years for which scientific data is available with the Indian Meteoreological Department.
View of central
Kolkata after a monsoon rain.
North-East Monsoon (Retreating Monsoon)
Around September, with the sun fast retreating south, the northern land mass of the
Indian Subcontinent begins to cool off rapidly. With this air pressure begins to build over
northern India. The
Indian Ocean and its surrounding atmosphere still holds its heat. This causes the cold wind to sweep down from the
Himalayas and
Indo-Gangetic Plain towards the vast spans of the
Indian Ocean south of the
Deccan peninsular. This is known as the 'North-East Monsoon' or 'Retreating Monsoon'.
While traveling towards the
Indian Ocean, the dry cold wind picks up some moisture from the
Bay of Bengal and pours it over peninsular
India. Cities like
Chennai, which get less rain from the South-West Monsoon, receives rain from the Retreating Monsoon. About 50% - 60% of the rain received by the state of
Tamil Nadu is from the North-East Monsoon.
It is worth noting that North-East Monsoon (or the Retreating Monsoon) is not able to bring as much rain as the South-West Monsoon.
North American Monsoon
The North American Monsoon (NAM) occurs from late June or early July 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. Because of the monsoons, the
Sonoran and
Mojave are considered relatively "wet" when ranked among other deserts such as the
Sahara.
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.
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.
ing 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].
African Monsoon
The monsoon of western sub-Saharan
Africa is the result of the seasonal shifts of the
Intertropical Convergence Zone and the great seasonal temperature differences between the
Sahara and the equatorial
Atlantic Ocean. The dry, northeasterly
trade winds, and their more extreme form, the
harmattan, are interrupted by the northern shift in the
ITCZ and resultant southerly, rain-bearing winds during the summer. The semiarid
Sahel and
Sudan depend upon this pattern for most of their precipitation.
South American Monsoon
Much of
Brazil experiences seasonal wind patterns that bring a summer maximum to precipitation.
Rio de Janeiro is infamous for flooding as a result of monsoon rains.
See also
★
Climate of India
★
Monsoon trough
★
Monsoon Cup
References
1. Monsoon
★ Initial text from the
Goddard Space Flight Center's public domain
Distributed Active Archive Center
★
American Heritage Dictionary definition and Etymology of "monsoon"
External links
★
National Weather Service: The North American Monsoon
★
North American Monsoon Experiment
★
Influence of monsoon winds
★
Meet the Indian monsoons at PBS.org
★
Arizona Central monsoon page
★
AMMA - African Monsoon Multidisciplinary Analyses
★
Ontario Weather Service: North American Monsoon Forecasting Project
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