) is traditionally 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[according to whom?]
used for locally heavy but short-term rains.
Advancing monsoon clouds and showers in Aralvaimozhy, near Nagercoil
The etymology of the word monsoon is not wholly certain.
The English monsoon
came from Portuguese monção
, ultimately from Arabicmawsim
"season"), "perhaps partly via early modern Dutch monson
Strengthening of the Asian monsoon has been linked to the uplift of the Tibetan Plateau
after the collision of the Indian sub-continent
and Asia around 50 million years ago.
Because of studies of records from the Arabian Sea
and that of the wind-blown dust in the Loess Plateau
, many geologists believe the monsoon first became strong around 8 million years ago. More recently, studies of plant fossils in China and new long-duration sediment
records from the South China Sea
led to a timing of the monsoon beginning 15–20 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
A study of marine plankton suggested that the Indian Monsoon strengthened around 5 million years ago. Then, during ice periods, the sea level fell and the Indonesian Seaway
closed. When this happened, cold waters in the Pacific were impeded from flowing into the Indian Ocean. It is believed that the resulting increase in sea surface temperatures in the Indian Ocean increased the intensity of monsoons.
Five episodes during the Quaternary
at 2.22 Ma
(PL-1), 1.83 Ma (PL-2), 0.68 Ma (PL-3), 0.45 Ma (PL-4) and 0.04 Ma (PL-5) were identified which showed a weakening of the Leeuwin Current (LC)
. The weakening of the LC would have an effect on the sea surface temperature (SST) field
in the Indian Ocean, as the Indonesian Throughflow
generally warms the Indian Ocean. Thus these five intervals could probably be those of considerable lowering of SST in the Indian Ocean and would have influenced Indian monsoon intensity. During the weak LC, there is the possibility of reduced intensity of the Indian winter monsoon and strong summer monsoon, because of change in the Indian Ocean dipole
due to reduction in net heat input to the Indian Ocean through the Indonesian Throughflow. Thus a better understanding of the possible links between El Niño
, Western Pacific Warm Pool, Indonesian Throughflow, wind pattern off western Australia, and ice volume expansion and contraction can be obtained by studying the behaviour of the LC during Quaternary at close stratigraphic intervals.
Strength of impact
On May 28, in the dry season
On August 28, in the rainy season
This visualization shows the Asian monsoon and how it develops using observational and modeled data. It also shows some of the impacts.
The impact of monsoon on the local weather is different from place to place. In some places there is just a likelihood of having a little more or less rain. In other places, quasi semi-deserts are turned into vivid green grasslands where all sorts of plants and crops can flourish.
The Indian Monsoon turns large parts of India from a kind of semi-desert into green lands. See photos only taken 3 months apart in the Western Ghats. In places like this it is crucial for farmers to have the right timing for putting the seeds on the fields, as it is essential to use all the rain that is available for growing crops.
Monsoons are large-scale sea breezes
which occur when the temperature on land is significantly warmer or cooler than the temperature of the ocean. These temperature imbalances happen because oceans and land absorb heat in different ways. Over oceans, the air temperature remains relatively stable for two reasons: water has a relatively high heat capacity
(3.9 to 4.2 J g−1
and because both conduction
will equilibrate a hot or cold surface with deeper water (up to 50 metres). In contrast, dirt, sand, and rocks have lower heat capacities (0.19 to 0.35 J g−1
and they can only transmit heat into the earth by conduction and not by convection. Therefore, bodies of water stay at a more even temperature, while land temperature are more variable.
During warmer months sunlight heats the surfaces of both land and oceans, but land temperatures rise more quickly. As the land's surface becomes warmer, the air above it expands and an area of low pressure
develops. Meanwhile, the ocean remains at a lower temperature than the land, and the air above it retains a higher pressure. This difference in pressure causes sea breezes
to blow from the ocean to the land, bringing moist air inland. This moist air rises to a higher altitude over land and then it flows back toward the ocean (thus completing the cycle). However, when the air rises, and while it is still over the land, the air cools
. This decreases the air's ability to hold water
, and this causes precipitation
over the land. This is why summer monsoons cause so much rain over land.
In the colder months, the cycle is reversed. Then the land cools faster than the oceans and the air over the land has higher pressure than air over the ocean. This causes the air over the land to flow to the ocean. When humid air rises over the ocean, it cools, and this causes precipitation over the oceans. (The cool air then flows towards the land to complete the cycle.)
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 vapor 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.
is caused when moist ocean air is lifted upwards by mountains,
convergence at the surface,
divergence aloft, or from storm-produced outflows at the surface.
However the lifting occurs, the air cools due to expansion in lower pressure, and this produces condensation
Africa (West African and Southeast African)
Southeast African monsoon clouds, over Mayotte
The monsoon of western Sub-Saharan Africa
is the result of the seasonal shifts of the Intertropical Convergence Zone
and the great seasonal temperature and humidity differences between the Sahara
and the equatorial Atlantic Ocean
The ITCZ migrates northward from the equatorial Atlantic in February, reaches western Africa on or near June 22, then moves back to the south by October.
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
depend upon this pattern for most of their precipitation.
The North American monsoon
) 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
, West Texas
. 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
It is also sometimes called the Desert monsoon
as a large part of the affected area are the Mojave
and Sonoran deserts
. However, it is debatable whether the North
and South American
weather patterns with incomplete wind reversal should be counted as true monsoons.
The Asian monsoons may be classified into a few sub-systems, such as the Indian Subcontinental Monsoon which affects the Indian subcontinent and surrounding regions including Nepal, and the East Asian Monsoon which affects southern China
and parts of Japan
South Asian monsoon
Onset dates and prevailing wind currents of the southwest summer monsoons in India
The southwestern summer monsoons occur from July through September. The Thar Desert
and adjoining areas of the northern and central Indian subcontinent heat up considerably during the hot summers. This causes a low pressure area over the northern and central Indian subcontinent. To fill this void, the moisture-laden winds from the Indian Ocean
rush into the subcontinent. These winds, rich in moisture, are drawn towards the Himalayas
. The Himalayas act like a high wall, blocking the winds from passing into Central Asia
, and forcing them to rise. As the clouds rise, their temperature
drops, and precipitation occurs
. Some areas of the subcontinent receive up to 10,000 mm (390 in) of rain annually.
The southwest monsoon is generally expected to begin around the beginning of June and fade away by the end of September. The moisture-laden winds on reaching the southernmost point of the Indian Peninsula
, due to its topography, become divided into two parts: the Arabian Sea Branch
and the Bay of Bengal Branch
The Arabian Sea Branch
of the Southwest Monsoon first hits the Western Ghats
of the coastal state of Kerala, India
, thus making this area the first state in India to receive rain from the Southwest Monsoon. This branch of the monsoon moves northwards along the Western Ghats
) with precipitation on coastal areas, west of the Western Ghats. The eastern areas of the Western Ghats do not receive much rain from this monsoon as the wind does not cross the Western Ghats.
The Bay of Bengal Branch
of Southwest Monsoon flows over the Bay of Bengal
heading towards North-East India
, picking up more moisture from the Bay of Bengal. The winds arrive at the Eastern Himalayas
with large amounts of rain. Mawsynram
, situated on the southern slopes of the Khasi Hills
in Meghalaya, India
, is one of the wettest places on Earth. After the arrival at the Eastern Himalayas, the winds turns towards the west
, travelling over the Indo-Gangetic Plain
at a rate of roughly 1–2 weeks per state,
pouring rain all along its way. June 1 is regarded as the date of onset of the monsoon in India, as indicated by the arrival of the monsoon in the southernmost state of Kerala.
The monsoon accounts for nearly 80% of the rainfall in India.
Indian agriculture (which accounts for 25% of the GDP and employs 70% of the population) is heavily dependent on the rains, for growing crops especially like cotton
and coarse grains. A delay of a few days in the arrival of the monsoon can badly affect the economy, as evidenced in the numerous droughts in India in the 1990s.
The monsoon is widely welcomed and appreciated by city-dwellers as well, for it provides relief from the climax of summer heat in June.
However, the roads take a battering every year. Often houses and streets are waterlogged and slums
are flooded despite drainage systems. A lack of city infrastructure coupled with changing climate patterns causes severe economic loss including damage to property and loss of lives, as evidenced in the 2005 flooding in Mumbai
that brought the city to a standstill. Bangladesh
and certain regions of India like Assam
and West Bengal
, also frequently experience heavy floods
during this season. Recently, 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.
The influence of the Southwest Monsoon is felt as far north as in China's Xinjiang
. It is estimated that about 70% of all precipitation in the central part of the Tian Shan Mountains
falls during the three summer months, when the region is under the monsoon influence; about 70% of that is directly of "cyclonic" (i.e., monsoon-driven) origin (as opposed to "local convection
Around September, with the sun retreating south, the northern landmass of the Indian subcontinent begins to cool off rapidly, and air pressure begins to build over northern India. The Indian Ocean and its surrounding atmosphere still hold their heat, causing cold wind to sweep down from the Himalayas
and Indo-Gangetic Plain
towards the vast spans of the Indian Ocean south of the Deccan
peninsula. This is known as the Northeast Monsoon or Retreating Monsoon.
While travelling towards the Indian Ocean, the cold dry wind picks up some moisture from the Bay of Bengal
and pours it over peninsular India and parts of Sri Lanka
. Cities like Chennai
, which get less rain from the Southwest Monsoon, receive rain from this Monsoon. About 50% to 60% of the rain received by the state of Tamil Nadu
is from the Northeast Monsoon.
In Southern Asia
, the northeastern monsoons take place from October to December when the surface high-pressure system
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 southern Asia, creating dry air streams
which produce clear skies over India. Meanwhile, a low pressure system known as a monsoon trough
develops over South-East Asia
and winds are directed toward Australia
East Asian Monsoon
Monsoon floods in the Philippines
The East Asian monsoon affects large parts of Indochina
, the Philippines
. It is characterised by a warm, rainy summer monsoon and a cold, dry winter monsoon. The rain occurs in a concentrated belt that stretches east–west except in East China where it is tilted east-northeast over Korea and Japan. The seasonal rain is known as Meiyu
in China, Jangma
in Korea, and Bai-u
in Japan, with the latter two resembling frontal rain.
The onset of the summer monsoon is marked by a period of premonsoonal rain over South China and Taiwan in early May. From May through August, the summer monsoon shifts through a series of dry and rainy phases as the rain belt moves northward, beginning over Indochina
and the South China Sea
(May), to the Yangtze River Basin
and Japan (June) and finally to North China
and Korea (July). When the monsoon ends in August, the rain belt moves back to South China
Also known as the Indo-Australian Monsoon. The rainy season occurs from September to February and it is a major source of energy for the Hadley circulation during boreal winter. The Maritime Continent Monsoon and the Australian Monsoon may be considered to be the same system, the Indo-Australian Monsoon.
The onset of the monsoon over the Maritime Continent tends to follow the heating maxima down Vietnam
and the Malay Peninsula
(September), to Sumatra
and the Philippines
(October), to Java
(November), Irian Jaya
and Northern Australia
(December, January). However, the monsoon is not a simple response to heating but a more complex interaction of topography, wind and sea, as demonstrated by its abrupt rather than gradual withdrawal from the region. The Australian monsoon (the "Wet") occurs in the southern summer when the monsoon trough develops over Northern Australia. Over three-quarters of annual rainfall in Northern Australia falls during this time.
The European Monsoon
(more commonly known as the return of the westerlies
) is the result of a resurgence of westerly winds from the Atlantic
, where they become loaded with wind and rain.
These westerly winds are a common phenomenon during the European winter, but they ease as spring approaches in late March and through April and May. The winds pick up again in June, which is why this phenomenon is also referred to as "the return of the westerlies".
The rain usually arrives in two waves, at the beginning of June, and again in mid- to late June. The European monsoon is not a monsoon in the traditional sense in that it doesn't meet all the requirements to be classified as such. Instead, the return of the westerlies is more regarded as a conveyor belt that delivers a series of low-pressure centres to Western Europe
where they create unsettled weather. These storms generally feature significantly lower-than-average temperatures, fierce rain or hail, thunder, and strong winds.
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