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A monsoon () 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 annual latitudinal oscillation of the Intertropical Convergence Zone (ITCZ) between its limits to the north and south of the equator. Usually, the term monsoon is used to refer to the Wet season of a seasonally changing pattern, although technically there is also a dry phase. The term is also sometimes used to describe locally heavy but short-term rains.
The major monsoon systems of the world consist of the West African, Asian–Australian, the North American, and South American monsoons.
The term was first used in English in British India and neighboring countries to refer to the big seasonal winds blowing from the Bay of Bengal and Arabian Sea in the southwest bringing heavy rainfall to the area.International Committee of the Third Workshop on Monsoons. The Global Monsoon System: Research and Forecast. Retrieved on 2008-03-16.
A study of marine plankton suggested that the South Asian Monsoon (SAM) 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. In 2018, a study of the SAM's variability over the past million years found that precipitation resulting from the monsoon was significantly reduced during compared to interglacial periods like the present day. The Indian Summer Monsoon (ISM) underwent several intensifications during the warming following the Last Glacial Maximum, specifically during the time intervals corresponding to 16,100–14,600 BP, 13,600–13,000 BP, and 12,400–10,400 BP as indicated by vegetation changes in the Tibetan Plateau displaying increases in humidity brought by an intensifying ISM. Though the ISM was relatively weak for much of the Late Holocene, significant glacial accumulation in the Himalayas still occurred due to cold temperatures brought by westerlies from the west.
During the Middle Miocene, the July ITCZ, the zone of rainfall maximum, migrated northwards, increasing precipitation over southern China during the East Asian Summer Monsoon (EASM) while making Indochina drier. During the Late Miocene Global Cooling (LMCG), from 7.9 to 5.8 million years ago, the East Asian Winter Monsoon (EAWM) became stronger as the subarctic front shifted southwards. An abrupt intensification of the EAWM occurred 5.5 million years ago. The EAWM was still significantly weaker relative to today between 4.3 and 3.8 million years ago but abruptly became more intense around 3.8 million years ago as crustal stretching widened the Tsushima Strait and enabled greater inflow of the warm Tsushima Current into the Sea of Japan. Circa 3.0 million years ago, the EAWM became more stable, having previously been more variable and inconsistent, in addition to being enhanced further amidst a period of global cooling and sea level fall. The EASM was weaker during cold intervals of glacial periods such as the Last Glacial Maximum (LGM) and stronger during interglacials and warm intervals of glacial periods. Another EAWM intensification event occurred 2.6 million years ago, followed by yet another one around 1.0 million years ago. During Dansgaard–Oeschger events, the EASM grew in strength, but it has been suggested to have decreased in strength during . The EASM expanded its influence deeper into the interior of Asia as sea levels rose following the LGM; it also underwent a period of intensification during the Middle Holocene, around 6,000 years ago, due to orbital forcing made more intense by the fact that the Sahara at the time was much more vegetated and emitted less dust. This Middle Holocene interval of maximum EASM was associated with an expansion of temperate deciduous forest steppe and temperate mixed forest steppe in northern China. By around 5,000 to 4,500 BP, the East Asian monsoon's strength began to wane, weakening from that point until the present day. A particularly notable weakening took place ~3,000 BP. The location of the EASM shifted multiple times over the course of the Holocene: first, it moved southward between 12,000 and 8,000 BP, followed by an expansion to the north between approximately 8,000 and 4,000 BP, and most recently retreated southward once more between 4,000 and 0 BP.
Five episodes during the Quaternary at 2.22 Megaannum (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.
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 thermal low 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 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.
Similar rainfall is caused when moist ocean air is lifted upwards by mountains,Dr. Michael Pidwirny (2008). CHAPTER 8: Introduction to the Hydrosphere (e). Cloud Formation Processes. Physical Geography. Retrieved on 2009-01-01. surface heating,Bart van den Hurk and Eleanor Blyth (2008). Global maps of Local Land–Atmosphere coupling. KNMI. Retrieved on 2009-01-02. convergence at the surface,Robert Penrose Pearce (2002). Meteorology at the Millennium. Academic Press, p. 66. . Retrieved on 2009-01-02. 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.
| Northern Mexico | North American/Gulf of California-Southwest USA | late May | September | incomplete wind reversal, waves |
| Tucson, Arizona, USA | North American/Gulf of California-Southwest USA | early July | September | incomplete wind reversal, waves |
| Central America | Central/South American Monsoon | April | October | true monsoon |
| Amazon Brazil | South American monsoon | September | May | waves |
| Southeast Brazil | South American monsoon | November | March | waves |
| West Africa | West African | June 22Innovations Report. Monsoon in West Africa: Classic continuity hides a dual-cycle rainfall regime. Retrieved on 2008-05-25. | Sept /October | waves |
| Southeast Africa | Southeast Africa monsoon w/ Harmattan | Jan | March | waves |
| Kerala, India | Indo-Australian/Indian-Indochina/East Asian monsoon | Jun 1 | Dec 1 | persistent |
| Mumbai, India | Indo-Australian/Indian-Indochina/East Asian monsoon | June 10 | Oct 1 | persistent |
| Karachi, Pakistan | Indo-Australian/Indian-Indochina/East Asian monsoon | late June - early July | September | abrupt |
| Lahore, Pakistan | Indo-Australian/Indian-Indochina/East Asian monsoon | late June | end of September | abrupt |
| Phuket, Thailand | Indo-Australian/Indian-Indochina/East Asian monsoon | February/March | December | persistent |
| Colombo, Sri Lanka | Indo-Australian/Indian-Indochina/East Asian monsoon | May 25 | Dec 15 | persistent |
| Bangkok, Thailand | Indo-Australian/Indian-Indochina/East Asian monsoon | April–May | October/November | persistent |
| Yangon, Myanmar | Indo-Australian/Indian-Indochina/East Asian monsoon | May 25 | Nov 1 | persistent |
| Dhaka, Bangladesh | Indo-Australian/Indian-Indochina/East Asian monsoon | mid-June | October | abrupt |
| Cebu, Philippines | Indo-Australian/Borneo-Australian/Australian monsoon | October | March | abrupt |
| Kelantan, Malaysia | Indo-Australian/Borneo-Australian/Australian monsoon | October | March | waves |
| Jakarta, Indonesia | Indo-Australian/Borneo-Australian/Australian monsoon | November | March | abrupt |
| Kaohsiung, Taiwan | Indo-Australian/Indian-Indochina/East Asian monsoon | May 10 | October | abrupt |
| Taipei, Taiwan | Indo-Australian/Indian-Indochina/East Asian monsoon | May 20 | October | abrupt |
| Hanoi, Vietnam | Indo-Australian/Indian-Indochina/East Asian monsoon | May 20 | October | abrupt |
| Kagoshima, Japan | Indo-Australian/Indian-Indochina/East Asian monsoon | Jun 10 | October | abrupt |
| Seoul, South Korea | Indo-Australian/Indian-Indochina/East Asian monsoon | July 10 | September | abrupt |
| Beijing, China | Indo-Australian/Indian-Indochina/East Asian monsoon | July 20 | September | abrupt |
| Darwin, Australia | Indo-Australian/Borneo-Australian/Australian monsoon | Oct | April | persistent |
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 (Konkan and Goa) 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 and Bengal, picking up more moisture from the Bay of Bengal. The winds arrive at the Himalaya 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, rice, oilseeds 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.Official Web Site of District Sirsa, India. District Sirsa. Retrieved on 2008-12-27. 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 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 "convection rain").Felix P. Blumer (1998). 9781901502459, International Association of Hydrological Sciences. . ISBN 9781901502459 The effects also extend westwards to the Mediterranean, where however the impact of the monsoon is to induce drought via the Rodwell-Hoskins mechanism.
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 is strongest. 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 which produce clear skies over India. Meanwhile, a low pressure system known as a monsoon trough develops over South-East Asia and Australasia and winds are directed toward Australia. In the Philippines, northeast monsoon is called Amihan.
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 and Japan (June) and finally to northern China and Korea (July). When the monsoon ends in August, the rain belt moves back to southern China.
The onset of the monsoon over Australia tends to follow the heating maxima down Vietnam and the Malay Peninsula (September), to Sumatra, Borneo and the Philippines (October), to Java, Sulawesi (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 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.
The return of the westerlies affects Europe's Northern Atlantic coastline, more precisely Ireland, Great Britain, the Benelux, western Germany, northern France and parts of Scandinavia.
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