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A '''monsoon''' is a seasonal prevailing [[wind]] that lasts for several months. The term was first used in English in [[India]], [[Bangladesh]], [[Pakistan]], and neighboring countries to refer to the big seasonal winds blowing from the [[Indian Ocean]] and [[Arabian Sea]] in the southwest bringing heavy [[rainfall]] to the area.<ref>[[American Meteorological Society]] Glossary of Meteorology. [http://amsglossary.allenpress.com/glossary/search?p=1&query=monsoon&submit=Search Monsoon.] Retrieved on [[2008-03-14]].</ref> In [[hydrology]], monsoon rainfall is considered to be that which occurs in any region that receives the majority of its rain during a particular season. This allows other regions of the world such as [[North America]], [[South America]], [[Sub-Saharan Africa]], [[Australia]] and [[East Asian monsoon|East Asia]] to qualify as monsoon regions.<ref>International Committee of the Third Workshop on Monsoons. [http://caos.iisc.ernet.in/faculty/bng/IWM-III-BNG_overview.pdf The Global Monsoon System: Research and Forecast.] Retrieved on [[2008-03-16]].</ref> In terms of total [[Precipitation (meteorology)|precipitation]] and total area covered, the monsoons affecting the Indian subcontinent dwarf the North American monsoon, and the South Asian monsoon affects |
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==Definition== |
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[[Image:Monsoon clouds Lucknow.JPG|thumb|right|250px|Monsoon clouds over [[Lucknow]], [[India]]]] |
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The English ''monsoon'' came from Portuguese ''monção'', ultimately from Arabic ''mawsim'' (موسم "season"), "perhaps partly via early modern Dutch ''monsun''".<ref>''[[Oxford English Dictionary|OED]]'' online</ref> The definition includes major wind systems that change direction seasonally. |
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: "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."<ref>{{cite web |
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| title = Monsoon |
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| publisher = Britannica |
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| url = http://www.britannica.com/eb/article-9053445/monsoon |
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| accessdate = 2007-05-15}}</ref> |
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==History== |
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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.<ref>ROYDEN, L.H., BURCHFIEL, B.C., VAN DER HILST, Rob, WHIPPLE, K.X., HODGES, K.V., KING, R.W., and CHEN, Zhiliang. [http://gsa.confex.com/gsa/2006AM/finalprogram/abstract_109662.htm UPLIFT AND EVOLUTION OF THE EASTERN TIBETAN PLATEAU.] Retrieved on [[2008-05-11]].</ref> 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 to a timing of the monsoon starting 15-20 million years ago and linked to early Tibetan uplift.<ref>P. D. Clift, M. K. Clark, and L. H. Royden. [http://www.cosis.net/abstracts/EAE03/04300/EAE03-J-04300.pdf An Erosional Record of the Tibetan Plateau Uplift and Monsoon Strengthening in the Asian Marginal Seas.] Retrieved on [[2008-05-11]].</ref> Testing of this hypothesis awaits deep ocean sampling by the [[Integrated Ocean Drilling Program]].<ref>[[Integrated Ocean Drilling Program]]. [http://www.iodp.org/index.php?option=com_docman&task=doc_download&gid=2 Earth, Oceans, and Life.] Retrieved on [[2008]]-[[05-11]].</ref> The monsoon has varied significantly in strength since this time, largely linked to global climate change, especially the cycle of the [[Pleistocene]] ice ages.<ref>Anil K. Gupta and Ellen Thomas. [http://geology.geoscienceworld.org/cgi/content/abstract/31/1/47 Initiation of Northern Hemisphere glaciation and strengthening of the northeast Indian monsoon: Ocean Drilling Program Site 758, eastern equatorial Indian Ocean.] Retrieved on [[2008-05-11]].</ref> Timing of the monsoon strengthening of the Indian Monsoon of around 5 million years ago was suggested due to an interval of closing of the Indonesian Seaway to cold thermocline waters passage from the Pacific to the Indian Ocean which is believed to have resulted in an increased sea surface temperature in the Indian Ocean, which increased [[gyre|gyral]] circulation and then caused an increased intensity of the monsoon.<ref>M. S. SRINIVASAN and D. K. SINHA. [http://cat.inist.fr/?aModele=afficheN&cpsidt=924989 Ocean circulation in the tropical Indo-Pacific during early Pliocene (5.6-4.2 Ma) : Paleobiogeographic and isotopic evidence.] Retrieved on [[2008-05-11]].</ref> Sinha et al. (2006) identified five episodes during the Quaternary at 2.22 (PL-1), 1.83 (PL-2), 0.68 (PL-3), 0.45 (PL-4) and 0.04 Ma (PL-5), of weakening of [[Leeuwin Current]] ([[Western Australia]]) and postulated that the weakening of the LC would have an effect on the sea surface temperature (SST) 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 definitely have influenced Indian monsoon intensity. They ( Sinha et al., 2006) stated that that during the weak LC there is the possibility of reduced intensity of 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 Nino, Western Pacific Warm Pool (WPWP), 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.<ref>Sinha,D.K., Singh,A.K. and Tiwari,M (2006). Palaeoceanographic and palaeoclimatic history of ODP site 763A (Exmouth Plateau), South-east Indian Ocean: 2.2 Ma record of planktic foraminifera. CURRENT SCIENCE, VOL. 90, NO. 10, 25 May 2006</ref> |
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== Process == |
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[[Image:Howrah Bridge 01.jpg|thumb|right|250px|View of the Indian Ocean Monsoon clouds over [[Howrah Bridge]], [[Kolkata]].]] |
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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 meters deep, through the action of wind and buoyancy-generated [[turbulence]], whereas the land surface conducts heat slowly, with the seasonal signal penetrating perhaps a meter or so. Additionally, the [[specific heat capacity]] 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 the air over the land warms faster and reaches a higher temperature than the air over the ocean.<ref name="Oracle">Oracle Thinkquest Education Foundation. [http://library.thinkquest.org/C003603/english/monsoons/causesofmonsoons.shtml monsoons: causes of monsoons.] Retrieved on [[2008-05-22]].</ref> Heating of the air over the land reduces the air's density, creating an area of [[low pressure]]. This produces a wind blowing toward the land, bringing moist near-surface air from over the ocean. 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 to expansion, which in turn produces [[condensation]]. |
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In winter, the land cools off quickly, but the ocean retains heat longer. The cold air over the land creates a high pressure area which produces a breeze from land to ocean.<ref name="Oracle"/> Monsoons are similar to [[sea breezes|sea and land breezes]], a term usually referring to the localized, diurnal (daily) cycle of circulation near coastlines, but they are much larger in scale, stronger and seasonal.<ref>[[BBC]] Weather. [http://www.bbc.co.uk/weather/features/understanding/monsoon.shtml The Asian Monsoon.] Retrieved on [[2008-05-22]].</ref> |
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== Monsoon Systems == |
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[[Image:Evening monsoonal squall.jpg|thumb|right|Monsoonal squall nears [[Darwin, Northern Territory|Darwin]], [[Northern Territory]]]] |
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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. |
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Even more broadly, it is now understood that in the geological past, monsoon systems must have always accompanied the formation of [[supercontinent]]s such as [[Pangaea]], with their extreme [[continental climate]]s.{{Fact|date=September 2008}} |
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=== Northeast Monsoon (Southern Asia and Australasia)=== |
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{{See also|Monsoon trough}} |
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In Southern [[Asia]], the northeastern monsoons take place from December to early March. The temperature over central Asia is less than 25°C 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 southern Asia, creating dry [[air stream]]s which produce clear skies over India. Meanwhile, a low pressure system develops over [[South-East Asia]] and [[Australasia]] and winds are directed toward [[Australia]] known as a [[monsoon trough]]. |
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=== Northern Indian Ocean Monsoon=== <!-- This section is linked from [[Thiruvananthapuram]] --> |
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[[Image:India southwest summer monsoon onset map en.svg|thumb|right|Onset dates and prevailing wind currents of the southwest summer monsoon.]] |
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==== South-West Summer Monsoon ==== |
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The southwestern summer monsoons occur from June through September. The [[Thar Desert|Great Indian Desert (Thar Desert)]] and adjoining areas of the northern and central [[Indian subcontinent]] heats 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 in to the subcontinent. These winds, rich in moisture, are drawn towards the [[Himalaya]]s, 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. |
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The southwest monsoon is generally expected to begin around the start of June and dies down by the end of September. The moisture-laden winds on reaching the southernmost point of the [[Indian peninsula]], due to its topology, become divided into two parts: |
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* ''Arabian Sea Branch of the SW Monsoon'' |
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* ''Bay of Bengal Branch of the SW Monsoon'' |
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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 [[east]]ern parts of the [[Western Ghats]] do not receive much rain from this monsoon as the wind does not cross the [[Western Ghats]]. |
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[[Image:Rain in Kerela 01.jpg|thumb|left|250px|View of south-west monsoon rain in [[Kerala]].]] |
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The '''Bay of Bengal Branch of SW Monsoon''' flows over the [[Bay of Bengal]] heading towards [[North-East India|North-Eastern India]] and [[Bengal]], picking up more moisture from the [[Bay of Bengal]]. Its hits the [[Himalaya|Eastern Himalaya]] and provides a huge amount of rain to the regions of [[North-East India]], [[Bangladesh]] and [[West Bengal]]. [[Mawsynram]], situated on the southern slopes of the [[Himalaya|Eastern Himalaya]] in [[Shillong]], [[India]] is one of the wettest places on Earth. After striking the [[Himalaya|Eastern Himalaya]] it turns towards the [[West]], travels over the [[Indo-Gangetic Plain]], at a rate of roughly 1–2 weeks per state{{Fact|date=February 2007}}, pouring rain all along its way. |
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The monsoon accounts for 80 percent of the rainfall in the country{{Fact|date=February 2007}}. [[India]]n [[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. |
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The monsoon is widely welcomed and appreciated by city-dwellers as well, for it provides relief from the climax of summer heat in June.<ref>Official Web Site of District Sirsa, India. [http://sirsa.gov.in/htfiles/location.html District Sirsa.] Retrieved on [[2008-12-27]].</ref> 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, [[slum]]s 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 [[2005 Maharashtra floods|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. |
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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. |
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[[Image:Central Calcutta after Rain.jpg|thumb|right|250px|View of central [[Kolkata]] after a monsoon rain.]] |
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==== North-East Monsoon (Retreating Monsoon) ==== |
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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 [[North India|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'''. |
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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{{Fact|Apr 2007|date=April 2007}}. |
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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. |
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=== North American Monsoon === |
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{{main|North American Monsoon}} |
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[[Image:azmonsoon.jpg|thumb|right|250px|[[Arizona]] monsoon season clouds]] |
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'''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.<ref>[[Arizona State University]] Department of Geography. [http://geography.asu.edu/aztc/monsoon.html Basics of Arizona Monsoon.] Retrieved on [[2008-02-29]].</ref><ref>New Mexico Tech. [http://www.ees.nmt.edu/vivoni/hydromet/lectures/Lecture17.pdf Lecture 17: 1. North American Monsoon System.] Retrieved on [[2008-02-29]].</ref> It is also sometimes called the ''Desert Monsoon'' as a large part of the affected area are the [[Mojave Desert|Mojave]] and [[Sonoran Desert]]s. |
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=== African Monsoon === |
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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]]. It migrates northward from the equatorial Atlantic in February, reaches western Africa on [[June 22]], then moves back to the south by October.<ref>Innovations Report. [http://www.innovations-report.de/html/berichte/umwelt_naturschutz/bericht-16061.html Monsoon in West Africa: Classic continuity hides a dual-cycle rainfall regime.] Retrieved on [[2008-05-25]].</ref> The dry, northeasterly [[trade winds]], and their more extreme form, the [[harmattan]], are interrupted by the northern shift in the [[Intertropical Convergence Zone|ITCZ]] and resultant southerly, rain-bearing winds during the summer. The semiarid [[Sahel]] and [[Sudan (region)|Sudan]] depend upon this pattern for most of their precipitationed area is desert. |
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==See also== |
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* [[Climate of India#Monsoons|Climate of India]] |
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* [[Monsoon trough]] |
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* [[Monsoon Cup]] |
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* [[United States rainfall climatology]] |
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==References == |
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{{reflist}} |
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* Initial text from the [[Goddard Space Flight Center]]'s public domain [http://daac.gsfc.nasa.gov/ Distributed Active Archive Center] |
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== External links == |
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{{wikinews|Asian monsoon rains force millions to flee}} |
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* [http://www.wrh.noaa.gov/fgz/science/monsoon.php?wfo=fgz National Weather Service: The North American Monsoon] |
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* [http://earthobservatory.nasa.gov/Newsroom/Campaigns/NAME_Mission.html North American Monsoon Experiment] |
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* [http://eamex.iastate.edu/ East Asian Monsoon Experiment] |
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* [http://nasrani.net/2007/02/18/influence-of-monsoon-winds/ Influence of monsoon winds] |
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* [http://www.pbs.org/wnet/nature/monsoon/html/intro.html Meet the Indian monsoons at PBS.org] |
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* [http://www.azcentral.com/weather/monsoon/ Arizona Central monsoon page] |
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* [http://www.pepperridgenorthvalley.com/monsoon_basics.php Basics of the Arizona Monsoon] |
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* [http://www.amma-international.org/rubrique.php3?id_rubrique=109 AMMA - African Monsoon Multidisciplinary Analyses] |
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* [http://www.youtube.com/watch?v=mAdhjKglM3A/ Flooding of urban areas due to Monsoon showers, Phnom Penh-Cambodia > Video attachment] |
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{{Koppen}} |
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[[Category:Weather hazards]] |
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[[Category:Wind]] |
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[[Category:Arabic words and phrases]] |
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[[Category:Flood]] |
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[[Category:Climate pattern]] |
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[[Category:Climate]] |
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[[Category:Portuguese loanwords]] |
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[[ar:ريح موسمي]] |
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[[bs:Monsun]] |
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[[bg:Мусон]] |
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[[ca:Monsó]] |
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[[cs:Monzun]] |
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[[da:Monsun]] |
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[[de:Monsun]] |
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[[et:Mussoon]] |
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[[el:Μουσώνας]] |
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[[es:Monzón]] |
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[[eo:Musono]] |
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[[eu:Montzoi]] |
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[[fr:Mousson]] |
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[[gl:Monzón]] |
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[[ko:계절풍]] |
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[[hi:मॉनसून]] |
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[[hr:Monsun]] |
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[[io:Musono]] |
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[[ilo:Nepnep]] |
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[[id:Muson]] |
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[[it:Monsone]] |
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[[he:מונסון]] |
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[[kk:Муссондар]] |
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[[lt:Musonas]] |
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[[hu:Monszun]] |
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[[ml:മണ്സൂണ്]] |
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[[nl:Moesson (regentijd)]] |
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[[ja:モンスーン]] |
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[[no:Monsun]] |
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[[pl:Monsun]] |
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[[pt:Monção]] |
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[[ro:Muson]] |
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[[ru:Муссон]] |
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[[sk:Monzún]] |
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[[sl:Monsun]] |
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[[sh:Monsunska klima]] |
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[[fi:Monsuuni]] |
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[[sv:Monsun]] |
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[[tl:Balaklaot]] |
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[[ta:பருவப் பெயர்ச்சிக் காற்று]] |
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[[te:ఋతుపవనాలు]] |
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[[th:มรสุม]] |
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[[uk:Мусон]] |
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[[vi:Gió mùa]] |
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[[zh:季风]] |
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