View source for Earth:Monsoon trough

{{short description|Weather phenomenon}}
[[File:Auguststreamlinesnavy.jpg|thumb|right|230px|August position of the ITCZ and monsoon trough in the Pacific Ocean, depicted by area of convergent streamlines in the northern Pacific]]

The '''monsoon trough''' is a portion of the [[Earth:Intertropical Convergence Zone|Intertropical Convergence Zone]] in the Western Pacific,<ref name="ams"/><ref name="Wang">Bin Wang. [https://books.google.com/books?id=yRT57TENzT8C&dq=weather+along+the+monsoon+trough&pg=PT207 The Asian Monsoon.] Retrieved 2008-05-03.</ref> as depicted by a line on a [[Earth:Weather map|weather map]] showing the locations of minimum sea level pressure,<ref name="ams">{{cite web|title=Monsoon trough|url=http://amsglossary.allenpress.com/glossary/search?p=1&query=monsoon+trough&submit=Search|work=Glossary of Meteorology|publisher=American Meteorological Society|access-date=2009-06-04|url-status=dead|archive-url=https://web.archive.org/web/20090617192300/http://amsglossary.allenpress.com/glossary/search?p=1&query=monsoon+trough&submit=Search|archive-date=17 June 2009}}</ref> and as such, is a [[Earth:Convergence zone|convergence zone]] between the wind patterns of the southern and northern hemispheres.

Westerly [[Earth:Monsoon|monsoon]] winds lie in its equatorward portion while easterly [[Earth:Trade winds|trade winds]] exist poleward of the trough.<ref>[[Organization:World Meteorological Organization|World Meteorological Organization]]. [http://severe.worldweather.wmo.int/tc/sp/acronyms.html Severe Weather Information Centre.] Retrieved 2008-05-03.</ref> Right along its axis, heavy rains can be found which usher in the peak of a location's respective rainy season. The monsoon trough plays a role in creating many of the world's [[Earth:Rainforest|rainforest]]s.<ref>Hobgood (2008)[http://geog-www.sbs.ohio-state.edu/courses/G230/hobgood/ASP230Lecture24.ppt Global Pattern of Surface Pressure and Wind.] {{webarchive|url=https://web.archive.org/web/20090318021749/http://geog-www.sbs.ohio-state.edu/courses/G230/hobgood/ASP230Lecture24.ppt |date=18 March 2009}} Ohio State University. Retrieved 2009-03-08.</ref>

The term ''monsoon trough'' is most commonly used in monsoonal regions of the Western Pacific such as Asia and {{wipe|Australia}}. The migration of the ITCZ/monsoon trough into a landmass heralds the beginning of the annual rainy season during summer months. Depressions and [[Physics:Tropical cyclone|tropical cyclone]]s often form in the vicinity of the monsoon trough, with each capable of producing a year's worth of rainfall in a matter of days.

==Movement and strength==
[[File:Februarystreamlinesnavy.jpg|thumb|right|200 px|February position of the ITCZ and monsoon trough in the Pacific Ocean, depicted by area of convergent streamlines offshore Australia and in the equatorial eastern Pacific]]
Monsoon troughing in the western Pacific reaches its zenith in [[Earth:Latitude|latitude]] during the late summer when the wintertime surface ridge in the opposite hemisphere is the strongest.  It can reach as far as the 40th parallel in East Asia during August and the 20th parallel in Australia during February. Its poleward progression is accelerated by the onset of the summer monsoon which is characterized by the development of lower air pressure over the warmest part of the various continents.<ref name="NCFMRF">National Centre for Medium Range Forecasting. [http://www.ncmrwf.gov.in/Chapter-II.pdf Chapter-II Monsoon-2004: Onset, Advancement and Circulation Features.] {{webarchive|url=https://web.archive.org/web/20110721161408/http://www.ncmrwf.gov.in/Chapter-II.pdf |date=21 July 2011}} Retrieved 2008-05-03.</ref><ref>Australian Broadcasting Corporation. [http://www.abc.net.au/storm/monsoon/print.htm Monsoon.] {{Webarchive|url=https://web.archive.org/web/20010223103812/http://www.abc.net.au/storm/monsoon/print.htm |date=23 February 2001}} Retrieved 2008-05-03.</ref><ref>Dr. Alex DeCaria. [http://snowball.millersville.edu/~adecaria/ESCI344/esci344_lesson04_seasonal_mean_wind_fields.html Lesson 4 – Seasonal-mean Wind Fields.] {{webarchive|url=https://web.archive.org/web/20090822081957/http://snowball.millersville.edu/~adecaria/ESCI344/esci344_lesson04_seasonal_mean_wind_fields.html |date=22 August 2009 }} Retrieved 2008-05-03.</ref> In the [[Earth:Southern Hemisphere|Southern Hemisphere]], the monsoon trough associated with the Australian monsoon reaches its most southerly latitude in February,<ref name="NAVY2"/> oriented along a west-northwest/east-southeast axis.

===Effect of wind surges===
Increases in the relative [[Vorticity|vorticity]], or spin, with the monsoon trough are normally a product of increased wind convergence within the [[Earth:Convergence zone|convergence zone]] of the monsoon trough. Wind surges can lead to this increase in convergence. A strengthening or equatorward movement in the [[Earth:Subtropical ridge|subtropical ridge]] can cause a strengthening of a monsoon trough as a wind surge moves towards the location of the monsoon trough. As [[Earth:Surface weather analysis|front]]s move through the [[Earth:Subtropics|subtropics]] and tropics of one hemisphere during their winter, normally as shear lines when their temperature gradient becomes minimal, wind surges can cross the equator in oceanic regions and enhance a monsoon trough in the other hemisphere's summer.<ref>Chih-Lyeu Chen. [http://stinet.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA232106 Effects of the Northeast Monsoon on the Equatorial Westerlies Over Indonesia.]{{dead link|date=June 2022|bot=medic}}{{cbignore|bot=medic}} Retrieved 2008-05-03.</ref> A key way of detecting whether a wind surge has reached a monsoon trough is the formation of a burst of thunderstorms within the monsoon trough.<ref name="NAVY3">U. S. Navy. [http://www.nrlmry.navy.mil/~chu/chap3/se301.htm SECTION 3. DYNAMIC CONTRIBUTORS TO TROPICAL CYCLONE FORMATION.] Retrieved 2006-11-26.</ref>

===Monsoon depressions===
[[File:MonsoonDepression2004091600.jpg|thumb|left|upright=0.85|Monsoon depression near Bangladesh]]
If a circulation forms within the monsoon trough, it is able to compete with the neighboring [[Physics:Thermal low|thermal low]] over the continent, and a wind surge will occur at its periphery.  Such a circulation which is broad in nature within a monsoon trough is known as a monsoon depression. In the Northern Hemisphere, monsoon depressions are generally asymmetric, and tend to have their strongest winds on their eastern periphery.<ref name="NAVY3"/> Light and variable winds cover a large area near their center, while bands of showers and thunderstorms develop within their area of circulation.<ref>Chip Guard. [http://research.eastwestcenter.org/climate/risa/Climate_Var_&_Chg-CNMI_27_Apr_06.pdf Climate Variability on CNMI.] {{webarchive|url=https://web.archive.org/web/20070622115803/http://research.eastwestcenter.org/climate/risa/Climate_Var_%26_Chg-CNMI_27_Apr_06.pdf |date=22 June 2007}} Retrieved 2008-05-03.</ref>

The presence of an upper level [[Earth:Jet stream|jet stream]] poleward and west of the system can enhance its development by leading to increased diverging air aloft over the monsoon depression, which leads to a corresponding drop in surface pressure.<ref>Sixiong Zhao and Graham A. Mills. [http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F1520-0493(1991)119%3C2074%3AASOAMD%3E2.0.CO%3B2 A Study of a Monsoon Depression Bringing Record Rainfall over Australia. Part II: Synoptic–Diagnostic Description.] Retrieved 2008-05-03.</ref> Even though these systems can develop over land, the outer portions of monsoon depressions are similar to tropical cyclones.<ref>N.E. Davidson and G.J. Holland. [http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F1520-0493(1987)115%3C0380%3AADAOTI%3E2.0.CO%3B2 A Diagnostic Analysis of Two Intense Monsoon Depressions over Australia.] Retrieved 2008-05-03.</ref> In India, for example, 6 to 7 monsoon depressions move across the country yearly,<ref name="NCFMRF"/> and their numbers within the Bay of Bengal increase during July and August of [[Earth:El Niño|El Niño]] events.<ref>O. P. Singh, Tariq Masood Ali Khan, and Md. Sazedur Rahman. [https://doi.org/10.1023%2FA%3A1013736923929 Impact of Southern Oscillation on the Frequency of Monsoon Depressions in the Bay of Bengal.] Retrieved 2008-05-03.</ref> Monsoon depressions are efficient rainfall producers, and can generate a year's worth of rainfall when they move through drier areas, such as the outback of Australia.<ref>Bureau of Meteorology. [https://web.archive.org/web/20060921044100/http://www.bom.gov.au/bmrc/clfor/cfstaff/jmb/TWPICE_reports/synoptic_01_Feb_2006.pdf TWP-ICE Synoptic Overview, 1 February 2006.] Retrieved 2008-05-03.</ref>

Some tropical cyclones recognised by Regional Specialized Meteorological Centres would have characteristics of a monsoon depression throughout their lifetime. The [[Earth:Joint Typhoon Warning Center|Joint Typhoon Warning Center]] (JTWC) added ''monsoon depression'' as a category in 2015, and Cyclone Komen is the first system recognised as a fully monsoon depression by JTWC.<ref>{{cite web |title=North Indian Ocean Best Track Data |url=https://www.metoc.navy.mil/jtwc/jtwc.html?north-indian-ocean |publisher=Joint Typhoon Warning Center |access-date=25 May 2020}}</ref>

==Roles==
===In rainy season===
[[File:Central Calcutta after Rain.jpg|thumb|right|200px|View of central [[Place:Kolkata|Kolkata]] after a monsoon rain.]]
Since the monsoon trough is an area of convergence in the wind pattern, and an elongated area of low pressure at the surface, the trough focuses low level moisture and is defined by one or more elongated [[Earth:Rainband|bands]] of [[Physics:Thunderstorm|thunderstorm]]s when viewing [[Astronomy:Satellite imagery|satellite imagery]]. Its abrupt movement to the north between May and June is coincident with the beginning of the monsoon regime and rainy seasons across South and East Asia. This convergence zone has been linked to prolonged heavy rain events in the Yangtze river as well as northern China.<ref name="Wang"/> Its presence has also been linked to the peak of the rainy season in locations within Australia.<ref>Bureau of Meteorology. [http://www.bom.gov.au/weather/sa/giles/climate.shtml Climate of Giles.] {{webarchive|url=https://web.archive.org/web/20080811064933/http://www.bom.gov.au/weather/sa/giles/climate.shtml |date=11 August 2008 }} Retrieved 2008-05-03.</ref>

===In tropical cyclogenesis===
[[File:WorldwideTCpeaks.gif|thumb|right|200px|Months of peak tropical cyclone activity worldwide]]
{{Main|Earth:Tropical cyclogenesis}}
A monsoon trough is a significant genesis region for [[Physics:Tropical cyclone|tropical cyclone]]s. [[Vorticity]]-rich low level environments, with significant low level spin, lead to a better than average chance of tropical cyclone formation due to their inherent rotation. This is because a pre-existing near-surface disturbance with sufficient spin and convergence is one of the six requirements for [[Earth:Tropical cyclogenesis|tropical cyclogenesis]].<ref>Christopher Landsea. [http://www.aoml.noaa.gov/hrd/Landsea/climvari/index.html Climate Variability of Tropical Cyclones: Past, Present and Future.] Retrieved 2006-11-26.</ref> There appears to be a 15- to 25-day cycle in thunderstorm activity associated with the monsoon trough, which is roughly half the wavelength of the [[Earth:Madden–Julian oscillation|Madden–Julian oscillation]], or MJO.<ref>Patrick A. Harr. [http://www.onr.navy.mil/sci_tech/32/reports/docs/05/mmharr.pdf Tropical Cyclone Formation/Structure/Motion Studies.] {{webarchive|url=https://web.archive.org/web/20071129132952/http://www.onr.navy.mil/sci_tech/32/reports/docs/05/mmharr.pdf |date=29 November 2007}} Retrieved 2006-11-26.</ref> This mirrors tropical cyclone genesis near these features, as genesis clusters in 2–3&nbsp;weeks of activity followed by 2–3&nbsp;weeks of inactivity. Tropical cyclones can form in outbreaks around these features under special circumstances, tending to follow the next cyclone to its poleward and west.<ref>Joint Typhoon Warning Center. [http://www.npmoc.navy.mil/jtwc/atcr/1995atcr/pdf/wnp/18w.pdf Typhoon Polly.] {{webarchive|url=https://web.archive.org/web/20060919032944/http://www.npmoc.navy.mil/jtwc/atcr/1995atcr/pdf/wnp/18w.pdf |date=19 September 2006}} Retrieved 2006-11-26.</ref>

Whenever the monsoon trough on the eastern side of the summertime Asian monsoon is in its normal orientation (oriented east-southeast to west-northwest), [[Physics:Tropical cyclone|tropical cyclone]]s along its periphery will move with a westward motion. If it reverses its orientation, orienting southwest to northeast, tropical cyclones will move more poleward. Tropical cyclone tracks with S-shapes tend to be associated with reverse-oriented monsoon troughs.<ref>Mark A. Lander. [http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F1520-0434(1996)011%3C0170:STCTTA%3E2.0.CO%3B2 Specific Tropical Cyclone Track Types and Unusual Tropical Cyclone Motions Associated with a Reverse-Oriented Monsoon Trough in the Western North Pacific.] Retrieved 2006-11-26.</ref> The [[Earth:South Pacific convergence zone|South Pacific convergence zone]] and South American convergence zones are generally reverse oriented.<ref name="NAVY2">U. S. Navy. [http://www.nrlmry.navy.mil/~chu/chap2/se102.htm 1.2 Pacific Ocean Surface Streamline Pattern.] Retrieved 2006-11-26.</ref> The failure of the monsoon trough, or the ITCZ, to move south of the equator in the eastern Pacific Ocean and Atlantic Ocean during the southern hemisphere summer, is considered one of the factors causing tropical cyclones to not normally form in those regions.<ref name="NAVY3"/> It has also been noted that when the monsoon trough lies near 20 degrees north latitude in the Pacific, the frequency of tropical cyclones is 2 to 3 times greater than when it lies closer to 10&nbsp;degrees north.<ref name="Wang"/>

==References==
{{reflist}}


[[Category:Tropical meteorology]]
[[Category:Atmospheric dynamics]]

{{Sourceattribution|Monsoon trough}}