Monsoon trough

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August position of the ITCZ and monsoon trough in the Pacific Ocean, depicted by area of convergent streamlines in the northern Pacific Auguststreamlinesnavy.jpg
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 Intertropical Convergence Zone in the Western Pacific, [1] [2] as depicted by a line on a weather map showing the locations of minimum sea level pressure, [1] and as such, is a convergence zone between the wind patterns of the southern and northern hemispheres.

Intertropical Convergence Zone

The Intertropical Convergence Zone (ITCZ), known by sailors as the doldrums or the calms, is the area encircling Earth near the Equator, where the northeast and southeast trade winds converge.

Weather map

A weather map displays various meteorological features across a particular area at a particular point in time and has various symbols which all have specific meanings. Such maps have been in use since the mid-19th century and are used for research and weather forecasting purposes. Maps using isotherms show temperature gradients, which can help locate weather fronts. Isotach maps, analyzing lines of equal wind speed, on a constant pressure surface of 300 or 250 hPa show where the jet stream is located. Use of constant pressure charts at the 700 and 500 hPa level can indicate tropical cyclone motion. Two-dimensional streamlines based on wind speeds at various levels show areas of convergence and divergence in the wind field, which are helpful in determining the location of features within the wind pattern. A popular type of surface weather map is the surface weather analysis, which plots isobars to depict areas of high pressure and low pressure. Cloud codes are translated into symbols and plotted on these maps along with other meteorological data that are included in synoptic reports sent by professionally trained observers.

Convergence zone

A convergence zone in meteorology is a region in the atmosphere where two prevailing flows meet and interact, usually resulting in distinctive weather conditions. This causes a mass accumulation that eventually leads to a vertical movement and to the formation of clouds and precipitation. Large-scale convergence, called synoptic-scale convergence, is associated with weather systems such as baroclinic troughs, low-pressure areas, and cyclones. Small-scale convergence will give phenomena from isolated cumulus clouds to large areas of thunderstorms.


Westerly monsoon winds lie in its equatorward portion while easterly trade winds exist poleward of the trough. [3] Right along its axis, heavy rains can be found which usher in the peak of a location's respective rainy season. As it passes poleward of a location, hot and dry conditions develop. The monsoon trough plays a role in creating many of the world's rainforests.

Monsoon seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea

Monsoon is traditionally defined as 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 used for locally heavy but short-term rains, although these rains meet the dictionary definition of monsoon.

Trade winds

The trade winds are the prevailing pattern of surface winds from the east toward the west (easterly) found in the tropics, within the lower portion of the Earth's atmosphere, in the lower part of the troposphere near the Earth's equator. The trade winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere, strengthening during the winter and when the Arctic oscillation is in its warm phase. Trade winds have been used by captains of sailing ships to cross the world's oceans for centuries, and enabled colonial expansion into the Americas and trade routes to become established across the Atlantic and Pacific oceans.

Wet season yearly period of high rainfall, especially in the tropics

The monsoon season is the time of year when most of a region's average annual rainfall occurs. Generally the season lasts at least a month. The term "green season" is also sometimes used as a euphemism by tourist authorities. Areas with wet seasons are dispersed across portions of the tropics and subtropics.

The term monsoon trough is most commonly used in monsoonal regions of the Western Pacific such as Asia and Australia. The migration of the ITCZ/monsoon trough into a landmass heralds the beginning of the annual rainy season during summer months. Depressions and tropical cyclones 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.

Tropical cyclone Is a rotating storm system

A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain. Depending on its location and strength, a tropical cyclone is referred to by different names, including hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, and simply cyclone. A hurricane is a tropical cyclone that occurs in the Atlantic Ocean and northeastern Pacific Ocean, and a typhoon occurs in the northwestern Pacific Ocean; in the south Pacific or Indian Ocean, comparable storms are referred to simply as "tropical cyclones" or "severe cyclonic storms".

Movement and strength

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 Februarystreamlinesnavy.jpg
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 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. [4] [5] [6] In the Southern Hemisphere, the monsoon trough associated with the Australian monsoon reaches its most southerly latitude in February, [7] oriented along a west-northwest/east-southeast axis.

Latitude The angle between zenith at a point and the plane of the equator

In geography, latitude is a geographic coordinate that specifies the north–south position of a point on the Earth's surface. Latitude is an angle which ranges from 0° at the Equator to 90° at the poles. Lines of constant latitude, or parallels, run east–west as circles parallel to the equator. Latitude is used together with longitude to specify the precise location of features on the surface of the Earth. On its own, the term latitude should be taken to be the geodetic latitude as defined below. Briefly, geodetic latitude at a point is the angle formed by the vector perpendicular to the ellipsoidal surface from that point, and the equatorial plane. Also defined are six auxiliary latitudes which are used in special applications.

40th parallel north circle of latitude

The 40th parallel north is a circle of latitude that is 40 degrees north of the Earth's equatorial plane. It crosses Europe, the Mediterranean Sea, Asia, the Pacific Ocean, North America, and the Atlantic Ocean.

20th parallel south circle of latitude

The 20th parallel south is a circle of latitude that is 20 degrees south of the Earth's equatorial plane. It crosses the Atlantic Ocean, Africa, the Indian Ocean, Australasia, the Pacific Ocean and South America.

Effect of wind surges

Increases in the relative vorticity, or spin, with the monsoon trough are normally a product of increased wind convergence within the convergence zone of the monsoon trough. Wind surges can lead to this increase in convergence. A strengthening or equatorward movement in the subtropical ridge can cause a strengthening of a monsoon trough as a wind surge moves towards the location of the monsoon trough. As fronts move through the 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. [8] 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. [9]

In continuum mechanics, the vorticity is a pseudovector field that describes the local spinning motion of a continuum near some point, as would be seen by an observer located at that point and traveling along with the flow.

Surface weather analysis

Surface weather analysis is a special type of weather map that provides a view of weather elements over a geographical area at a specified time based on information from ground-based weather stations.


The subtropics are geographic and climate zones located roughly between the tropics at latitude 23.5° and temperate zones north and south of the Equator.

Embedded depressions

Monsoon Depression near Bangladesh MonsoonDepression2004091600.jpg
Monsoon Depression near Bangladesh

If a circulation forms within the monsoon trough, it is able to compete with the neighboring 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. [9] Light and variable winds cover a large area near their center, while bands of showers and thunderstorms develop within their area of circulation. [10]

Thermal low

Thermal lows, or heat lows, are non-frontal low-pressure areas that occur over the continents in the subtropics during the warm season, as the result of intense heating when compared to their surrounding environments. Thermal lows occur near the Sonoran Desert, on the Mexican plateau, in California's Great Central Valley, the Sahara, over north-west Argentina in South America, over the Kimberley region of north-west Australia, the Iberian peninsula, and the Tibetan plateau.

The presence of an upper level 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. [11] Even though these systems can develop over land, the outer portions of monsoon depressions are similar to tropical cyclones. [12] In India, for example, 6 to 7 monsoon depressions move across the country yearly, [4] and their numbers within the Bay of Bengal increase during July and August of El Niño events. [13] 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. [14]


In rainy season

View of central Kolkata after a monsoon rain. Central Calcutta after Rain.jpg
View of central 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 bands of thunderstorms when viewing 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. [2] Its presence has also been linked to the peak of the rainy season in locations within Australia. [15] As it progresses poleward of a particular location, clear, hot, and dry conditions develop as winds become westerly. [16] Many of the world's rainforests are associated with these climatological low-pressure systems. [17]

In tropical cyclogenesis

Months of peak tropical cyclone activity worldwide WorldwideTCpeaks.gif
Months of peak tropical cyclone activity worldwide

A monsoon trough is a significant genesis region for tropical cyclones. 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 tropical cyclogenesis. [18] 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 Madden–Julian oscillation, or MJO. [19] This mirrors tropical cyclone genesis near these features, as genesis clusters in 2–3 weeks of activity followed by 2–3 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. [20]

Whenever the monsoon trough on the eastern side of the summertime Asian monsoon is in its normal orientation (oriented east-southeast to west-northwest), tropical cyclones 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. [21] The South Pacific convergence zone and South American convergence zones are generally reverse oriented. [7] 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. [9] 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 degrees north. [2]

Related Research Articles

Cyclone large scale air mass that rotates around a strong center of low pressure

In meteorology, a cyclone is a large scale air mass that rotates around a strong center of low atmospheric pressure. Cyclones are characterized by inward spiraling winds that rotate about a zone of low pressure. The largest low-pressure systems are polar vortices and extratropical cyclones of the largest scale. Warm-core cyclones such as tropical cyclones and subtropical cyclones also lie within the synoptic scale. Mesocyclones, tornadoes and dust devils lie within smaller mesoscale. Upper level cyclones can exist without the presence of a surface low, and can pinch off from the base of the tropical upper tropospheric trough during the summer months in the Northern Hemisphere. Cyclones have also been seen on extraterrestrial planets, such as Mars and Neptune. Cyclogenesis is the process of cyclone formation and intensification. Extratropical cyclones begin as waves in large regions of enhanced mid-latitude temperature contrasts called baroclinic zones. These zones contract and form weather fronts as the cyclonic circulation closes and intensifies. Later in their life cycle, extratropical cyclones occlude as cold air masses undercut the warmer air and become cold core systems. A cyclone's track is guided over the course of its 2 to 6 day life cycle by the steering flow of the subtropical jet stream.

Horse latitudes

Horse latitudes, subtropical ridges or subtropical highs are the subtropical latitudes between 30 and 35 degrees both north and south where Earth's atmosphere is dominated by the subtropical high, an area of high pressure, which suppresses precipitation and cloud formation, and has variable winds mixed with calm winds.

Anticyclone opposite to a cyclone

An anticyclone is a weather phenomenon defined by the United States National Weather Service's glossary as "a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere". Effects of surface-based anticyclones include clearing skies as well as cooler, drier air. Fog can also form overnight within a region of higher pressure. Mid-tropospheric systems, such as the subtropical ridge, deflect tropical cyclones around their periphery and cause a temperature inversion inhibiting free convection near their center, building up surface-based haze under their base. Anticyclones aloft can form within warm core lows such as tropical cyclones, due to descending cool air from the backside of upper troughs such as polar highs, or from large scale sinking such as the subtropical ridge. The evolution of an anticyclone depends on a few variables such as its size, intensity, moist-convection, Coriolis force etc.

A low-pressure area, low, depression or cyclone is a region on the topographic map where the atmospheric pressure is lower than that of surrounding locations. Low-pressure systems form under areas of wind divergence that occur in the upper levels of the troposphere. The formation process of a low-pressure area is known as cyclogenesis. Within the field of meteorology, atmospheric divergence aloft occurs in two areas. The first area is on the east side of upper troughs, which form half of a Rossby wave within the Westerlies. A second area of wind divergence aloft occurs ahead of embedded shortwave troughs, which are of smaller wavelength. Diverging winds aloft ahead of these troughs cause atmospheric lift within the troposphere below, which lowers surface pressures as upward motion partially counteracts the force of gravity.

The synoptic scale in meteorology is a horizontal length scale of the order of 1000 kilometers or more. This corresponds to a horizontal scale typical of mid-latitude depressions. Most high and low-pressure areas seen on weather maps such as surface weather analyses are synoptic-scale systems, driven by the location of Rossby waves in their respective hemisphere. Low-pressure areas and their related frontal zones occur on the leading edge of a trough within the Rossby wave pattern, while high-pressure areas form on the back edge of the trough. Most precipitation areas occur near frontal zones. The word synoptic is derived from the Greek word συνοπτικός, meaning seen together.


The westerlies, anti-trades, or prevailing westerlies, are prevailing winds from the west toward the east in the middle latitudes between 30 and 60 degrees latitude. They originate from the high-pressure areas in the horse latitudes and trend towards the poles and steer extratropical cyclones in this general manner. Tropical cyclones which cross the subtropical ridge axis into the westerlies recurve due to the increased westerly flow. The winds are predominantly from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere.

Tropical wave type of atmospheric trough

Tropical waves, easterly waves, or tropical easterly waves, also known as African easterly waves in the Atlantic region, are a type of atmospheric trough, an elongated area of relatively low air pressure, oriented north to south, which moves from east to west across the tropics, causing areas of cloudiness and thunderstorms. West-moving waves can also form from the tail end of frontal zones in the subtropics and tropics, and may be referred to as easterly waves, but these waves are not properly called tropical waves; they are a form of inverted trough sharing many characteristics with fully tropical waves. All tropical waves form in the easterly flow along the equatorward side of the subtropical ridge or belt of high pressure which lies north and south of the Intertropical Convergence Zone (ITCZ). Tropical waves are generally carried westward by the prevailing easterly winds along the tropics and subtropics near the equator. They can lead to the formation of tropical cyclones in the north Atlantic and northeastern Pacific basins. A tropical wave study is aided by Hovmöller diagrams, a graph of meteorological data.

Pressure system relative peak or lull in the sea level pressure distribution

A pressure system is a relative peak or lull in the sea level pressure distribution. The surface pressure at sea level varies minimally, with the lowest value measured 87 kilopascals (26 inHg) and the highest recorded 108.57 kilopascals (32.06 inHg). High- and low-pressure systems evolve due to interactions of temperature differentials in the atmosphere, temperature differences between the atmosphere and water within oceans and lakes, the influence of upper-level disturbances, as well as the amount of solar heating or radiationized cooling an area receives. Pressure systems cause weather to be experienced locally. Low-pressure systems are associated with clouds and precipitation that minimize temperature changes throughout the day, whereas high-pressure systems normally associate with dry weather and mostly clear skies with larger diurnal temperature changes due to greater radiation at night and greater sunshine during the day. Pressure systems are analyzed by those in the field of meteorology within surface weather maps.

Weather front boundary separating two masses of air of different densities

A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena outside the tropics. In surface weather analyses, fronts are depicted using various colored triangles and half-circles, depending on the type of front. The air masses separated by a front usually differ in temperature and humidity.

Mesoscale convective system complex of thunderstorms organized on a larger scale

A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more. A mesoscale convective system's overall cloud and precipitation pattern may be round or linear in shape, and include weather systems such as tropical cyclones, squall lines, lake-effect snow events, polar lows, and Mesoscale Convective Complexes (MCCs), and generally form near weather fronts. The type that forms during the warm season over land has been noted across North America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.

Trough (meteorology) elongated region of low atmospheric pressure

A trough is an elongated (extended) region of relatively low atmospheric pressure, often associated with fronts. Troughs may be at the surface, or aloft, or both under various conditions. Most troughs bring clouds, showers, and a wind shift, particularly following the passage of the trough. This results from convergence or "squeezing" which forces lifting of moist air behind the trough line.

1984 Pacific typhoon season typhoon season in the Pacific Ocean

The 1984 Pacific typhoon season has no official bounds, but most tropical cyclones tend to form in the northwestern Pacific Ocean between May and November. These dates conventionally delimit the period of each year when most tropical cyclones form in the northwestern Pacific Ocean. Tropical Storms formed in the entire west pacific basin were assigned a name by the Joint Typhoon Warning Center. Tropical depressions that enter or form in the Philippine area of responsibility are assigned a name by the Philippine Atmospheric, Geophysical and Astronomical Services Administration or PAGASA. This can often result in the same storm having two names.

1981 Pacific typhoon season typhoon season in the Pacific Ocean

The 1981 Pacific typhoon season was a slightly above average season that produced 29 tropical storms, 13 typhoons and two intense typhoons. The season ran throughout 1981, though most tropical cyclones typically develop between May and October. The season's first named storm, Freda, developed on March 12 while the final storm, Lee, dissipated on December 29. Tropical cyclones only accounted for 12 percent of the rainfall in Hong Kong this season, the lowest percentage for the protectorate since 1972.

Extratropical cyclone type of cyclone

Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are low-pressure areas which, along with the anticyclones of high-pressure areas, drive the weather over much of the Earth. Extratropical cyclones are capable of producing anything from cloudiness and mild showers to heavy gales, thunderstorms, blizzards, and tornadoes. These types of cyclones are defined as large scale (synoptic) low pressure weather systems that occur in the middle latitudes of the Earth. In contrast with tropical cyclones, extratropical cyclones produce rapid changes in temperature and dew point along broad lines, called weather fronts, about the center of the cyclone.

Upper tropospheric cyclonic vortex

An upper tropospheric cyclonic vortex is a vortex, or a circulation with a definable center, that usually moves slowly from east-northeast to west-southwest and is prevalent across Northern Hemisphere's warm season. Its circulations generally do not extend below 6,080 metres (19,950 ft) in altitude, as it is an example of a cold-core low. A weak inverted wave in the easterlies is generally found beneath it, and it may also be associated with broad areas of high-level clouds. Downward development results in an increase of cumulus clouds and the appearance of circulation at ground level. In rare cases, a warm-core cyclone can develop in its associated convective activity, resulting in a tropical cyclone and a weakening and southwest movement of the nearby upper tropospheric cyclonic vortex. Symbiotic relationships can exist between tropical cyclones and the upper level lows in their wake, with the two systems occasionally leading to their mutual strengthening. When they move over land during the warm season, an increase in monsoon rains occurs.

Cyclone Cilla Category 1 South Pacific cyclone in 2003

Tropical Cyclone Cilla was a tropical cyclone that brought minor damage to several islands in the South Pacific in January 2003. The fifth cyclone of the 2002–03 South Pacific cyclone season, Cyclone Cilla developed from a monsoon trough on January 26 northwest of Fiji. Initially, Cilla moved east, and due to decreased wind shear, Cilla was able to intensify. On January 28, Cilla reached its peak intensity of 75 km/h (45 mph). After slightly weakening, Cilla briefly re-intensified the next day. However, Cilla transitioned into an extratropical cyclone on January 30. Along its path, Cilla dropped heavy rainfall over islands it passed. During its formative stages, the low dropped heavy rain over Fiji, which had already been affected by Cyclone Ami two weeks prior. Damage in Tonga was mostly limited to vegetation and fruit trees; infrastructural damage was also relatively minor. Cilla also brought moderate rain to American Samoa.

Earth rainfall climatology

Earth rainfall climatology Is the study of rainfall, a sub-field of Meteorology. Formally, a wider study includes water falling as ice crystals, i.e. hail, sleet, snow. The aim of rainfall climatology is to measure, understand and predict rain distribution across different regions of planet Earth, a factor of air pressure, humidity, topography, cloud type and raindrop size, via direct measurement and remote sensing data acquisition. Current technologies accurately predict rainfall 3–4 days in advance using numerical weather prediction. Geostationary orbiting satellites gather IR and visual wavelength data to measure realtime localised rainfall by estimating cloud albedo, water content, and the corresponding probability of rain. Geographic distribution of rain is largely governed by climate type, topography and habitat humidity. In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation. On the leeward side of mountains, desert climates can exist due to the dry air caused by compressional heating. The movement of the monsoon trough, or intertropical convergence zone, brings rainy seasons to savannah climes. The urban heat island effect leads to increased rainfall, both in amounts and intensity, downwind of cities. Global warming may also cause changes in the precipitation pattern globally, including wetter conditions at high latitudes and in some wet tropical areas, and drier conditions in parts of the subtropics and middle latitudes. Precipitation is a major component of the water cycle, and is responsible for depositing most of the fresh water on the planet. Approximately 505,000 cubic kilometres (121,000 cu mi) of water falls as precipitation each year; 398,000 cubic kilometres (95,000 cu mi) of it over the oceans. Given the Earth's surface area, that means the globally averaged annual precipitation is 990 millimetres (39 in). Climate classification systems such as the Köppen climate classification system use average annual rainfall to help differentiate between differing climate regimes.


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