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.

Contents

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. The monsoon trough plays a role in creating many of the world's rainforests. [4]

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.

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. [5] [6] [7] In the Southern Hemisphere, the monsoon trough associated with the Australian monsoon reaches its most southerly latitude in February, [8] oriented along a west-northwest/east-southeast axis. North-south-oriented mountain barriers, like the Rockies and the Andes, and large massifs, such as the Plateau of Tibet, also influence atmospheric flow. [9]

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. [10] 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. [11]

Monsoon 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. [11] Light and variable winds cover a large area near their center, while bands of showers and thunderstorms develop within their area of circulation. [12]

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. [13] Even though these systems can develop over land, the outer portions of monsoon depressions are similar to tropical cyclones. [14] In India, for example, 6 to 7 monsoon depressions move across the country yearly, [5] and their numbers within the Bay of Bengal increase during July and August of El Niño events. [15] 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. [16]

Some tropical cyclones recognised by Regional Specialized Meteorological Centres would have characteristics of a monsoon depression throughout their lifetime. The 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. [17]

Roles

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. [18]

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. [19] 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. [20] 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. [21]

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. [22] The South Pacific convergence zone and South Atlantic convergence zones are generally reverse oriented. [8] 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. [11] 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

<span class="mw-page-title-main">Cyclone</span> Large scale rotating air mass

In meteorology, a cyclone is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above. 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 the smaller mesoscale.

<span class="mw-page-title-main">Horse latitudes</span> Latitudes 30–35 degrees north and south of the Equator

The horse latitudes are the latitudes about 30 degrees north and south of the Equator. They are characterized by sunny skies, calm winds, and very little precipitation. They are also known as subtropical ridges or highs. It is a high-pressure area at the divergence of trade winds and the westerlies.

<span class="mw-page-title-main">Anticyclone</span> Weather phenomenon of high pressure, as opposed to a cyclone

An anticyclone is a weather phenomenon defined as a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere as viewed from above. 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.

<span class="mw-page-title-main">Intertropical Convergence Zone</span> Meteorological phenomenon

The Intertropical Convergence Zone, known by sailors as the doldrums or the calms because of its monotonous windless weather, is the area where the northeast and the southeast trade winds converge. It encircles Earth near the thermal equator though its specific position varies seasonally. When it lies near the geographic Equator, it is called the near-equatorial trough. Where the ITCZ is drawn into and merges with a monsoonal circulation, it is sometimes referred to as a monsoon trough.

<span class="mw-page-title-main">Low-pressure area</span> Area with air pressures lower than adjacent areas

In meteorology, a low-pressure area, low area or low is a region where the atmospheric pressure is lower than that of surrounding locations. Low-pressure areas are commonly associated with inclement weather, while high-pressure areas are associated with lighter winds and clear skies. Winds circle anti-clockwise around lows in the northern hemisphere, and clockwise in the southern hemisphere, due to opposing Coriolis forces. Low-pressure systems form under areas of wind divergence that occur in the upper levels of the atmosphere (aloft). The formation process of a low-pressure area is known as cyclogenesis. In meteorology, atmospheric divergence aloft occurs in two kinds of places:

<span class="mw-page-title-main">Synoptic scale meteorology</span> 1000-km-order method of measuring weather systems

In meteorology, the synoptic scale is a horizontal length scale of the order of 1,000 km (620 mi) or more. This corresponds to a horizontal scale typical of mid-latitude depressions. Most high- and low-pressure areas seen on weather maps 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 Ancient Greek word συνοπτικός (sunoptikós), meaning "seen together".

<span class="mw-page-title-main">Westerlies</span> Prevailing winds from the west

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.

<span class="mw-page-title-main">Pressure system</span> Relative peak or lull in the sea level pressure distribution

A pressure system is a 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.

<span class="mw-page-title-main">Mesoscale convective system</span> 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 forms near weather fronts. The type that forms during the warm season over land has been noted across North and South America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.

<span class="mw-page-title-main">1996 Pacific typhoon season</span>

The 1996 Pacific typhoon season was a relatively active season, producing 30 tropical storms, 16 typhoons and six super typhoons. It has no official bounds; it ran year-round in 1996, 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. However the first named storm, Ann, did not develop until April 3. The final named storm, Greg, dissipated on December 27.

<span class="mw-page-title-main">1994 Pacific typhoon season</span>

The 1994 Pacific typhoon season was an extremely active season in the annual cycle of tropical cyclone formation in the Western North Pacific, being the second most active season in the basin, only behind 1964. With a total of 36 tropical storms, where 20 of them reached typhoon strength and 6 further strengthening into super typhoons, during the course of the season, much like the Pacific hurricane season. The season had no official bounds and it ran year-round in 1994, but most tropical cyclones tend to form in the northwestern Pacific Ocean between May and November. Tropical storms that formed west of the date line were assigned a name by the Joint Typhoon Warning Center. The World Meteorological Organization-designated Regional Specialized Meteorological Centre for tropical cyclones for the region is the Japan Meteorological Agency. Tropical depressions that entered or formed in the Philippine area of responsibility were assigned a name by the Philippine Atmospheric, Geophysical and Astronomical Services Administration or PAGASA. This can result in some storms having two names.

<span class="mw-page-title-main">1987 Pacific typhoon season</span>

The 1987 Pacific typhoon season was a below average season, though it featured a relatively high amount of typhoons. It had no official bounds; it ran year-round in 1987, but most tropical cyclones formed between May and November. Tropical storms that formed in the entire 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.

<span class="mw-page-title-main">1985 Pacific typhoon season</span>

The 1985 Pacific typhoon season was a slightly above average season in terms of named storms, though most storms were weak and short-lived. The season had 28 named storms, 15 typhoons and 1 super typhoon. It ran year-round, thus ending the two-season run which began in 1983 that only had tropical cyclones forming between June and December. These dates conventionally delimit the period of each year when most tropical cyclones form in the northwestern Pacific Ocean. A total of 57 tropical depressions formed this year, of which only 28 became tropical storms and were assigned a name by the Joint Typhoon Warning Center. This made the season well above average. Additionally, tropical depressions that enter or form in the Philippine area of responsibility were 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. This year, a total of 17 storms were named this way.

<span class="mw-page-title-main">1984 Pacific typhoon season</span>

The 1984 Pacific typhoon season had the second-latest start in the basin on record, only behind the previous year, 1983, by one day. Despite this, it was above average in terms of named storms and featured many intense storms. It ran year-round in 1984, but again all tropical cyclones formed between June and December. These dates conventionally delimit the period of each year when most tropical cyclones form in the northwestern Pacific Ocean. A total of 44 tropical depressions formed this year, of which only 27 became tropical storms and were assigned a name by the Joint Typhoon Warning Center. This made the season above average when compared to the long term mean of 25 storms per season. Additionally, 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. This year, a total of 20 storms were named this way.

<span class="mw-page-title-main">1982 Pacific typhoon season</span>

The 1982 Pacific typhoon season was an average season in terms of total tropical storms, though it featured a very high amount of typhoons. It ran year-round in 1982, but most tropical cyclones formed between June and December. These dates conventionally delimit the period of each year when most tropical cyclones form in the northwestern Pacific Ocean. A total of 37 tropical depressions formed this year, of which 25 became tropical storms and were assigned a name by the Joint Typhoon Warning Center. Additionally, 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. This year, a total of 23 storms were named this way.

<span class="mw-page-title-main">1981 Pacific typhoon season</span>

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.

<span class="mw-page-title-main">2002–03 South Pacific cyclone season</span> Tropical cyclone season

The 2002–03 South Pacific cyclone season was the most active and longest tropical cyclone season since 1997–98, with ten tropical cyclones occurring within the South Pacific basin between 160°E and 120°W. The season started earlier than normal, with two systems developing before the official start of the season on November 1, 2002, while the final system dissipated on June 9, 2003, after the season had officially ended on April 30. During the season, tropical cyclones were officially monitored by the Regional Specialized Meteorological Centre (RSMC) in Nadi, Fiji and the Tropical Cyclone Warning Centres in Brisbane, Australia and Wellington, New Zealand. The United States Armed Forces through the Joint Typhoon Warning Center (JTWC), also monitored the basin and issued unofficial warnings for American interests. RSMC Nadi attaches a number and an F suffix to tropical disturbances that occur within the basin, while the JTWC designates significant tropical cyclones with a number and a P suffix. RSMC Nadi, TCWC Wellington and TCWC Brisbane all use the Australian Tropical Cyclone Intensity Scale and estimate windspeeds over a ten-minute period, while the JTWC estimates sustained winds over a one-minute period, which are subsequently compared to the Saffir–Simpson Hurricane Scale (SSHS).

<span class="mw-page-title-main">Upper tropospheric cyclonic vortex</span>

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 cloudy 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

<span class="mw-page-title-main">Earth rainfall climatology</span> Study of rainfall

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.

<span class="mw-page-title-main">Glossary of tropical cyclone terms</span>

The following is a glossary of tropical cyclone terms.

References

  1. 1 2 "Monsoon trough". Glossary of Meteorology. American Meteorological Society. Archived from the original on 17 June 2009. Retrieved 4 June 2009.
  2. 1 2 3 Bin Wang. The Asian Monsoon. Retrieved 2008-05-03.
  3. ↑ World Meteorological Organization. Severe Weather Information Centre. Retrieved 2008-05-03.
  4. ↑ Hobgood (2008)Global Pattern of Surface Pressure and Wind. Archived 18 March 2009 at the Wayback Machine Ohio State University. Retrieved 2009-03-08.
  5. 1 2 National Centre for Medium Range Forecasting. Chapter-II Monsoon-2004: Onset, Advancement and Circulation Features. Archived 21 July 2011 at the Wayback Machine Retrieved 2008-05-03.
  6. ↑ Australian Broadcasting Corporation. Monsoon. Archived 23 February 2001 at the Wayback Machine Retrieved 2008-05-03.
  7. ↑ Dr. Alex DeCaria. Lesson 4 – Seasonal-mean Wind Fields. Archived 22 August 2009 at the Wayback Machine Retrieved 2008-05-03.
  8. 1 2 U. S. Navy. 1.2 Pacific Ocean Surface Streamline Pattern. Retrieved 2006-11-26.
  9. ↑ "Atmosphere - Effect of continents on air movement | Britannica". www.britannica.com. Retrieved 13 May 2024.
  10. ↑ Chih-Lyeu Chen. Effects of the Northeast Monsoon on the Equatorial Westerlies Over Indonesia. [ dead link ] Retrieved 2008-05-03.
  11. 1 2 3 U. S. Navy. SECTION 3. DYNAMIC CONTRIBUTORS TO TROPICAL CYCLONE FORMATION. Retrieved 2006-11-26.
  12. ↑ Chip Guard. Climate Variability on CNMI. Archived 22 June 2007 at the Wayback Machine Retrieved 2008-05-03.
  13. ↑ Sixiong Zhao and Graham A. Mills. A Study of a Monsoon Depression Bringing Record Rainfall over Australia. Part II: Synoptic–Diagnostic Description. [ permanent dead link ] Retrieved 2008-05-03.
  14. ↑ N.E. Davidson and G.J. Holland. A Diagnostic Analysis of Two Intense Monsoon Depressions over Australia. [ permanent dead link ] Retrieved 2008-05-03.
  15. ↑ O. P. Singh, Tariq Masood Ali Khan, and Md. Sazedur Rahman. Impact of Southern Oscillation on the Frequency of Monsoon Depressions in the Bay of Bengal. Retrieved 2008-05-03.
  16. ↑ Bureau of Meteorology. TWP-ICE Synoptic Overview, 1 February 2006. Retrieved 2008-05-03.
  17. ↑ "North Indian Ocean Best Track Data". Joint Typhoon Warning Center. Retrieved 25 May 2020.
  18. ↑ Bureau of Meteorology. Climate of Giles. Archived 11 August 2008 at the Wayback Machine Retrieved 2008-05-03.
  19. ↑ Christopher Landsea. Climate Variability of Tropical Cyclones: Past, Present and Future. Retrieved 2006-11-26.
  20. ↑ Patrick A. Harr. Tropical Cyclone Formation/Structure/Motion Studies. Archived 29 November 2007 at the Wayback Machine Retrieved 2006-11-26.
  21. ↑ Joint Typhoon Warning Center. Typhoon Polly. Archived 19 September 2006 at the Wayback Machine Retrieved 2006-11-26.
  22. ↑ Mark A. Lander. 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.

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