Monsoon circulation: due to the surfaceland、oceanDistribution and formation oflandandoceanThe speed of heat absorption and dissipation is different, and when this situation extends to a long season, due to different seasons and weatherVogueThe monsoon circulation is formed when there are obvious seasonal changes in the direction or pressure system.
The main monsoon systems include the West African monsoon system, the South Asian monsoon system, the East Asian monsoon system and the Australian monsoon system.Each monsoon system is composed of some circulation systems, which are called members of the monsoon system.Members of each monsoon system come from different sources. Some are mid latitude systems, some are cross equatorial airflow, and others areSubtropical zoneandtropicThe circulation system of.[1]
The abbreviation of monsoon circulation ismonsoonThat is, the circulation system with winter and summer wind or nearly opposite wind direction.In each monsoon circulation system, the onset of the monsoon is often explosive or abrupt, and the outbreak time is different.For example, the West African summer monsoon system breaks northward in the middle of May every year;IndiaThe summer monsoon and the East Asian summer monsoon moved northward from mid May to mid July, with rapid progress and stagnation at each stage;AustraliaThe monsoon usually breaks out suddenly in December every year.[2]
Causes of monsoon circulation
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Thermal difference between land and seaThe seasonal displacement of the planetary wind belt and the dynamic and thermal effects of the huge terrain such as the Qinghai Tibet Plateau are the main factors for the formation of the monsoon.[3]
Sea land monsoon circulation
The monsoon circulation caused by the thermal difference between land and sea is called sea land monsoon.In summer, the heating of the continent is more intense than that of the ocean, and the pressure changes more slowly with height than that over the ocean. Therefore, at a certain height, a horizontal pressure gradient from the continent to the ocean will be generated. The air will point from the continent to the ocean, forming a high pressure on the ocean, forming a low pressure on the continent, and the air will flow from the ocean to the continent, forming an air flow opposite to the upper air flow, forming a summer monsoon circulation.[3]
In winter, the continent cools rapidly, and the temperature on the ocean is higher than that on the land. Therefore, the continent is high pressure, and the ocean is low pressure. The low-level airflow flows from the continent to the ocean, and the high-level airflow flows from the ocean to the continent, forming the winter monsoon circulation.[3]
The sea land monsoon is related to the thermal difference between sea and land, so where the temperature difference between sea and land is large, the sea land monsoon is very popular.The strongest monsoon regions on the earth are in the tropics and subtropics.This is because the sea land temperature difference near the equator is very small all year round. With the increase of latitude, the sea land temperature difference increases, and the monsoon force increases.However, above the mid latitude, the activity of cyclones increases and the wind direction changes complicatedly.The regularity of monsoon will be disturbed.[3]
Planetary monsoon circulation
planetThe wind belt has the rule of south-north movement with seasonal changes. The monsoon circulation formed by the seasonal change of wind direction is called the Planetary Monsoon.The five wind belts on the earth move northward in summer in the northern hemisphere and southward in summer in the southern hemisphere.In this way, the southern edge of the westerlies in winter may become the easterlies in summer;The prevailing wind in winter and summer will change about 180 °.
The regions where the planetary wind direction changes are basically banded, which can occur in coastal, inland and central oceans.In terms of latitude, this kind of monsoon is most obvious in equatorial and tropical regions, so it is often called equatorial monsoon or tropical monsoon.For example, inthe pacific oceanIn the east, the equatorial depression stays in the southern hemisphere in winter and moves to the northern hemisphere in summer, so the area between the equator and 10 ° N is affected by the northern hemisphere in wintertrade windIt is controlled by the northeasterly wind, and in summer, it is controlled by the trade winds crossing the equator in the southern hemisphere, and it is driven by the southwest wind.[3]
Role of Qinghai Tibet Plateau and other large landforms
Qinghai Tibet PlateauThe impact on the monsoon circulation has both thermodynamic and topographic dynamic effects.Huge and toweringQinghai Tibet PlateauThere is also seasonal thermal difference with the surrounding free atmosphere.For the atmosphere over the Qinghai Tibet Plateau, it is a heat source from March to September, leading to the formation of the Indian low pressure, which plays an important role in maintaining and strengthening the South Asian monsoon.The simulation experiment shows that if there is no Tibetan Plateau, the South Asian monsoon phenomenon will be significantly weakened.Its existence has played an important role in maintaining and strengthening the South Asian monsoon, and is one of the important reasons for the strong southwest monsoon.In winter, due to intense radiation cooling,Qinghai Tibet PlateauIt is not only a cold source, but also because of the blocking effect of the large terrain, the intensity of cold air entering South Asia is significantly weakened, so the intensity of the South Asian winter monsoon is weak.[3]
The actual monsoon in a region is often the result of the combined action of many factors, such as the local distribution characteristics of land and sea, the seasonal displacement of the planetary wind belt, and topography. One or several factors play a dominant role in the formation and maintenance of the monsoon, while other factors play an auxiliary role.For example, the formation of the temperate and subtropical monsoon usually includes the seasonal displacement of the planetary wind belt in addition to the thermal difference between land and sea.In addition to the seasonal displacement of the planetary wind belt, the formation of the equatorial and tropical monsoon also includes the role of the thermal difference between land and sea.Larger terrain is often a factor that can not be ignored to change the intensity and direction of the monsoon.In addition, due to the different latitude and geographical conditions of each region, the intensity and characteristics of the monsoon are also different.[3]
Category of monsoon circulation system
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There are many monsoon regions in the world, but the most important monsoon climate regions are mainly located inEastern Hemisphere。Each monsoon region has a unique circulation system.According to the different regions where the monsoon system occurs and the circulation characteristics, the monsoon system has different names.The main monsoon systems include the West African monsoon system, the South Asian monsoon system, the East Asian monsoon system and the Australian monsoon system.Each monsoon system is composed of some circulation systems, which are called members of the monsoon system.The members of each monsoon system come from different sources. Some are mid latitude systems, some are cross equatorial airflow, and some are subtropical and tropical circulation systems.[2]
South Asian summer monsoon circulation system
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Around the middle of May every year, the southeast airflow from the Mascarene High in the South Indian Ocean forms a strongSomaliaThe jet stream flows directly towards the Indian subcontinent in South Asia and causes continuous precipitation, which is called the South Asian summer monsoon.The weather systems that cause precipitation are mainly monsoon trough and monsoon low pressure.The monsoon trough is basically along theHimalayasSouth, from northwest to southeastBay of Bengal(BOB) andIndochina PeninsulaStretched in the direction.When it is in the active stage, there are a series of low-pressure activities along the trough line, resulting in repeated heavy precipitation processes.These low pressures areBay of BengalIt has been further developed and strengthened.So inBay of BengalThe region has the highest frequency of low pressure in South Asia.The South Asian monsoon trough can sometimes extend eastward to the South China Sea.Especially when there are many typhoons, they sometimes resembleEquatorial convergence zone(ITCZ).[4]
East Asian summer monsoon circulation system
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Composition of east asian summer monsoon system
Although the East Asian monsoon region is connected with the South Asian monsoon region, many scholars also believe that the East Asian summer monsoon is the extension of the South Asian summer wind direction to the east, but according to a large number of research results at home and abroad in recent years, the East Asian summer monsoon system and the South Asian Indian summer monsoon system are both interconnected and independent.Basically, the East Asian summer monsoon system is a relatively independent monsoon system compared with the Indian summer monsoon.On the contrary, the East Asian summer monsoon is more closely related to the Indonesia North Australia winter monsoon in the southern hemisphere.When the East Asian summer monsoon prevails, it is also the prevailing period of the Indonesia North Australia winter monsoon.The study also believes that the East Asian monsoon system includes the South China Sea monsoon system (occurring in the tropical monsoon area of the West Pacific in the South China Sea, northeast monsoon in winter and southwest monsoon in summer), and the subtropical monsoon system (occurring in the subtropical monsoon area of the East Asian continent and Japan, northwest monsoon or northeast monsoon prevails in winter; southwest monsoon or southeast monsoon prevails in summer).[4]
Many scholars regard East Asia and Indonesia Northern Australia summer monsoon (Northern Hemisphere) as a unified circulation system.Its members mainly include: low level Australian cold anticyclone, north cross equatorial flow in East Asia, South China Sea West Pacific Tropical Convergence Zone (ITCZ) (or tropical monsoon convergence zone, South China Sea monsoon trough, etc.)Western Pacific subtropical high, Meiyu convergence zone (or subtropical monsoon convergence zone, Meiyu front, etc.);The eastern ridge of the South Asian anticyclone in the upper air, the easterly jet (including the southern and northern easterly jet streams), the southward cross equatorial flow in East Asia, and the southern hemisphere upper subtropical ridge.Under the control of these circulation systems, there are three low-level monsoon currents, namely, from Australiacold anticyclone The winter southeast monsoon that emanates from the middle, the South China Sea West Pacific tropical southwest monsoon that turns after crossing the equator, and the East Asian continent Japan subtropical southwest monsoon that turns northward from the west side of the Western Pacific subtropical ridge.To the north of the two southwest monsoons in East Asia are two convergence zones, and the upper level is a divergence zone, corresponding to two monsoon rain zones.It can be inferred that there are two closed meridional vertical circulations in East Asia in summer.One is that the northward airflow diverging from the Australian anticyclone converges and rises in the South China Sea West Pacific ITCZ, turns southward and sinks over Australia after reaching high altitude, and then returns to the Australian anticyclone, forming a closed meridional circulation.This circulation circle is associated with the tropical monsoon and is called the tropical monsoon meridional circulation.The other is associated with the subtropical monsoon, fromSubtropical ridgeThe northward airflow on the west side rises to high altitude in the subtropical convergence zone and then turns southward, sinking in the subtropical ridge along the coast of South China, forming a small closureMeridional circulation, called subtropical monsoon meridional circulation.[4]
The flow of the South China Sea West Pacific tropical monsoon mainly comes from the southern hemisphere.The flow of the East Asian continent Japan subtropical monsoon consists of three parts, namelySubtropical highThe south-west airflow, the tropical southwest monsoon of the western Pacific in the South China Sea and the tropical southwest monsoon of India converge at the west side of the subtropical high.The South China Sea Western Pacific ITCZ is composed of a single tropical ocean air mass, which is not frontal.The subtropical monsoon convergence zone is composed of tropical air mass and northern polar continental metamorphic air mass. The humidity contrast is obvious, and at least there is an obvious frontal structure at high altitude.
Characteristics of High and Low Level Circulation in Drought and Flood Years
The change of strength and position of a member of the summer monsoon circulation system can affect the change of the whole circulation system, thus affecting the strength, advance and retreat of the summer monsoon, and further affecting drought and flood in various regions.This can be seen from the comparison between 1978 and 1980 in Figure 1.The solid line part of Figure 1 is a comprehensive map based on the East Asian monsoon circulation system in the summer of 1980 (a typical waterlogging year in the Yangtze Huaihe River basin).[4]
In Figure 1, in addition to the above mentioned circulation situation in 1980 (a typical waterlogging year in the Yangtze and Huaihe River basins), the characteristics of the East Asian summer monsoon circulation system in 1978 (a typical drought year in the Yangtze and Huaihe River basins) are also plotted (dotted line).It can be seen from the comparison between the solid line and the dotted line in Figure 1 thatCross equatorial flowCompared with the dry and dry year (1978), the tropical monsoon convergence zone in the flood year is located at 12 ° N, in which there are fewer typhoons. The subtropical monsoon convergence zone (or Meiyu front) is located at about 30 ° N, and correspondingly there are two easterly jets in the upper air.In dry years, the tropical monsoon convergence zone is located in the north, about 22 ° N, where there are many typhoons, and the subtropical monsoon convergence zone is also located in the north, about 40 ° N. Accordingly, there is only one south branch easterly jet in the upper air.In short, the East Asian monsoon system in the northern hemisphere is about 8 latitudes south of the dry and dry years in flood years.At the same time, the location of the westerly jet axis in the mid latitude of the Northern Hemisphere is also 5 latitudes south of the dry and dry years, at 40 ° - 45 ° N.The position of the Australian cold high in the southern hemisphere only slightly deviates in the east-west direction.[4]
Fig. 1 Comprehensive Diagram of East Asian Monsoon Circulation System in Summer 1980 and Summer 1978
Thermal Properties of the East Asian Subtropical Summer Monsoon
Because the three currents that make up the East Asian subtropical monsoon (i.eAustraliaThe winter southeast monsoon air flow radiated from the cold anticyclone, the tropical southwest monsoon air flow in the western Pacific of the South China Sea that turns after crossing the equator, and the East Asian continent Japan subtropical southwest monsoon air flow that turns from the west side of the subtropical ridge of the western Pacific to the north) all come from the tropical ocean and contain rich water vapor. When they enter the mainland,It is also affected by the radiative heating of the mainland in summer and the sinking warming under the subtropical high ridge, and the temperature rises, thus forming the characteristics of high temperature and high humidity.[4]
East Asian winter monsoon circulation system
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Just as the East Asian summer monsoon is closely related to the Indonesia-North Australia winter monsoon, the East Asian winter monsoon is also closely related to the Indonesia-North Australia summer monsoon in the southern hemisphere. The prevailing period of the East Asian winter monsoon is the prevailing period of the Indonesia-North Australia summer monsoon.
Characteristics of high and low level circulation
The low level members of the East Asia and Indonesia North Australia winter monsoon (Northern Hemisphere) circulation system include: the cold anticyclone over the Asian continent, the East Asia southward equatorial flow, the Indonesia North Australia summer monsoon convergence zone, orTropical convergence zone(northwest monsoon and southeast trade wind) and Australian thermal depression;The upper air members include: the southern hemisphere upper subtropical high ridge, the northward cross equatorial flow and the western ridge of the northern hemisphere upper subtropical high. Under the control of these circulation systems, there are two monsoon flows, one is the East Asian winter monsoon emanating from the inside of the Asian cold anticyclone, the north of 30 ° N is the northwest monsoon, and the south is the northeast monsoon,The other is the indonesian north australia summer northwest monsoon, whose air flow comes from the east asia northeast monsoon in the northern hemisphere and the western pacific subtropical high in the northern hemispherenortheast trades。The main precipitation area during the East Asian winter monsoon has moved from the northern hemisphere to the equator and Indonesia to the south, which is also the strongest precipitation area in the world in winter.[4]
The meridional vertical circulation of the winter monsoon in East Asia and Indonesia North Australia (Northern Hemisphere) flows southward from the cold Asian anticyclone across the equator, rises on the convergence zone of the summer monsoon in the Southern Hemisphere, reaches the upper troposphere, then turns into the divergent southeast airflow, flows northward across the equator and enters the western wind zone of the Northern Hemisphere, where it sinks back into the cold anticyclone,Thus forming a closed meridional vertical circulation.[4]
Circulation characteristics of winter monsoon anomaly
The East Asian circulation system and weather characteristics in strong and weak winter monsoon years are obviously different.In the strong winter monsoon year, the subtropical high over the western Pacific is weak at 500 hPa, the westerly circulation in Asia is weak, the East Asia long wave trough extends southward, and the 115 ° E westerly jet at 200 hPa is strong and northward.On the contrary, the circulation characteristics of weak winter monsoon year are as follows: 500 hPa Western Pacific subtropical high is strong, Asian westerly circulation is strong, East Asia trough is weak, and 115 ° E westerly jet at 200 hPa level is weak and southward.[4]
