Monsoon
The monsoon is a seasonal wind produced by the displacement of the Intertropical Convergence Zone. In summer the winds blow from south to north, loaded with rain. In winter, they are dry and cold inland winds, especially in the Indian Ocean and South Asia. The southwest monsoon, which ripped off the coast of Kerala, India, generally begins in the first half of June.
Despite this original definition of the term, it has subsequently been used as a synonym for rain in summer, regardless of its cause or place where it occurs.
Etymology
Monzón derives from the Portuguese monção, taken from the Arabic mawsim (موسم "season / season").
Processes
Monsoons are caused by land cooling and heating faster than water, according to the process of alternating air. Therefore, in summer, the land reaches a higher temperature than the ocean. As the wind blows from areas of high pressure (anticyclones) to areas of low pressure (cyclones) in order to equalize both pressures, rain is produced by moist air rising and cooling by rising in the mountains. When the sun warms the land, the breezes blow in the opposite direction, from the sea to the land.
It's at a higher temperature, so the air rises, causing an area of low pressure in the ocean. The wind now blows from the land towards the ocean. But since the temperature difference is less than during the summer, the wind that blows from the anticyclone to the squall is not as constant.
The monsoons typically occur on the southern Asian coasts in the Indian Ocean and, above all, on the southern slopes of the highest mountain ranges in the world (Himalayas and Karakorum) where the most intense rains on our planet occur, with more than 10 m of water per year (Cherrapunji, Assam), only comparable to those registered in the northwest of Colombia, in the Quibdó depression and in the Darién jungle, already on the border with Panama.
Monsoon systems
As monsoons have become better understood, their definition has been broadened to include nearly all events associated with the annual weather cycle on the tropical and subtropical continents of Asia, Australia, South America, and Africa along with their adjacent seas and oceans. It is in these regions that the most dramatic climate changes on Earth occur.
In a broader sense, in the geological past, monsoon systems have always accompanied the formation of supercontinents like Pangea, with their extreme continental climates.
Monsoon in India
Summer Monsoon
In the Thar desert in the northern Indian subcontinent, the daytime temperature in summer is very high and the surface air rises in altitude causing a local depression. This is the origin (place) of the circulation that is established with the coasts of the Indian Ocean. Warm, moist air from the sea comes from both the east and west and converges on the Himalayas. This mountain range forces the air to rise and is cooled by the ideal gas law and moisture condenses in the form of clouds and rain. The constant flow of humid air produces abundant rainfall and up to 10,000 mm of rain can be collected per year in some places [citation needed].
This monsoon, which arrives from the southwest, is divided into two branches due to the orography of India. These are: the southwest monsoon of the Arabian Sea and that of the Bay of Bengal. The wind first reaches the region of the Ghats mountains on the coast of the state of Kerala in southwestern India. The wind circulation is divided into two: the first branch moves north along the western slopes of the mountains, while the second passes along the eastern side of the Deccan Plateau and suffers a foehn effect, which dries it out. and produces only light rains and variable distribution in the Deccan peninsula.
The monsoon wind of this second branch passes through the Bay of Bengal, where it is moistened by evaporation from the sea surface, and then runs to the mouth of the Ganges and up the slope of the Himalayas east of the Burmese mountains. This branch of the monsoon brings rain to northeastern India, the State of West Bengal, Bangladesh and Burma.
The uplift of the monsoon wind is accentuated in this region by the funnel-shaped Ganges delta and rugged mountains. The monsoon wind, blocked by the mountains, must turn west on the Indo-Gangetic Plain and waters it abundantly. Cherrapunji, in the state of Meghalaya, located on the southern slopes of the Himalayas, is one of the wettest places on Earth. The moisture contained in the monsoon gradually drains off along its path and north-west India receives almost no rain, being a very arid region.
This process of development of summer monsoon rains is gradually established in the Indian subcontinent, so the date of its beginning can vary between March and June depending on the region, and that of its end, September to November. Sometimes it happens that it weakens for a few years, or that it is interrupted for different periods.
Winter Monsoon
Starting in September, daytime temperatures decrease in the north of the subcontinent, with shorter days, and the temperature drops at night in these desert areas. A large thermal anticyclone called the Siberian anticyclone forms in the Lake Baikal region. The overlying subsidence areas are fed aloft by upwelling winds which then maintain the ITCZ in the humid regions of the Southern Hemisphere, mainly above Indonesia, northeast of Australia and the eastern coasts of Africa.
Under these conditions, the trade winds originate south of the Siberian anticyclone and go southeast to head towards the ITCZ, which is south of the equator. Because the Indian Ocean cools more slowly than the surrounding continent, these trade winds mix with the advection of polar air surrounding the Siberian high to form northeasterly currents that blow from land to sea. Before reaching India, the air must cross the Himalayas and therefore suffers a strong foehn effect that dries it out even more and heats it considerably. The circulation of the winds is thus established by the same corridors that the summer monsoon used during the summer in the Ganges and Indus valleys, giving rise to the northeast monsoon or "dry monsoon".
This wind clears the sky in the north of the continent, but once it passes over the Indian Ocean it accumulates moisture by evaporation from the surface of the Bay of Bengal. This winter monsoon will then pass over the islands and south-eastern India and form clouds as it climbs the slopes of these regions. These rains are less abundant than during the summer monsoon, but cities like Madras and states such as Tamil Nadu benefit from it. These places receive 50% to 60% of the annual precipitation during this monsoon.
Impact on Indian economy and culture
The summer monsoon produces 80 percent of the total precipitation in the affected areas. The return of the monsoon has an uneven rhythm since, from one year to the next, the rains have a different duration and intensity. The monsoon is beneficial, since it waters the land, and at the same time harmful, when it floods the villages. It is irregular and unpredictable.
The eternal return of the monsoons is a permanent surprise: will it be early or late, abundant or weak, regular or brutal? Thus, agriculture in India, which accounts for twenty-five percent of the gross national product and seventy percent of employment, depends on the monsoon. Crops such as cotton or rice have a high demand for water. A weak monsoon, delayed monsoon or prolonged interruptions become a dramatic turnaround for hundreds of millions of Indians and Bangalis, whose economic lives are entirely dependent on the contribution of these monsoon rains. During the 1990s, drought caused by a change in the classical pattern of the monsoon season caused significant humanitarian and financial damage.
In their prayers, a nation of farmers asks for a good monsoon, without which the country will plunge into famine. The pilgrimage is used to obtain a doubly better, richer and fairer world. For, all year round subject to the caste hierarchy, the pilgrims want, during the time of their devotion, to live in an egalitarian world in which all believers are equal in the eyes of the gods.
The monsoon is also popular with city dwellers as it cools the atmosphere. Indeed, cloudy skies let in less solar radiation and keep the temperature slightly lower than during the period preceding the monsoon. However, the humidity increases considerably and the rain floods the streets. The rains, in this way, damage many buildings, especially on sloping streets. Deaths occur each year from drowning and from diseases carried by insects that reproduce well in these conditions. Some years, like 2005, there were thousands of deaths due to flooding. Recently, some arid areas such as the Thar desert have suffered from flooding when the monsoon season has extended.
Extension of the concept
Since the monsoon system is better understood, its definition has been broadened to include almost all phenomena related to the annual cycle of Climate in the tropics and subtropics of Asia, Australia, South America, Africa and in regional seas and oceans. All of these regions have the strongest and most spectacular weather cycles on our planet, Earth, and it is especially the summer monsoon that dominates these areas. The monsoon in South China and South Asia falls into the same cycle as that of India. It occurs with some difference in other regions and one cannot generally speak of winter monsoons in any of these regions except India. Finally, monsoon events remain marginal in the American tropics and subtropics, but the term is used quite frequently by the National Weather Service to refer to the rainy season in the deserts of the American West.
African Monsoon
The most striking case in this regard is that of sub-Saharan Africa. In the southwest of this region of Africa, there is a monsoon that is related to the semi-annual displacement of the Intertropical Convergence Zone (ITCZ) and the difference in warming of the Sahara and the equatorial Atlantic coast in the Gulf of Guinea. The dry northeasterly trade winds, and especially its more intense form of harmattan, are cut by the summer northward movement of the ITCZ where the winds are light. The humid belt of the African coast is expanding, without entering the interior of the continent, unlike what happens in India or China.
The West African monsoon differs in many ways from the Asian monsoon. The phenomenon is very symmetrical from west to east on a large scale, whereas in India the flow is more complex at the beginning. Another important difference, among many others, is found in the fact that the Indian monsoon seems more constant in terms of rainfall than the African monsoon. India has never known more than two consecutive years of drought during the 20th century, while the Sahel region has suffered from drought since the early 1990s.
The African monsoon is still a subject of study. In fact, it varies by up to 40% from year to year, while the Indian monsoon oscillates by only 10%. The semi-arid regions of the Sahel and Sudan have a highly random rainfall period within a southerly flow, on which the survival of the population depends.
Monsoon of Southeast Asia and Oceania
In South Asia, monsoons occur from June to September, with north-easterly winds. The temperature in central Asia is less than 25 °C because it is winter, which creates an anticyclone over the region. The jet stream in this region is divided into a subtropical and a polar branch. The first current blows mainly from the northeast, bringing dry air to India and South Asia. At the same time, a low pressure is developing in Southeast Asia and Australia, whose winds are heading towards Australia, forming a wet convergence.
South American Monsoon
The Argentine coast is affected by the summer monsoon, especially the province of Corrientes. Most of Brazil is influenced by a summer monsoon. Rio de Janeiro is famous for its floods during it.
North American Monsoon
In North America, the temperature difference between the great deserts of the western United States and Mexico and the Gulf of California drives a monsoon that runs from late June to late September. It starts along the coast and extends to the desert during this period. It affects the Sierra Madre Occidental in Mexico, and in the United States the states of Arizona, New Mexico, Nevada, Utah, Colorado, Texas and even the southeastern part of California. It rarely makes it to the Pacific coast.
This is associated with episodes of brief but torrential storms and not with continuous rains. In fact, it causes large amounts of moisture from the Gulf of Mexico to create warm, unstable air. This moisture is not distributed over large territories, and storms occur when additional triggers occur. In general, storms occur and cause flash floods of dry streams in these areas if the "precipitable" is greater than 34mm. Up to seventy percent of the total annual precipitation in these regions falls during the monsoon. Plants have adapted to this rainfall and the Sonoran and Mojave deserts are considered "wet" deserts. These rains also play a role in the control of forest fires.
Additional bibliography
- International Committee of the Third Workshop on Monsoons. The Global Monsoon System: Research and Forecast. (in English)
- Chang, C.P., Wang, Z., Hendon, H., 2006, The Asian Winter Monsoon. The Asian Monsoon, Wang, B. (ed.), Praxis, Berlin, pp. 89-127. (in English)
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