Monsoon of South Asia

Last updated

A visualisation of the South Asian Monsoon based on the Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS) 30+ year quasi-global rainfall dataset, analysed and visualised using Google Earth Engine. The South Asian Monsoon.gif
A visualisation of the South Asian Monsoon based on the Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS) 30+ year quasi-global rainfall dataset, analysed and visualised using Google Earth Engine.
Annual average monsoon precipitation in India over 110 years. The long-term average has been 899 millimeters of precipitation. However, the monsoon varies over the Indian subcontinent within a +-20% range. Rains that exceed 10% typically lead to major floods, while a 10% shortfall is a significant drought. Monsoon annual trends for India over 110 years, average precipitation over the nation.png
Annual average monsoon precipitation in India over 110 years. The long-term average has been 899 millimeters of precipitation. However, the monsoon varies over the Indian subcontinent within a ±20% range. Rains that exceed 10% typically lead to major floods, while a 10% shortfall is a significant drought.

The Monsoon of South Asia is among several geographically distributed global monsoons. It affects the Indian subcontinent, where it is one of the oldest and most anticipated weather phenomena and an economically important pattern every year from June through September, but it is only partly understood and notoriously difficult to predict. Several theories have been proposed to explain the origin, process, strength, variability, distribution, and general vagaries of the monsoon, but understanding and predictability are still evolving.

Contents

The unique geographical features of the Indian subcontinent, along with associated atmospheric, oceanic, and geographical factors, influence the behavior of the monsoon. Because of its effect on agriculture, on flora and fauna, and on the climates of nations such as Bangladesh, Bhutan, India, Nepal, Pakistan, and Sri Lanka – among other economic, social, and environmental effects – the monsoon is one of the most anticipated, tracked, [3] and studied weather phenomena in the region. It has a significant effect on the overall well-being of residents and has even been dubbed the "real finance minister of India". [4] [5]

Definition

The word monsoon (derived from the Arabic "mausim", meaning "seasonal reversal of winds"), although generally defined as a system of winds characterized by a seasonal reversal of direction, [6] lacks a consistent, detailed definition. Some examples are:

Background

Observed initially by sailors in the Arabian Sea [10] traveling between Africa, India, and Southeast Asia, the monsoon can be categorized into two branches based on their spread over the subcontinent:

Southwest monsoon clouds over Tamil Nadu. Monsoon season beach Tamil Nadu India.jpg
Southwest monsoon clouds over Tamil Nadu.

Alternatively, it can be categorized into two segments based on the direction of rain-bearing winds:

Based on the time of year that these winds bring rain to India, the monsoon can also be categorized into two periods:

The complexity of the monsoon of South Asia is not completely understood, making it difficult to accurately predict the quantity, timing, and geographic distribution of the accompanying precipitation. These are the most monitored components of the monsoon, and they determine the water availability in India for any given year. [11]

Changes of the Monsoon

Monsoon over India Aug2011monsoon.JPG
Monsoon over India

Monsoons typically occur in tropical areas. One area that monsoons impact greatly is India. In India monsoons create an entire season in which the winds reverse completely.

The rainfall is a result of the convergence of wind flow from the Bay of Bengal and reverse winds from the South China Sea. [12]

The onset of the monsoon occurs over the Bay of Bengal in May, [12] arriving at the Indian Peninsula by June, [13] and then the winds move towards the South China Sea. [12]

Effect of geographical relief features

Although the southwest and northeast monsoon winds are seasonally reversible, they do cause precipitation on their own.

Two factors are essential for rain formation:

  1. Moisture-laden winds
  2. Droplet formation

Additionally, one of the causes of rain must happen. In the case of the monsoon, the cause is primarily orographic, due to the presence of highlands in the path of the winds. Orographic barriers force wind to rise. Precipitation then occurs on the windward side of the highlands because of adiabatic cooling and condensation of the moist rising air.

The unique geographic relief features of the Indian subcontinent come into play in allowing all of the above factors to occur simultaneously. The relevant features in explaining the monsoon mechanism are as follows:

  1. The presence of abundant water bodies around the subcontinent: the Arabian Sea, Bay of Bengal, and Indian Ocean. These help moisture accumulate in the winds during the hot season.
  2. The presence of abundant highlands like the Western Ghats and the Himalayas right across the path of the southwest monsoon winds. These are the main cause of the substantial orographic precipitation throughout the subcontinent. [Note 2]
    1. The Western Ghats are the first highlands of India that the southwest monsoon winds encounter. [Note 3] The Western Ghats rise abruptly from the Western Coastal Plains of the subcontinent, making effective orographic barriers for the monsoon winds.
    2. The Himalayas play more than the role of orographic barriers for the monsoon. They also help confine it to the subcontinent. Without them, the southwest monsoon winds would blow right over the Indian subcontinent into Tibet, Afghanistan, and Russia without causing any rain. [Note 4]
    3. For the northeast monsoon, the highlands of the Eastern Ghats play the role of orographic barrier.

Features of monsoon rains

There are some unique features of the rains that the monsoon brings to the Indian subcontinent.

"Bursting"

"Monsoon burst" over Mumbai Mumbai india monsoon clouds.jpg
"Monsoon burst" over Mumbai

Bursting of monsoon refers to the sudden change in weather conditions in India (typically from hot and dry weather to wet and humid weather during the southwest monsoon), characterized by an abrupt rise in the mean daily rainfall. [14] [15] Similarly, the burst of the northeast monsoon refers to an abrupt increase in the mean daily rainfall over the affected regions. [16]

Rain variability ("vagaries")

One of the most commonly used words to describe the erratic nature of the monsoon is "vagaries", used in newspapers, [17] magazines, [18] books, [19] web portals [20] to insurance plans, [21] and India's budget discussions. [22] In some years, it rains too much, causing floods in parts of India; in others, it rains too little or not at all, causing droughts. In some years, the rain quantity is sufficient but its timing arbitrary. Sometimes, despite average annual rainfall, the daily distribution or geographic distribution of the rain is substantially skewed. In the recent past, rainfall variability in short time periods (about a week) were attributed to desert dust over the Arabian Sea and Western Asia. [23]

Ideal and normal monsoon rains

Annual average rainfall in India India annual rainfall map en.svg
Annual average rainfall in India

Normally, the southwest monsoon can be expected to "burst" onto the western coast of India (near Thiruvananthapuram) at the beginning of June and to cover the entire country by mid-July. [11] [24] [25] Its withdrawal from India typically starts at the beginning of September and finishes by the beginning of October. [26] [27]

The northeast monsoon usually "bursts" around 20 October and lasts for about 50 days before withdrawing. [16]

However, a rainy monsoon is not necessarily a normal monsoon – that is, one that performs close to statistical averages calculated over a long period. A normal monsoon is generally accepted to be one involving close to the average quantity of precipitation over all the geographical locations under its influence (mean spatial distribution) and over the entire expected time period (mean temporal distribution). Additionally, the arrival date and the departure date of both the southwest and northeast monsoon should be close to the mean dates. The exact criteria for a normal monsoon are defined by the India Meteorological Department with calculations for the mean and standard deviation of each of these variables. [28]

Monsoon clouds over Taraganj, Rangpur, Bangladesh Moonsoon taragonj 1.jpg
Monsoon clouds over Taraganj, Rangpur, Bangladesh

Theories for mechanism of monsoon

Theories of the mechanism of the monsoon primarily try to explain the reasons for the seasonal reversal of winds and the timing of their reversal.

Traditional theory

Because of differences in the specific heat capacity of land and water, continents heat up faster than seas. Consequently, the air above coastal lands heats up faster than the air above seas. These create areas of low air pressure above coastal lands compared with pressure over the seas, causing winds to flow from the seas onto the neighboring lands. This is known as sea breeze.

Process of monsoon creation

A: sea breeze; B: land breeze Diagrama de formacion de la brisa-breeze.svg
A: sea breeze; B: land breeze

Also known as the thermal theory or the differential heating of sea and land theory, the traditional theory portrays the monsoon as a large-scale sea breeze. It states that during the hot subtropical summers, the massive landmass of the Indian Peninsula heats up at a different rate than the surrounding seas, resulting in a pressure gradient from south to north. This causes the flow of moisture-laden winds from sea to land. On reaching land, these winds rise because of the geographical relief, cooling adiabatically and leading to orographic rains. This is the southwest monsoon. The reverse happens during the winter, when the land is colder than the sea, establishing a pressure gradient from land to sea. This causes the winds to blow over the Indian subcontinent toward the Indian Ocean in a northeasterly direction, causing the northeast monsoon. Because the southwest monsoon flows from sea to land, it carries more moisture, and therefore causes more rain, than the northeast monsoon. Only part of the northeast monsoon passing over the Bay of Bengal picks up moisture, causing rain in Andhra Pradesh and Tamil Nadu during the winter months.

However, many meteorologists argue that the monsoon is not a local phenomenon as explained by the traditional theory, but a general weather phenomenon along the entire tropical zone of Earth. This criticism does not deny the role of differential heating of sea and land in generating monsoon winds, but casts it as one of several factors rather than the only one.

Dynamic theory

The atmospheric circulation system with associated pressure belts and latitudes. Earth Global Circulation.svg
The atmospheric circulation system with associated pressure belts and latitudes.

The prevailing winds of the atmospheric circulation arise because of the difference in pressure at various latitudes and act as means for distribution of thermal energy on the planet. This pressure difference is because of the differences in solar insolation received at different latitudes and the resulting uneven heating of the planet. Alternating belts of high pressure and low pressure develop along the equator, the two tropics, the Arctic Circle and Antarctic Circle, and the two polar regions, giving rise to the trade winds, the westerlies, and the polar easterlies. However, geophysical factors like Earth's orbit, its rotation, and its axial tilt cause these belts to shift gradually north and south, following the Sun's seasonal shifts.

Process of monsoon creation

The dynamic theory explains the monsoon on the basis of the annual shifts in the position of global belts of pressure and winds. According to this theory, the monsoon is a result of the shift of the Intertropical Convergence Zone (ITCZ) under the influence of the vertical sun. Though the mean position of the ITCZ is taken as the equator, it shifts north and south with the migration of the vertical sun toward the Tropics of Cancer and Capricorn during the summer of the respective hemispheres (Northern and Southern Hemisphere). As such, during the northern summer (May and June), the ITCZ moves north, along with the vertical sun, toward the Tropic of Cancer. The ITCZ, as the zone of lowest pressure in the tropical region, is the target destination for the trade winds of both hemispheres. Consequently, with the ITCZ at the Tropic of Cancer, the southeast trade winds of the Southern Hemisphere have to cross the equator to reach it. [Note 5] However, because of the Coriolis effect (which causes winds in the Northern Hemisphere to turn right, whereas winds in the Southern Hemisphere turn left), these southeast trade winds are deflected east in the Northern Hemisphere, transforming into southwest trades. [Note 6] These pick up moisture while traveling from sea to land and cause orographic rain once they hit the highlands of the Indian Peninsula. This results in the southwest monsoon.

The dynamic theory explains the monsoon as a global weather phenomenon rather than just a local one. And when coupled with the traditional theory (based on the heating of sea and land), it enhances the explanation of the varying intensity of monsoon precipitation along the coastal regions with orographic barriers.

Jet stream theory

Jet streams of Earth Jetcrosssection.jpg
Jet streams of Earth

This theory tries to explain the establishment of the northeast and southwest monsoons, as well as unique features like "bursting" and variability.

The jet streams are systems of upper-air westerlies. They give rise to slowly moving upper-air waves, with 250-knot winds in some air streams. First observed by World War II pilots, they develop just below the tropopause over areas of steep pressure gradient on the surface. The main types are the polar jets, the subtropical westerly jets, and the less common tropical easterly jets. They follow the principle of geostrophic winds. [Note 7]

Process of monsoon creation

The Tibetan Plateau lies north of the Himalayas Tibet Karte Topograpisch.png
The Tibetan Plateau lies north of the Himalayas

Over India, a subtropical westerly jet develops in the winter season and is replaced by the tropical easterly jet in the summer season. The high temperature during the summer over the Tibetan Plateau, as well as over Central Asia in general, is believed to be the critical factor leading to the formation of the tropical easterly jet over India.

The mechanism affecting the monsoon is that the westerly jet causes high pressure over northern parts of the subcontinent during the winter. This results in the north-to-south flow of the winds in the form of the northeast monsoon. With the northward shift of the vertical sun, this jet shifts north, too. The intense heat over the Tibetan Plateau, coupled with associated terrain features like the high altitude of the plateau, generate the tropical easterly jet over central India. This jet creates a low-pressure zone over the northern Indian plains, influencing the wind flow toward these plains and assisting the development of the southwest monsoon[ clarification needed ].

Theories for "bursting"

The "bursting" [14] of the monsoon is primarily explained by the jet stream theory and the dynamic theory.

Dynamic theory

According to this theory, during the summer months in the Northern Hemisphere, the ITCZ shifts north, pulling the southwest monsoon winds onto the land from the sea. However, the huge landmass of the Himalayas restricts the low-pressure zone onto the Himalayas themselves. It is only when the Tibetan Plateau heats up significantly more than the Himalayas that the ITCZ rises abruptly and swiftly shifts north, leading to the bursting of monsoon rains over the Indian subcontinent. The reverse shift takes place for the northeast monsoon winds, leading to a second, minor burst of rainfall over the eastern Indian Peninsula during the Northern Hemisphere winter months.

Jet stream theory

According to this theory, the onset of the southwest monsoon is driven by the shift of the subtropical westerly jet north from over the plains of India toward the Tibetan Plateau. This shift is due to the intense heating of the plateau during the summer months. The northward shift is not a slow and gradual process, as expected for most changes in weather pattern. The primary cause is believed to be the height of the Himalayas. As the Tibetan Plateau heats up, the low pressure created over it pulls the westerly jet north. Because of the lofty Himalayas, the westerly jet's movement is inhibited. But with continuous dropping pressure, sufficient force is created for the movement of the westerly jet across the Himalayas after a significant period. As such, the shift of the jet is sudden and abrupt, causing the bursting of southwest monsoon rains onto the Indian plains. The reverse shift happens for the northeast monsoon.

Theories for monsoon variability

The jet stream effect

The jet stream theory also explains the variability in timing and strength of the monsoon.

Timing: A timely northward shift of the subtropical westerly jet at the beginning of summer is critical to the onset of the southwest monsoon over India. If the shift is delayed, so is the southwest monsoon. An early shift results in an early monsoon.
Strength: The strength of the southwest monsoon is determined by the strength of the easterly tropical jet over central India. A strong easterly tropical jet results in a strong southwest monsoon over central India, and a weak jet results in a weak monsoon.

El Niño–Southern Oscillation effect

Effects of El Nino on the weather of the subcontinent El Nino regional impacts.png
Effects of El Niño on the weather of the subcontinent

El Niño is a warm ocean current originating along the coast of Peru that replaces the usual cold Humboldt Current. The warm surface water moving toward the coast of Peru with El Niño is pushed west by the trade winds, thereby raising the temperature of the southern Pacific Ocean. The reverse condition is known as La Niña.

Southern Oscillation, a phenomenon first observed by Sir Gilbert Walker, director general of observatories in India, refers to the seesaw relationship of atmospheric pressures between Tahiti and Darwin, Australia. [29] Walker noticed that when pressure was high in Tahiti, it was low in Darwin, and vice versa. [29] A Southern Oscillation Index (SOI), based on the pressure difference between Tahiti and Darwin, has been formulated by the Bureau of Meteorology (Australia) to measure the strength of the oscillation. [30] Walker noticed that the quantity of rainfall in the Indian subcontinent was often negligible in years of high pressure over Darwin (and low pressure over Tahiti). Conversely, low pressure over Darwin bodes well for precipitation quantity in India. Thus, Walker established the relationship between southern oscillation and quantities of monsoon rains in India. [29]

Ultimately, the southern oscillation was found to be simply an atmospheric component of the El Niño/La Niña effect, which happens in the ocean. [29] Therefore, in the context of the monsoon, the two together came to be known as the El Niño–Southern Oscillation (ENSO) effect. The effect is known to have a pronounced influence on the strength of the southwest monsoon over India, with the monsoon being weak (causing droughts) during El Niño years, while La Niña years bring particularly strong monsoons. [29]

Indian Ocean dipole effect

Although the ENSO effect was statistically effective in explaining several past droughts in India, in recent decades, its relationship with the Indian monsoon seemed to weaken. [31] For example, the strong ENSO of 1997 did not cause drought in India. [29] However, it was later discovered that, just like ENSO in the Pacific Ocean, a similar seesaw ocean-atmosphere system in the Indian Ocean was also in play. This system was discovered in 1999 and named the Indian Ocean Dipole (IOD). An index to calculate it was also formulated. IOD develops in the equatorial region of the Indian Ocean from April to May and peaks in October. [29] With a positive IOD, winds over the Indian Ocean blow from east to west. This makes the Arabian Sea (the western Indian Ocean near the African coast) much warmer and the eastern Indian Ocean around Indonesia colder and drier. [29] In negative dipole years, the reverse happens, making Indonesia much warmer and rainier.

A positive IOD index often negates the effect of ENSO, resulting in increased monsoon rains in years such as 1983, 1994, and 1997. [29] Further, the two poles of the IOD – the eastern pole (around Indonesia) and the western pole (off the African coast) — independently and cumulatively affect the quantity of monsoon rains. [29]

Equatorial Indian Ocean oscillation

As with ENSO, the atmospheric component of the IOD was later discovered and the cumulative phenomenon named Equatorial Indian Ocean oscillation (EQUINOO). [29] When EQUINOO effects are factored in, certain failed forecasts, like the acute drought of 2002, can be further accounted for. [29] The relationship between extremes of the Indian summer monsoon rainfall, along with ENSO and EQUINOO, [32] have been studied, and models to better predict the quantity of monsoon rains have been statistically derived. [32]

Impact of climate change

Since 1950s, the South Asian summer monsoon has been exhibiting large changes, especially in terms of droughts and floods. [33] The observed monsoon rainfall indicates a gradual decline over central India, with a reduction of up to 10%. [34] This is primarily due to a weakening monsoon circulation as a result of the rapid warming in the Indian Ocean, [35] [36] and changes in land use and land cover, [37] while the role of aerosols remains elusive. Since the strength of the monsoon is partially dependent on the temperature difference between the ocean and the land, higher ocean temperatures in the Indian Ocean have weakened the moisture bearing winds from the ocean to the land. The reduction in the summer monsoon rainfall has grave consequences over central India because at least 60% of the agriculture in this region is still largely rain-fed.

A recent assessment of the monsoonal changes indicate that the land warming has increased during 2002–2014, possibly reviving the strength of the monsoon circulation and rainfall. [38] Future changes in the monsoon will depend on a competition between land and ocean—on which is warming faster than the other.

Meanwhile, there has been a three-fold rise in widespread extreme rainfall events during the years 1950 to 2015, over the entire central belt of India, leading to a steady rise in the number of flash floods with significant socioeconomic losses. [39] [40] Widespread extreme rainfall events are those rainfall events which are larger than 150 mm/day and spread over a region large enough to cause floods.

Monsoon rain prediction models

Since the Great Famine of 1876–1878 in India, various attempts have been made to predict monsoon rainfall. [41] At least five prediction models exist. [42]

Seasonal Prediction of Indian Monsoon (SPIM)

The Centre for Development of Advanced Computing (CDAC) at Bengaluru facilitated the Seasonal Prediction of Indian Monsoon (SPIM) experiment on the PARAM Padma supercomputing system. [43] This project involved simulated runs of historical data from 1985 to 2004 to try to establish the relationship of five atmospheric general circulation models with monsoon rainfall distribution. [42]

India Meteorological Department model

The department has tried to forecast the monsoon for India since 1884, [41] and is the only official agency entrusted with making public forecasts about the quantity, distribution, and timing of the monsoon rains. Its position as the sole authority on the monsoon was cemented in 2005 [42] by the Department of Science and Technology (DST), New Delhi. In 2003, IMD substantially changed its forecast methodology, model, [44] and administration. [45] A sixteen-parameter monsoon forecasting model used since 1988 was replaced in 2003. [44] However, following the 2009 drought in India (worst since 1972), [46] The department decided in 2010 that it needed to develop an "indigenous model" [47] to further improve its prediction capabilities.

Significance

Western Ghats, Maharashtra on 28 May in dry season MatheranPanoramaPointDrySeasonCrop.jpg
Western Ghats, Maharashtra on 28 May in dry season
Western Ghats, Maharashtra on 28 August in rainy season MatheranPanoramaPointMonsoonCrop.jpg
Western Ghats, Maharashtra on 28 August in rainy season

The monsoon is the primary delivery mechanism for fresh water in the Indian subcontinent. As such, it affects the environment (and associated flora, fauna, and ecosystems), agriculture, society, hydro-power production, and geography of the subcontinent (like the availability of fresh water in water bodies and the underground water table), with all of these factors cumulatively contributing to the health of the economy of affected countries.

The monsoon turns large parts of India from semi-deserts into green grasslands. See photos taken only three months apart in the Western Ghats.

Geographical (wettest spots on Earth)

Mawsynram and Cherrapunji, both in the Indian state of Meghalaya, alternate as the wettest places on Earth given the quantity of their rainfall, [48] though there are other cities with similar claims. They receive more than 11,000 millimeters of rain each from the monsoon.

Agricultural

In India, which has historically had a primarily agrarian economy, the services sector recently overtook the farm sector in terms of GDP contribution. However, the agriculture sector still contributes 17–20% of GDP [49] and is the largest employer in the country, with about 60% of Indians dependent on it for employment and livelihood. [49] About 49% of India's land is agricultural; that number rises to 55% if associated wetlands, dryland farming areas, etc., are included. Because more than half of these farmlands are rain-fed, the monsoon is critical to food sufficiency and quality of life.

Despite progress in alternative forms of irrigation, agricultural dependence on the monsoon remains far from insignificant. Therefore, the agricultural calendar of India is governed by the monsoon. Any fluctuations in the time distribution, spatial distribution, or quantity of the monsoon rains may lead to floods or droughts, causing the agricultural sector to suffer. This has a cascading effect on the secondary economic sectors, the overall economy, food inflation, and therefore the general population's quality and cost of living.

Economic

The economic significance of the monsoon is aptly described by Pranab Mukherjee's remark that the monsoon is the "real finance minister of India". [4] [5] A good monsoon results in better agricultural yields, which brings down prices of essential food commodities and reduces imports, thus reducing food inflation overall. [49] Better rains also result in increased hydroelectric production. [49] All of these factors have positive ripple effects throughout the economy of India. [49]

The down side however is that when monsoon rains are weak, crop production is low leading to higher food prices with limited supply. [50] As a result, the Indian government is actively working with farmers and the nation's meteorological department to produce more drought resistant crops. [50]

Health

The onset of the monsoon increases fungal and bacterial activity. A host of mosquito-borne, water-borne and air-borne infections become more common as a result of the change in the ecosystem. These include diseases such as dengue, malaria, cholera, and colds. [51]

Social

D. Subbarao, former governor of the Reserve Bank of India, emphasized during a quarterly review of India's monetary policy that the lives of Indians depend on the performance of the monsoon. [52] His own career prospects, his emotional well-being, and the performance of his monetary policy are all "a hostage" to the monsoon, he said, as is the case for most Indians. [52] Additionally, farmers rendered jobless by failed monsoon rains tend to migrate to cities. This crowds city slums and aggravates the infrastructure and sustainability of city life. [53]

Travel

In the past, Indians usually refrained from traveling during monsoons for practical as well as religious reasons. But with the advent of globalization, such travel is gaining popularity. Places like Kerala and the Western Ghats get a large number of tourists, both local and foreigners, during the monsoon season. Kerala is one of the top destinations for tourists interested in Ayurvedic treatments and massage therapy. One major drawback of traveling during the monsoon is that most wildlife sanctuaries are closed. Also, some mountainous areas, especially in Himalayan regions, get cut off when roads are damaged by landslides and floods during heavy rains. [54]

Environmental

The monsoon is the primary bearer of fresh water to the area. The peninsular/Deccan rivers of India are mostly rain-fed and non-perennial in nature, depending primarily on the monsoon for water supply. [55] Most of the coastal rivers of Western India are also rain-fed and monsoon-dependent. [55] [56] As such, the flora, fauna, and entire ecosystems of these areas rely heavily on the monsoon. [ citation needed ]

See also

Notes

  1. ↑ The name of the wind is based on the direction that it blows from. Southwest winds blow onto the land from the southwest. Northeast winds flow from the northeast toward the southwest, onto the land.
  2. ↑ The Aravalli Mountains also lie in the path of the southwest monsoon but do not result in much precipitation because they are in the direction of the path of the southwest winds and not across them, causing no orographic lift of the winds.
  3. ↑ Other major highlands, like the Cardamom Hills, Anaimalai Hills, and Nilgiri Mountains, that play active roles in the monsoon are considered major extensions of the Western Ghats and are thus not discussed separately.
  4. ↑ First, the Himalayas serve as orographic barriers to the southwest monsoon winds. Second, they help confine the winds to the subcontinent, hampering their northward progress. Third, they contribute to the convergence of the Bay of Bengal branch and Arabian Sea branch of the southwest monsoon winds, increasing the intensity of precipitation over the northern part of the subcontinent. Fourth, they are a major factor in the bursting of the monsoon under the jet stream theory. Fifth, they help determine the direction of the Bay of Bengal branch of the northeast monsoon. Their role is still a matter of active study, and understanding of them is evolving regularly.
  5. ↑ When the southeast trade winds cross the equator, they are perceived in the Northern Hemisphere as equatorial westerlies because they seem to blow from the equator toward the Tropic of Cancer. Similarly, when the ITCZ is at the Tropic of Cancer, the northeast trade winds are confined to the area north of the Tropic of Cancer.
  6. ↑ Change of direction or origin of winds alters their nomenclature as noted above.
  7. ↑ Geostrophic winds blow parallel to the isobars and keep low-pressure zones to their left in the Northern Hemisphere and to their right in the Southern Hemisphere. The reversal is a result of the Coriolis effect.

Related Research Articles

<span class="mw-page-title-main">Monsoon</span> Seasonal changes in atmospheric circulation and precipitation

A monsoon is traditionally 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 annual latitudinal oscillation of the Intertropical Convergence Zone (ITCZ) between its limits to the north and south of the equator. 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 also sometimes used to describe locally heavy but short-term rains.

<span class="mw-page-title-main">El Niño–Southern Oscillation</span> Physical oceanography

El Niño–Southern Oscillation (ENSO) is an irregular periodic variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean, affecting the climate of much of the tropics and subtropics. The warming phase of the sea temperature is known as El Niño and the cooling phase as La Niña. The Southern Oscillation is the accompanying atmospheric component, coupled with the sea temperature change: El Niño is accompanied by high air surface pressure in the tropical western Pacific and La Niña with low air surface pressure there. The two periods last several months each and typically occur every few years with varying intensity per period.

<span class="mw-page-title-main">Orographic lift</span> Air mass forced upwards as it moves over rising terrain

Orographic lift occurs when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain. As the air mass gains altitude it quickly cools down adiabatically, which can raise the relative humidity to 100% and create clouds and, under the right conditions, precipitation.

<span class="mw-page-title-main">Rain shadow</span> Leeward side of a mountain range

A rain shadow is an area of significantly reduced rainfall behind a mountainous region, on the side facing away from prevailing winds, known as its leeward side.

<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, a usage that is more common in Australia and parts of Asia.

<span class="mw-page-title-main">Prevailing winds</span> Strongest direction of wind on a region of Earths surface

In meteorology, prevailing wind in a region of the Earth's surface is a surface wind that blows predominantly from a particular direction. The dominant winds are the trends in direction of wind with the highest speed over a particular point on the Earth's surface at any given time. A region's prevailing and dominant winds are the result of global patterns of movement in the Earth's atmosphere. In general, winds are predominantly easterly at low latitudes globally. In the mid-latitudes, westerly winds are dominant, and their strength is largely determined by the polar cyclone. In areas where winds tend to be light, the sea breeze/land breeze cycle is the most important cause of the prevailing wind; in areas which have variable terrain, mountain and valley breezes dominate the wind pattern. Highly elevated surfaces can induce a thermal low, which then augments the environmental wind flow.

<span class="mw-page-title-main">Climate of India</span> Overview of climatic conditions in India

The climate of India consists of a wide range of weather conditions across a vast geographic scale and varied topography. Based on the Köppen system, India hosts six major climatic sub types, ranging from arid deserts in the west, alpine tundra and glaciers in the north, and humid tropical regions supporting rain forests in the southwest and the island territories. Many regions have starkly different microclimates, making it one of the most climatically diverse countries in the world. The country's meteorological department follows the international standard of four seasons with some local adjustments: winter, summer, monsoon or rainy season, and a post-monsoon period.

The South Pacific Convergence Zone (SPCZ), a reverse-oriented monsoon trough, is a band of low-level convergence, cloudiness and precipitation extending from the Western Pacific Warm Pool at the maritime continent south-eastwards towards French Polynesia and as far as the Cook Islands. The SPCZ is a portion of the Intertropical Convergence Zone (ITCZ) which lies in a band extending east–west near the Equator but can be more extratropical in nature, especially east of the International Date Line. It is considered the largest and most important piece of the ITCZ, and has the least dependence upon heating from a nearby landmass during the summer than any other portion of the monsoon trough. The SPCZ can affect the precipitation on Polynesian islands in the southwest Pacific Ocean, so it is important to understand how the SPCZ behaves with large-scale, global climate phenomenon, such as the ITCZ, El Niño–Southern Oscillation, and the Interdecadal Pacific oscillation (IPO), a portion of the Pacific decadal oscillation.

<span class="mw-page-title-main">2005 North Indian Ocean cyclone season</span> Cyclone season in the North Indian ocean

The 2005 North Indian Ocean cyclone season caused much devastation and many deaths in Southern India despite the storms’ weakness. The basin covers the Indian Ocean north of the equator as well as inland areas, sub-divided by the Arabian Sea and the Bay of Bengal. Although the season began early with two systems in January, the bulk of activity was confined from September to December. The official India Meteorological Department tracked 12 depressions in the basin, and the unofficial Joint Typhoon Warning Center (JTWC) monitored two additional storms. Three systems intensified into a cyclonic storm, which have sustained winds of at least 63 km/h (39 mph), at which point the IMD named them.

<span class="mw-page-title-main">Geography of South America</span> Overview of the geography of South America

The geography of South America contains many diverse regions and climates. Geographically, South America is generally considered a continent forming the southern portion of the landmass of the Americas, south and east of the Colombia–Panama border by most authorities, or south and east of the Panama Canal by some. South and North America are sometimes considered a single continent or supercontinent, while constituent regions are infrequently considered subcontinents.

<span class="mw-page-title-main">Monsoon trough</span> Weather phenomenon

The monsoon trough is a portion of the Intertropical Convergence Zone in the Western Pacific, as depicted by a line on a weather map showing the locations of minimum sea level pressure, and as such, is a convergence zone between the wind patterns of the southern and northern hemispheres.

<span class="mw-page-title-main">Drought in India</span> Natural calamity

Drought in India has resulted in millions of deaths over the years. Indian agriculture is heavily dependent on the country's climate: a favorable monsoon is critical to securing water for irrigating India's crops. In parts of India, failure of the monsoons causes water shortages, resulting in poor yields. This is particularly true of major drought-prone regions southeastern Maharashtra, northern Karnataka, Andhra Pradesh, Odisha, Gujarat, Telangana, and Rajasthan.

<span class="mw-page-title-main">Western Disturbance</span> Extratropical storm over north India

A western disturbance is an extratropical storm originating in the Mediterranean region that brings sudden winter rain to the northwestern parts of the Indian subcontinent, which extends as east as up to northern parts of Bangladesh and South eastern Nepal. It is a non-monsoonal precipitation pattern driven by the westerlies. The moisture in these storms usually originates over the Mediterranean Sea, the Caspian Sea and the Black Sea. Extratropical storms are a global phenomena with moisture usually carried in the upper atmosphere, unlike their tropical counterparts where the moisture is carried in the lower atmosphere. In the case of the Indian subcontinent, moisture is sometimes shed as rain when the storm system encounters the Himalayas. Western disturbances are more frequent and stronger in the winter season.

<span class="mw-page-title-main">North American monsoon</span> Pattern of thunderstorms and rainfall in the southwestern United States and northwestern Mexico

The North American monsoon, variously known as the Southwest monsoon, the Mexican monsoon, the New Mexican monsoon, or the Arizona monsoon is a pattern of pronounced increase in thunderstorms and rainfall over large areas of the southwestern United States and northwestern Mexico, centered roughly on the Rio Grande Valley, and typically occurring between June and mid-September. During the monsoon, thunderstorms are fueled by daytime heating and build up during the late afternoon and early evening. Typically, these storms dissipate by late night, and the next day starts out fair, with the cycle repeating daily. The monsoon typically loses its energy by mid-September when much drier conditions are reestablished over the region. Geographically, the North American monsoon precipitation region is centered over the Sierra Madre Occidental in the Mexican states of Sinaloa, Durango, Sonora and Chihuahua.

<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">Climate of Asia</span> Overview of the climate of Asia

The climate of Asia is dry across its southwestern region, with dry across much of the interior. Some of the largest daily temperature ranges on Earth occur in the western part of Asia. The monsoon circulation dominates across the southern and eastern regions, due to the Himalayas forcing the formation of a thermal low which draws in moisture during the summer. The southwestern region of the continent experiences low relief as a result of the subtropical high pressure belt; they are hot in summer, warm to cool in winter, and may snow at higher altitudes. Siberia is one of the coldest places in the Northern Hemisphere, and can act as a source of arctic air mass for North America. The most active place on Earth for tropical cyclone activity lies northeast of the Philippines and south of Japan, and the phase of the El Nino-Southern Oscillation modulates where in Asia landfall is more likely to occur. Many parts of Asia are being impacted by climate change.

<span class="mw-page-title-main">2014 North Indian Ocean cyclone season</span> North Indian Ocean tropical cyclone season

The 2014 North Indian Ocean cyclone season was an event in the annual cycle of tropical cyclone formation. The season included two very severe cyclonic storms, both in October, and one other named cyclonic storm, classified according to the tropical cyclone intensity scale of the India Meteorological Department. Cyclone Hudhud is estimated to have caused US$3.58 billion in damage across eastern India, and more than 120 deaths.

<span class="mw-page-title-main">2015 North Indian Ocean cyclone season</span> North Indian Ocean tropical cyclone season

The 2015 North Indian Ocean cyclone season was an event in the annual cycle of tropical cyclone formation. The North Indian Ocean cyclone season has no official bounds, but cyclones tend to form between months of April and December, with the peak from May to November. These dates conventionally delimit the period of each year when most tropical cyclones form in the northern Indian Ocean.

<span class="mw-page-title-main">1969 North Indian Ocean cyclone season</span> North Indian Ocean tropical cyclone season

The 1969 North Indian Ocean cyclone season was an active cyclone season. The season has no official bounds but cyclones in the northern Indian Ocean tend to form between April and December. There are two main seas in the North Indian Ocean—the Bay of Bengal to the east of the Indian subcontinent and the Arabian Sea to the west of India. The official Regional Specialized Meteorological Centre in this basin is the India Meteorological Department (IMD), while the Joint Typhoon Warning Center releases unofficial advisories. An average of four to six storms form in the North Indian Ocean every season with peaks in May and November. Cyclones occurring between the meridians 45°E and 100°E are included in the season by the IMD.

<span class="mw-page-title-main">Asiatic Low</span> A low-pressure trough which lies over southern Asia, during early summer

The Asiatic Low is a low-pressure trough which lies over southern Asia, during early summer. It is located roughly over India, heading over the Bay of Bengal. It is a major action centre for the Northern Hemisphere during that time of the year.

References

  1. ↑ India Meteorological Department, Monsoon data 1901-2010 Archived 24 December 2010 at the Wayback Machine , Ministry of Earth Sciences, Government of India
  2. ↑ Pal et al., Districtwise Drought Climatology Of The Southwest Monsoon Season over India Based on Standardized Precipitation Index Archived 24 September 2015 at the Wayback Machine National Climate Centre, Research Report No: 2/2010, India Meteorological Department Pune, Govt of India
  3. ↑ Alexander Frater (1 May 2005). Chasing the Monsoon . Picador. ISBN   978-0-330-43313-6 . Retrieved 2 March 2011.
  4. 1 2 News Service, Indo-Asian (31 May 2010). "India cheers as monsoon arrives; hopes of better farm output raised". Hindustan Times. Retrieved 5 April 2021.
  5. 1 2 "India cheers as monsoon arrives in Kerala". Indo-Asian News Service. 1 June 2010. Archived from the original on 5 June 2010. Retrieved 2 March 2011.
  6. 1 2 Glossary of Meteorology (June 2000). "Monsoon". American Meteorological Society. Archived from the original on 22 March 2008. Retrieved 14 March 2008.
  7. ↑ "IPCC Fourth Assessment Report: Climate Change 2007, Glossary". Intergovernmental Panel on Climate Change. Retrieved 2 March 2011.
  8. ↑ "IMD terminologies & glossary". Indian Meteorological Department. Retrieved 2 March 2011.
  9. ↑ Colin S. Ramage (1971). Monsoon meteorology. Academic Press. ISBN   978-0-12-576650-0 . Retrieved 3 March 2011.
  10. ↑ Helaine Selin, ed. (1997). Encyclopaedia of the history of science, technology, and medicine in non-western cultures. Springer. pp. 766–. ISBN   978-0-7923-4066-9 . Retrieved 3 March 2011.
  11. 1 2 "Indian Meteorological Department". imd.gov.in. Archived from the original on 24 December 2010. Retrieved 3 March 2011.
  12. 1 2 3 Zhang, Zuqiang; Chan, Johnny C. L.; Ding, Yihui (1 October 2004). "Characteristics, evolution and mechanisms of the summer monsoon onset over Southeast Asia". International Journal of Climatology. 24 (12): 1461–1482. Bibcode:2004IJCli..24.1461Z. doi:10.1002/joc.1082. ISSN   1097-0088. S2CID   56428695.
  13. ↑ "Class 9 CBSE Geography Climate The Mechanism of Monsoons - Wiki". www.nextgurukul.in. Archived from the original on 21 August 2021. Retrieved 22 August 2021.
  14. 1 2 Michael Allaby (2002). Encyclopedia of weather and climate. Infobase Publishing. pp. 373–. ISBN   978-0-8160-4801-4 . Retrieved 3 March 2011.
  15. ↑ M. Hanif (1 January 2005). Encyclopaedia of Agricultural Geography. Anmol Publications PVT. LTD. pp. 163–. ISBN   978-81-261-2482-4 . Retrieved 3 March 2011.
  16. 1 2 Bin Wang (2006). The Asian monsoon. Springer. pp. 188–. ISBN   978-3-540-40610-5 . Retrieved 3 March 2011.
  17. ↑ Kasabe, Nanda Dabhole (23 July 1997). "Vagaries apart, monsoon is normal". The Indian Express. Retrieved 5 March 2011.
  18. ↑ Pratiyogita Darpan (October 2007). Pratiyogita Darpan. Pratiyogita Darpan. pp. 93–. Retrieved 5 March 2011.
  19. ↑ Krishnamacharyulu (1 September 2003). Cases in Rural Marketing: An Integrated Approach. Pearson Education India. pp. 106–. ISBN   978-81-317-0188-1 . Retrieved 5 March 2011.
  20. ↑ "Agronomic Measures in Dryland Agriculture: An Overview". India Water Portal. Retrieved 5 March 2011.
  21. ↑ "Weather Insurance To Protect A Few". The Financial Express. 16 July 2004. Retrieved 5 March 2011.
  22. ↑ "Budget must focus on Growth, Food Inflation, FDI, Govt B/s: G Chokkalingam". myIris.com. 22 February 2011. Retrieved 5 March 2011.
  23. ↑ Vinoj, V.; Rasch, Philip J.; Wang, Hailong; Yoon, Jin-Ho; Ma, Po-Lun; Landu, Kiranmayi; Singh, Balwinder (2014). "Short-term modulation of Indian summer monsoon rainfall by West Asian dust". Nature Geoscience. 7 (4): 308–313. Bibcode:2014NatGe...7..308V. doi:10.1038/ngeo2107.
  24. ↑ "SW Monsoon Normal Onset Dates". Indian Meteorological Department. Retrieved 3 March 2011.
  25. ↑ "Monsoon Onset dates on Map of India". Indian Meteorological Department. Retrieved 3 March 2011.
  26. ↑ "SW Monsoon Normal Withdrawal Dates". Indian Meteorological Department. Retrieved 3 March 2011.
  27. ↑ "Monsoon withdrawal dates on Map of India". Indian Meteorological Department. Retrieved 3 March 2011.
  28. ↑ "Met. Terminologies and Glossary - Monsoon". Indian Meteorological Department . Retrieved 5 March 2011.
  29. 1 2 3 4 5 6 7 8 9 10 11 12 Gopal Raj, N (4 May 2004). "El Nino & the Indian Ocean Dipole". The Hindu.
  30. ↑ "Climate glossary - Southern Oscillation Index (SOI)". Bureau of Meteorology (Australia). 3 April 2007. Retrieved 4 March 2011.
  31. ↑ Kumar, K. K.; Balaji Rajagopalan; Mark A. Cane (25 June 1999). "On the Weakening Relationship Between the Indian Monsoon and ENSO". Science. 284 (5423): 2156–2159. doi:10.1126/science.284.5423.2156. PMID   10381876. S2CID   18499571.
  32. 1 2 Gadgil, Sulochana; P. N. Vinayachandran; P. A. Francis (1 January 2004). "Extremes of the Indian summer monsoon rainfall, ENSO and equatorial Indian Ocean oscillation". Geophysical Research Letters. 31 (12): L12213. Bibcode:2004GeoRL..3112213G. doi: 10.1029/2004GL019733 . Retrieved 5 March 2011.
  33. ↑ Singh, Deepti; Ghosh, Subimal; Roxy, Mathew K.; McDermid, Sonali (March 2019). "Indian summer monsoon: Extreme events, historical changes, and role of anthropogenic forcings". Wiley Interdisciplinary Reviews: Climate Change. 10 (2): e571. doi:10.1002/wcc.571. S2CID   133873220.
  34. ↑ Roxy, Mathew Koll; Ritika, Kapoor; Terray, Pascal; Murtugudde, Raghu; Ashok, Karumuri; Goswami, B. N. (16 June 2015). "Drying of Indian subcontinent by rapid Indian Ocean warming and a weakening land-sea thermal gradient" (PDF). Nature Communications. 6: 7423. Bibcode:2015NatCo...6.7423R. doi: 10.1038/ncomms8423 . PMID   26077934.
  35. ↑ Roxy, Mathew Koll; Ritika, Kapoor; Terray, Pascal; Masson, Sébastien (11 September 2014). "The Curious Case of Indian Ocean Warming" (PDF). Journal of Climate. 27 (22): 8501–8509. Bibcode:2014JCli...27.8501R. doi:10.1175/JCLI-D-14-00471.1. ISSN   0894-8755. S2CID   42480067.
  36. ↑ "Warming Indian Ocean, weakening monsoon". India Climate Dialogue. 16 June 2015. Retrieved 18 June 2016.
  37. ↑ Paul, Supantha; Ghosh, Subimal; Oglesby, Robert; Pathak, Amey; Chandrasekharan, Anita; Ramsankaran, RAAJ (24 August 2016). "Weakening of Indian Summer Monsoon Rainfall due to Changes in Land Use Land Cover". Scientific Reports. 6 (1): 32177. Bibcode:2016NatSR...632177P. doi:10.1038/srep32177. ISSN   2045-2322. PMC   4995379 . PMID   27553384.
  38. ↑ Roxy, Mathew Koll (2017). "Land warming revives monsoon". Nature Climate Change. 7 (8): 549–550. Bibcode:2017NatCC...7..549R. doi:10.1038/nclimate3356.
  39. ↑ Roxy, M. K.; Ghosh, Subimal; Pathak, Amey; Athulya, R.; Mujumdar, Milind; Murtugudde, Raghu; Terray, Pascal; Rajeevan, M. (3 October 2017). "A threefold rise in widespread extreme rain events over central India". Nature Communications. 8 (1): 708. Bibcode:2017NatCo...8..708R. doi:10.1038/s41467-017-00744-9. ISSN   2041-1723. PMC   5626780 . PMID   28974680.
  40. ↑ Simpkins, Graham (2 November 2017). "Hydroclimate: Extreme rain in India". Nature Climate Change. 7 (11): 760. Bibcode:2017NatCC...7..760S. doi: 10.1038/nclimate3429 . ISSN   1758-6798.
  41. 1 2 "Introduction". Indian Institute of Tropical Meteorology, Pune, India. Retrieved 5 March 2011.
  42. 1 2 3 Gadgil, Sulochana; J. Srinivasan (10 February 2011). "Seasonal prediction of the Indian monsoon" (PDF). Current Science. 3. 100: 343–353. Retrieved 5 March 2011.
  43. ↑ "Seasonal Prediction of Indian Monsoon(SPIM)". Centre for Development of Advanced Computing. Archived from the original on 2 March 2011. Retrieved 5 March 2011.
  44. 1 2 Rajeevapn, M.; D. S. Pai; S. K. Dikshit; R. R. Kelkar (10 February 2004). "IMD's new operational models for long-range forecast of southwest monsoon rainfall over India and their verification for 2003" (PDF). Current Science. 3. 86: 422–431. Retrieved 7 March 2011.
  45. ↑ BAGLA, PALLAVA (28 April 2003). "Man behind old monsoon model goes out quietly". Express India. Retrieved 7 March 2011.
  46. ↑ AFP (30 September 2009). "Drought in India worst since 1972". The Times of India. Retrieved 7 March 2011.
  47. ↑ "India needs indigenous monsoon model for better prediction, says IMD chief". ExpressIndia. 10 June 2010. Retrieved 7 March 2011.
  48. ↑ Philip, A J (24 August 2003). "Cherrapunji no longer wettest Challenge comes from nearby village". The Tribune. Retrieved 9 March 2011.
  49. 1 2 3 4 5 "How monsoon impacts the Indian economy". Rediff.com. Retrieved 6 March 2011.
  50. 1 2 "Monsoon affects economy, health in India". WORLD MONSOONS. Retrieved 15 February 2016.
  51. ↑ "Monsoon Illnesses in India – All You Need to Know".
  52. 1 2 "All Indians 'chasing the monsoon'". Thaindia.com. Indo-Asian News Service. 27 July 2010. Retrieved 2 March 2011.
  53. ↑ MITRA, SUBHANKAR (29 August 2009). "The monsoon effect". IndianExpress. Retrieved 7 March 2011.
  54. ↑ "Traveling in India during monsoons". 31 May 2013.
  55. 1 2 "Rivers-Profile-Know India". India.gov.in. Archived from the original on 4 February 2011. Retrieved 6 March 2011.
  56. ↑ "Indian Geography". IndiaBook.com. Retrieved 6 March 2011.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.