Monsoon of South Asia

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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.

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

Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea. Usually, the term monsoon is used to refer to the rainy phase of a seasonally changing pattern, although technically there is also a dry phase. The term is sometimes incorrectly used for locally heavy but short-term rains, although these rains meet the dictionary definition of monsoon.

Indian subcontinent Peninsular region in south-central Asia south of the Himalayas

The Indian subcontinent, is a southern region and peninsula of Asia, mostly situated on the Indian Plate and projecting southwards into the Indian Ocean from the Himalayas. Geologically, the Indian subcontinent is related to the land mass that rifted from Gondwana and merged with the Eurasian plate nearly 55 million years ago. Geographically, it is the peninsular region in south-central Asia delineated by the Himalayas in the north, the Hindu Kush in the west, and the Arakanese in the east. Politically, the Indian subcontinent includes Bangladesh, Bhutan, India, Maldives, Nepal, Pakistan and Sri Lanka.

Weather Short-term state of the atmosphere

Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the lowest level of the atmosphere, the troposphere, just below the stratosphere. Weather refers to day-to-day temperature and precipitation activity, whereas climate is the term for the averaging of atmospheric conditions over longer periods of time. When used without qualification, "weather" is generally understood to mean the weather of Earth.

Contents

The unique geographical features of the Indian subcontinent, along with associated atmospheric, oceanic, and geophysical 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]

Geography of India geography of the country of India

India lies on the Indian Plate, the northern portion of the Indo-Australian Plate, whose continental crust forms the Indian subcontinent. The country is situated north of the equator between 8°04' to 37°06' north latitude and 68°07' to 97°25' east longitude. It is the seventh-largest country in the world, with a total area of 3,287,263 square kilometres (1,269,219 sq mi). India measures 3,214 km (1,997 mi) from north to south and 2,933 km (1,822 mi) from east to west. It has a land frontier of 15,200 km (9,445 mi) and a coastline of 7,516.6 km (4,671 mi).

Ocean A body of water that composes much of a planets hydrosphere

An ocean is a body of water that composes much of a planet's hydrosphere. On Earth, an ocean is one of the major conventional divisions of the World Ocean. These are, in descending order by area, the Pacific, Atlantic, Indian, Southern (Antarctic), and Arctic Oceans. The word "ocean" is often used interchangeably with "sea" in American English. Strictly speaking, a sea is a body of water partly or fully enclosed by land, though "the sea" refers also to the oceans.

Agriculture Cultivation of plants and animals to provide useful products

Agriculture is the science and art of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people to live in cities. The history of agriculture began thousands of years ago. After gathering wild grains beginning at least 105,000 years ago, nascent farmers began to plant them around 11,500 years ago. Pigs, sheep and cattle were domesticated over 10,000 years ago. Plants were independently cultivated in at least 11 regions of the world. Industrial agriculture based on large-scale monoculture in the twentieth century came to dominate agricultural output, though about 2 billion people still depended on subsistence agriculture into the twenty-first.

Definition

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

American Meteorological Society

The American Meteorological Society (AMS) is the premier scientific and professional organization in the United States promoting and disseminating information about the atmospheric, oceanic, and hydrologic sciences. Its mission is to advance the atmospheric and related sciences, technologies, applications, and services for the benefit of society.

Arabian Sea A marginal sea of the northern Indian Ocean between the Arabian Peninsula and India

The Arabian Sea is a region of the northern Indian Ocean bounded on the north by Pakistan and Iran, on the west by the Gulf of Aden, Guardafui Channel and the Arabian Peninsula, on the southeast by the Laccadive Sea, on the southwest by the Somali Sea, and on the east by India. Its total area is 3,862,000 km2 (1,491,000 sq mi) and its maximum depth is 4,652 metres (15,262 ft). The Gulf of Aden in the west connects the Arabian Sea to the Red Sea through the strait of Bab-el-Mandeb, and the Gulf of Oman is in the northwest, connecting it to the Persian Gulf.

Intergovernmental Panel on Climate Change scientific intergovernmental body

The Intergovernmental Panel on Climate Change (IPCC) is an intergovernmental body of the United Nations, dedicated to providing the world with an objective, scientific view of climate change, its natural, political and economic impacts and risks, and possible response options.

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:

Africa The second largest and second most-populous continent, mostly in the Northern and Eastern Hemispheres

Africa is the world's second largest and second most-populous continent, being behind Asia in both categories. At about 30.3 million km2 including adjacent islands, it covers 6% of Earth's total surface area and 20% of its land area. With 1.2 billion people as of 2016, it accounts for about 16% of the world's human population. The continent is surrounded by the Mediterranean Sea to the north, the Isthmus of Suez and the Red Sea to the northeast, the Indian Ocean to the southeast and the Atlantic Ocean to the west. The continent includes Madagascar and various archipelagos. It contains 54 fully recognised sovereign states (countries), nine territories and two de facto independent states with limited or no recognition. The majority of the continent and its countries are in the Northern Hemisphere, with a substantial portion and number of countries in the Southern Hemisphere.

Southeast Asia Subregion of Asia

Southeast Asia or Southeastern Asia is a subregion of Asia, consisting of the countries that are geographically south of China and Japan, east of India, west of Papua New Guinea, and north of Australia. Southeast Asia is bordered to the north by East Asia, to the west by South Asia and the Bay of Bengal, to the east by Oceania and the Pacific Ocean, and to the south by Australia and the Indian Ocean. The region is the only part of Asia that lies partly within the Southern Hemisphere, although the majority of it is in the Northern Hemisphere. In contemporary definition, Southeast Asia consists of two geographic regions:

  1. Mainland Southeast Asia, also known historically as Indochina, comprising parts of Northeast India, Vietnam, Laos, Cambodia, Thailand, Myanmar and West Malaysia.
  2. Maritime Southeast Asia, also known historically as Nusantara, the East Indies and Malay Archipelago, comprises the Andaman and Nicobar Islands of India, Indonesia, East Malaysia, Singapore, the Philippines, East Timor, Brunei, Christmas Island, and the Cocos (Keeling) Islands. Taiwan is also included in this grouping by many anthropologists.
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. [12] One area that monsoons impact greatly is India. [12] In India monsoons create an entire season in which the winds reverse completely. [12]

Various atmospheric conditions influence the monsoon winds. The first condition is the differential heating and cooling of land and water. This creates low pressure on the landmass, while high pressure is created over the seas during daytime, but is reversed during the night time.

The second condition is the shift in the position of Inter-Tropical Convergence Zone (ITCZ). In summer, the equatorial trough normally positioned about 5°N of the equator moves over the Ganga plain creating a monsoon trough during the monsoon season.

The third condition is the presence of the high-pressure area that develops east of Madagascar. It is approximately at 20°S over the Indian Ocean. The intensity and position of this high-pressure area affects the Indian Monsoon.

The fourth condition develops during the summer. The Tibetan Plateau gets intensely heated resulting in strong vertical air currents and high pressure over the plateau about 9 km above sea level. The fifth condition develops during the summer due to the movement of the westerly jet streams to the north of the Himalayas and the presence of the tropical easterly jet stream over the Indian Peninsula.

Changes in pressure over the southern oceans also affect the monsoons. In certain years, there is a reversal in the pressure conditions. This periodic change in pressure conditions is known as the Southern Oscillation, or SO.

The Southern Oscillation is connected to la nina, which is a warm ocean current that flows past the Peruvian Coast. It flows every two to five years in place of the cold Peruvian current. The phenomenon is, referred to as ENSO (El Nino Southern Oscillations). In India, the monsoon lasts for 100 to 120 days from early June and to mid-September. The monsoon winds encounter various atmospheric conditions on their way and hence are pulsating in nature, and not steady.

The monsoon arrives with a sudden downpour of rainfall that continues for several days. This is known as the ‘burst’ of the monsoon.

The rainfall is a result of the convergence of wind flow from the Bay of Bengal and reverse winds from the South China Sea. [13]

The onset of the monsoon occurs over the Bay of Bengal in May, [13] arriving at the Indian Peninsula by June, [12] and then the winds move towards the South China Sea. [13] By early September, the monsoon starts to withdraw or retreat and is a more gradual process. By mid-October, it withdraws completely from the northern half of the peninsula. The withdrawal takes place progressively from north to south from the first week of December to the first week of January. This is the start of the winter season.

The retreating monsoon winds move over the Arabian Sea and the Bay of Bengal, and collect moisture on the way. These monsoon winds reach the southern states of India by October, and are responsible for a second round of rainfall. These are called the winter monsoons. The winter monsoon is experienced in the states of Karnataka, Tamil Nadu, Kerala and Andhra Pradesh in the first week of January.

Effect of geographical relief features

Although the southwest and northeast monsoon winds are seasonally reversible, they do not 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 sext 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 Indian Meteorological Department with calculations for the mean and standard deviation of each of these variables. [28]
A monsoon with excess rain can cause floods, and one with too little rain can lead to widespread drought, famine, and economic losses. Therefore, a normal monsoon with mean performance is the most desirable monsoon.

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 and Antarctic circles, 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 Inter Tropical 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 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 Thomas 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 on the monsoon

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 remain 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 have 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 1876–78 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]

Indian 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 MatheranPanoramaPointDrySeason.JPG
Western Ghats,Maharashtra on 28 May in dry season
Western Ghats,Maharashtra on 28 August in rainy season MatheranPanoramaPointMonsoon.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]

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. [51] 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. [51] 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. [52]

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. [53]

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. [54] Most of the coastal rivers of Western India are also rain-fed and monsoon-dependent. [54] [55] 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

El Niño Warm phase of a cyclic climatic phenomenon in the Pacific Ocean

El Niño is the warm phase of the El Niño–Southern Oscillation (ENSO) and is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific, including the area off the Pacific coast of South America. The ENSO is the cycle of warm and cold sea surface temperature (SST) of the tropical central and eastern Pacific Ocean. El Niño is accompanied by high air pressure in the western Pacific and low air pressure in the eastern Pacific. El Niño phases are known to occur close to four years, however, records demonstrate that the cycles have lasted between two and seven years. During the development of El Niño, rainfall develops between September–November. The cool phase of ENSO is La Niña, with SSTs in the eastern Pacific below average, and air pressure high in the eastern Pacific and low in the western Pacific. The ENSO cycle, including both El Niño and La Niña, causes global changes in temperature and rainfall.

El Niño–Southern Oscillation Irregularly periodic variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean

El Niño–Southern Oscillation (ENSO) is an irregularly 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.

Orographic lift

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.

Intertropical Convergence Zone

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

Climate of India

The Climate of India comprises a wide range of weather conditions across a vast geographic scale and varied topography, making generalisations difficult. Based on the Köppen system, India hosts six major climatic subtypes, ranging from arid desert in the west, alpine tundra and glaciers in the north, and humid tropical regions supporting rainforests in the southwest and the island territories. Many regions have starkly different microclimates. The country's meteorological department follows the international standard of four climatological seasons with some local adjustments: winter, summer, a monsoon 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.

Walker circulation

The Walker circulation, also known as the Walker cell, is a conceptual model of the air flow in the tropics in the lower atmosphere (troposphere). According to this model, parcels of air follow a closed circulation in the zonal and vertical directions. This circulation, which is roughly consistent with observations, is caused by differences in heat distribution between ocean and land. It was discovered by Gilbert Walker. In addition to motions in the zonal and vertical direction the tropical atmosphere also has considerable motion in the meridional direction as part of, for example, the Hadley Circulation.

Madden–Julian oscillation

The Madden–Julian oscillation (MJO) is the largest element of the intraseasonal variability in the tropical atmosphere. It was discovered in 1971 by Roland Madden and Paul Julian of the American National Center for Atmospheric Research (NCAR). It is a large-scale coupling between atmospheric circulation and tropical deep atmospheric convection. Unlike a standing pattern like the El Niño–Southern Oscillation (ENSO), the Madden–Julian oscillation is a traveling pattern that propagates eastward, at approximately 4 to 8 m/s, through the atmosphere above the warm parts of the Indian and Pacific oceans. This overall circulation pattern manifests itself most clearly as anomalous rainfall.

Cherrapunji Town in Meghalaya, India

Cherrapunji is a subdivisional town in the East Khasi Hills district in the Indian state of Meghalaya. It is the traditional capital of aNongkhlaw hima, both known as Sohra or Churra.

2005 North Indian Ocean cyclone season cyclone season in the North Indian ocean

The 2005 North Indian Ocean cyclone season was destructive and deadly to southern India, despite the weak storms. 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.

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 Panama–Colombia 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.

Monsoon trough

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.

United States rainfall climatology

The characteristics of United States rainfall climatology differ significantly across the United States and those under United States sovereignty. Late summer and fall extratropical cyclones bring a majority of the precipitation which falls across western, southern, and southeast Alaska annually. During the winter, and spring, Pacific storm systems bring Hawaii and the western United States most of their precipitation. Nor'easters moving down the East coast bring cold season precipitation to the Carolinas, Mid-Atlantic and New England states. Lake-effect snows add to precipitation potential downwind of the Great Lakes, as well as Great Salt Lake and the Finger Lakes during the cold season. The snow to liquid ratio across the contiguous United States averages 13:1, meaning 13 inches (330 mm) of snow melts down to 1 inch (25 mm) of water.

Indian Ocean Dipole irregular oscillation of sea-surface temperatures in which the western Indian Ocean becomes alternately warmer and then colder than the eastern part of the ocean

The Indian Ocean Dipole (IOD), also known as the Indian Niño, is an irregular oscillation of sea-surface temperatures in which the western Indian Ocean becomes alternately warmer and then colder than the eastern part of the ocean.

Drought in India

Drought in India has resulted in tens of millions of deaths over the course of the 18th, 19th, and 20th centuries. Indian agriculture is heavily dependent on the climate of India: a favorable southwest summer monsoon is critical in securing water for irrigating Indian crops. In some parts of India, the failure of the monsoons result in water shortages, resulting in below-average crop yields. This is particularly true of major drought-prone regions such as southern and eastern Maharashtra, northern Karnataka, Andhra Pradesh, Odisha, Gujarat, Telangana and Rajasthan.

Western Disturbance continuous rainfall and flood

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. It is a non-monsoonal precipitation pattern driven by the westerlies. The moisture in these storms usually originates over the Mediterranean Sea and the Atlantic Ocean. 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.

Earth rainfall climatology

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

Climate of Asia

The Climate of Asia is wet across southeast sections, and dry across much of the interior. Some of the largest daily temperature ranges on Earth occur in western sections of Asia. The monsoon circulation dominates across southern and eastern sections, due to the presence of the Himalayas forcing the formation of a thermal low which draws in moisture during the summer. Southwestern sections of the continent experience low relief as a result of the subtropical high pressure belt; they are hot in the 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 masses 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 Niño-Southern Oscillation modulates where in Asia landfall is more likely to occur.

2017 North Indian Ocean cyclone season

The 2017 North Indian Ocean cyclone season was a below average season in the annual cycle of tropical cyclone formation. This season produced only three named storms, of which one only intensified into a very severe cyclonic storm. The North Indian Ocean cyclone season has no official bounds, but cyclones tend to form between April and December, with the two peaks in May and November. These dates conventionally delimit the period of each year when most tropical cyclones form in the northern Indian Ocean. The season began with the formation Cyclone Maarutha on April 15, and ended with the dissipation of a deep depression on December 9.

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