Indian Monsoon

• Monsoons are seasonal winds that reverse their direction according to the change in season.
• The word “monsoon” originated from the Arabic word for season, “mawsim”.
• Monsoons are periodic winds.
• Monsoons travel from the sea to the land in summers and from land to the sea during winters, hence, are a double system of seasonal winds.
• Some scholars treat monsoon winds as land and sea breeze on a large scale.
• Historically, monsoons were important for traders and seafarers to move from place to place.
• Monsoons are most pronounced in the Indian subcontinent, but they also occur in central-western Africa, Southeast Asia, and a few other places.
• India gets southwest monsoon winds in the summers and northeast monsoons during the winters.
• Southwest monsoons bring intense rainfall to most regions in India.
• Northeast monsoons bring rainfall to mainly the south-eastern coast of India (Southern coast of Seemandhra and the coast of Tamil Nadu).
• Countries like India, Indonesia, Bangladesh, Myanmar etc. receive most of the annual rainfall during the southwest monsoon season.
• Southeast China, Japan, etc., receive most of the annual rainfall during the northeast rainfall season.

Mechanism of Indian Monsoon

• Monsoons occur in the tropical area roughly between 20° N and 20° S.
• Monsoons are a complex meteorological phenomenon with an unclear origin.
• There are several theories that try to explain the mechanism of monsoons.
• To understand the mechanism of monsoons, certain facts are important.

Thermal Concept

• Halley proposed that the primary cause of the Indian monsoon circulation was the differential heating effects of the land and the sea.
• In winter, the huge landmass of Asia cools more rapidly than the surrounding oceans, resulting in a strong high-pressure center over the continent. As a consequence, there is an outflow of air from the land towards the sea, bringing cold, dry air towards the low latitudes.
• In summer, the landmass of Asia heats quickly and develops a strong low-pressure center. The pressure over the adjacent oceans being high, a sea-to-land pressure gradient is established. The surface airflow is, therefore, from the highs over the oceans towards the lows over the heated land. The air that is attracted into the centers of low pressure from over the oceans is warm and moist.
• Halley’s concept is criticized for failing to explain the intricacies of monsoons, such as the sudden burst of monsoon, breaks in monsoon, and the spatial and temporal distribution of monsoon. The low-pressure areas are not stationary, and the rainfall is not only convectional but a mix of orographic, cyclonic, and convectional rainfall.

Recent Concept about the Origin of Indian Monsoon

• The recent concept of monsoon relies heavily on the role of:
  • Himalayas and Tibetan plateau as a physical barrier and a source of high-level heat.
  • Circulation of upper air jet streams in the troposphere.
  • Existence of upper air circum-polar whirl over north and south poles in the troposphere.
  • The occurrence of ENSO (El-Nino and Southern Oscillation) in the South Pacific ocean.
  • Walker cell in Indian Ocean.
  • Indian Ocean Dipole.

Role of the Himalayas and Tibetan Plateau

• Tibet plateau plays a crucial role in initiating monsoon circulation.
• Tibet plateau extends over an area of about 4.5 million sq. km and has an average height of 4000 m.
• Due to its height, Tibet plateau receives 2-3°C more insolation than neighboring areas.
• Heating of Tibet plateau leads to clockwise air circulation in the middle troposphere.
• Two wind streams originate from Tibet plateau: one blows southward and develops into the tropical easterly jet stream (TEJ), and the other blows in the opposite direction towards the North Pole and becomes the westerly jet stream over Central Asia.
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Role of Jet Stream

• Sub-tropical westerly jet stream is bifurcated by the Tibet plateau in winters.
• The northward branch of the jet stream extends up to 20N-35N.
• The tropical easterly jet stream (TEJ), which branches off from the anticyclone developed over Tibet, sometimes reaches the tip of Peninsular India.
• Jet speed winds are also reported over other parts of the Peninsular region.
• This jet stream descends over the Indian Ocean and intensifies the high-pressure cell known as Mascarene High.
• Onshore winds start blowing from the Mascarene High towards the thermally induced low-pressure area developed in the northern part of the Indian subcontinent.
• After crossing the equator, these winds become southwesterly and are known as the southwesterly summer monsoon.
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Role of ENSO

• El Nino, a reversal of normal conditions in the Pacific Ocean’s sea surface temperature, can influence the Indian monsoon.
• Although there is no direct correlation between a bad monsoon and El Nino, they are generally associated.
• Southern Oscillation is a see-saw pattern of atmospheric pressure between the eastern and western Pacific oceans, with a period varying from 2-7 years.
• Southern Oscillation Index (SOI) is used to measure the pressure difference between Tahiti and Darwin in the Pacific Ocean.
• A negative value of SOI implies high pressure over the north Indian Ocean during the winter season and may result in a poor monsoon in India.
• The Somalian current changes its direction of flow after every six months.
• During the North-East Monsoon, the Somali Current flows southwest, while during the South-West Monsoon it is a major western boundary current, comparable with the Gulf Stream.
• Normally, a low-pressure area remains along the eastern coast of Somalia.
• In exceptional years, after every six or seven years, the low-pressure area in the western Arabian Sea becomes a high-pressure area, resulting in a weaker monsoon in India.

Walker Cell

• East-west atmospheric circulation observed in tropical oceanic regions, known as the Walker cell
• Many scientists use the term “Walker cell” for all east-west circulations in different oceans
• Walker cell associated with Southern Oscillation and its strength fluctuates with the Southern Oscillation Index (SOI)
• High positive SOI creates a zone of low atmospheric pressure over Australia and Indonesian archipelago, with rising air deflecting towards Africa and South America in upper atmosphere
• In the Indian Ocean, air descends at high-pressure zone and surface winds blow as Southwest monsoon towards Indian subcontinent in summers
• During La-Nina, Indian Ocean branch of Walker cell gets strengthened and surface winds are more intense, associated with good monsoon
• During El-Nino or negative SOI, ascending branch of Walker cell shifts to central Pacific Ocean, causing Indian Ocean cell to shift east and surface winds or Southwest monsoon winds to weaken.
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Indian Ocean Dipole

• The Indian Ocean Dipole (IOD) is a phenomenon in the Indian Ocean that involves periodic oscillation of sea-surface temperatures (SST) between positive, neutral, and negative phases.
• It is defined by the difference in sea surface temperature between two areas or poles, a western pole in the Arabian Sea and an eastern pole in the eastern Indian Ocean south of Indonesia.
• A positive phase of IOD brings greater-than-average sea-surface temperatures and greater precipitation in the western Indian Ocean region, with corresponding cooling of waters in the eastern Indian Ocean leading to droughts in adjacent land areas of Indonesia and Australia.
• The negative phase of the IOD brings opposite conditions, with warmer water and greater precipitation in the eastern Indian Ocean, and cooler and drier conditions in the west.
• The IOD interacts with similar phenomena like the El Niño-Southern Oscillation (ENSO) in the Pacific Ocean, and both positive and negative IOD have been seen coupled with La Nina.
• The IOD also affects the strength of monsoons over the Indian subcontinent, with a positive IOD associated with warm sea-surface temperatures of the western Indian Ocean being favorable for monsoon.
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Nature of Indian Monsoon

• Systematic studies of the causes of rainfall in the South Asian region help to understand the salient features of the monsoon, particularly some of its important aspects, such as:
  • Onset and advance of monsoon
  • Rain-bearing systems and the relationship between their frequency and distribution of
  • monsoon rainfall.
  • Break in the monsoon
  • Retreat of the monsoon

Onset and Advance of Monsoon

• Monsoon is primarily caused by the differential heating of land and sea
• Low pressure at ITCZ in north India pulls trade winds of southern hemisphere northwards
• Southeast trade winds cross the equator and enter Bay of Bengal and Arabian Sea, carrying moisture
• Northwards shift of ITCZ leads to development of easterly jet stream over 15N
• Southwest monsoon rains begin abruptly, bringing down temperatures and termed as “break” or “burst”
• Southwest monsoon reaches Andaman-Nicobar Islands on 15th May, Kerala coast on 1st June, Mumbai and Kolkata between 10th and 13th June, and covers whole of India by 15th July.
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Rain Bearing Systems and Distribution of Rainfall

• The southwest monsoon splits into two branches, the Arabian Sea Branch and the Bay of Bengal Branch.
• The Bay of Bengal branch causes rainfall over the plains of north India, while the Arabian Sea branch brings rain to the west coast of India and extends toward a low-pressure area over the Thar Desert.
• The Arabian Sea monsoon winds further split into three branches:
  • One branch is obstructed by the Western Ghats, causing heavy rainfall on the windward side and a rain-shadow area on the leeward side.
  • Another branch strikes the coast north of Mumbai and causes rainfall in central India and the Chotanagpur plateau before mingling with the Bay of Bengal branch in the Ganga plains.
  • A third branch strikes the Saurashtra Peninsula and the Kutch, causing scanty rainfall in Punjab and Haryana before joining the Bay of Bengal branch to cause rains in the western Himalayas.
• The intensity of rainfall over the west coast of India is related to offshore meteorological conditions and the position of the equatorial jet stream along the eastern coast of Africa.
• The Bay of Bengal branch strikes the coast of Myanmar and part of southeast Bangladesh, but the Arakan Hills deflect a portion towards the Indian subcontinent.
• The Tamil Nadu coast remains dry during this season because it is situated in the rainshadow area of the Arabian Sea branch and lies parallel to the Bay of Bengal branch.
• The frequency and path of tropical depressions originating over the Bay of Bengal vary, affecting the amount of rainfall in north India.
• The amount of rainfall in Rajasthan desert is low due to the Arabian Sea branch blowing parallel to the Aravalis mountain chain without obstruction.

Break in the Monsoon

• Break in the monsoon occurs when there is a gap of one or more weeks without rain during the southwest monsoon period.
• Different regions experience breaks in the monsoon due to different reasons.
• In northern India, lack of frequent rain-bearing storms along the monsoon trough or the ITCZ can cause breaks in the monsoon.
• Over the west coast, dry spells are associated with days when winds blow parallel to the coast.

Retreating/Post Monsoons (The Transition Season)

• During October-November, the apparent movement of the sun towards the south weakens the monsoon trough or low-pressure trough over the northern plains, gradually replaced by a high-pressure system.
• The southwest monsoon winds weaken and start withdrawing gradually, leading to the retreat of the monsoon from the Northern Plains by the beginning of October.
• The months of October-November are a period of transition from hot rainy season to dry winter conditions, marked by clear skies and a rise in temperature.
• Day temperatures remain high, while nights become cool and pleasant, and the land is still moist from the monsoon rains.
• The weather during this time can be oppressive due to high temperature and humidity, commonly known as ‘October heat’.
• By early November, the low-pressure conditions over northwestern India get transferred to the Bay of Bengal, associated with the occurrence of cyclonic depressions originating over the Andaman Sea.
• These cyclones generally cross the eastern coasts of India and cause heavy and widespread rain, often leading to destruction of life and property.
• The deltas of the Godavari, Krishna, and Kaveri rivers are frequently struck by cyclones, and sometimes the coasts of Orissa, West Bengal, and Bangladesh also experience cyclonic impacts.
• The Coromandel Coast receives a bulk of its rainfall from depressions and cyclones during this period.

Features of Monsoon Rainfall

• Monsoon rain is seasonal and occurs between June and September in India.
• Spatial distribution of rainfall is largely influenced by relief or topography.
• The windward side of the Western Ghats receives heavy rainfall of over 250 cm, while the northeastern states receive heavy rainfall due to their hill ranges and the Eastern Himalayas.
• Rainfall ranges from 20 cm in western Rajasthan to over 400 cm in certain parts of the Western Ghats and North-East India.
• The monsoon rainfall has a declining trend with increasing distance from the sea. Rainfall decreases from east to west in plains as one branch of monsoon enters from the eastern side. For example, Kolkata receives 119 cm of rainfall, Allahabad 76 cm, and Delhi 56 cm only.
• Breaks in rainfall, as discussed earlier, are related to cyclonic depressions mainly formed at the head of the Bay of Bengal and their crossing into the mainland.
• The frequency, intensity, and passage followed by these depressions determine the spatial distribution of rainfall.
• Sometimes, monsoon rains end earlier than usual, causing damage to standing crops and making the sowing of winter crops difficult.

Monsoons and the Economic Life in India

• The monsoon is a crucial factor that influences the entire agricultural cycle of India, as about 64 percent of the population depends on agriculture for their livelihood, and agriculture is predominantly based on the southwest monsoon.
• Except for the Himalayas, most parts of India have temperatures above the threshold level for growing crops throughout the year.
• Regional variations in the monsoon climate, such as rainfall patterns and timing, help in growing various types of crops in different parts of the country.
• Agricultural prosperity in India relies heavily on timely and adequately distributed rainfall. If the monsoon fails or is inadequate, it adversely affects agriculture, particularly in areas where irrigation infrastructure is not well-developed.
• Sudden bursts of heavy rainfall during the monsoon season can also cause problems such as soil erosion over large areas in India, which can negatively impact agricultural productivity.

Monsoon as unifying factors:

• The Himalayas protect India from cold winds from central Asia
• Northern India has higher temperatures due to this protection
• The peninsular plateau is influenced by the sea and has moderate temperatures
• Despite moderating influences, there are variations in temperature
• The monsoon has a unifying influence on India
• Seasonal alteration of wind systems provide a cycle of seasons
• Uncertainties in rain and uneven distribution are typical of the monsoons
• The Indian landscape, animal and plant life, agricultural calendar, and people’s festivities revolve around the monsoon
• The monsoon provides water for agriculture and binds the country together
• River valleys carrying water unite as a single river valley unit