Mechanism of Indian Monsoon

• The word “monsoon” originates from the Arabic word “Mausam,” meaning season, referring to seasonal winds blowing predominantly in one direction during a specific season.
• Monsoons denote seasonal winds characterized by consistency and regularity during a portion of the year, absent or blowing from another direction for the remainder of the year.
• They play a vital role in national economies, particularly in agriculture, as the timely onset of monsoon is crucial for agricultural production and influences various sectors of the economy.
• Delay in monsoon onset can lead to food grain hoarding, artificial scarcity, and price rises.
• Monsoons constitute a dual system of seasonal winds, flowing from sea to land in summer and from land to sea in winter.
• Ancient text RIG-VEDA contains several hymns written in praise of PARJANYA, the God of rains, the creator of all living things -plants, animals and other creatures.
• In the tenth century, Al Masudi, an Arab explorer, gave an account of the reversal of ocean currents and the monsoon winds over the north Indian Ocean.
• Some scholars liken monsoon winds to large-scale land and sea breezes.
• Monsoons are unique to regions such as the Indian Subcontinent, Southeast Asia, and parts of Central Western Africa, with the Indian Subcontinent experiencing them most prominently.
• Indian monsoons are expansive convection cells, marked by periodic or secondary winds causing seasonal reversals in wind direction.
• India receives south-west monsoon winds in summer and north-east monsoon winds in winter.
• South-west monsoons form due to intense low-pressure systems over the Tibetan plateau, while north-east monsoons are associated with high-pressure cells over the Tibetan and Siberian plateaus.
• South-west monsoons bring heavy rainfall to most regions in India, while north-east monsoons primarily affect the southeastern coast of India, including the southern coast of Seemandhra and Tamil Nadu.
• Countries like India, Indonesia, Bangladesh, and Myanmar receive the bulk of their annual rainfall during the south-west monsoon season, while South East China, Japan, etc., receive theirs during the north-east monsoon season.

Indian Monsoons

Classical Theory:

• Monsoons are referenced in ancient texts like the Rig Veda and writings by Greek and Buddhist scholars, but the first systematic scientific studies of monsoon winds are attributed to the Arabs.
• Around the tenth century, Arab explorer Al Masudi from Baghdad provided an account of the reversal of ocean currents and the monsoon winds in the north Indian Ocean.
• In 1554 A.D., Sidi Ali reported the commencement dates of monsoons at various locations.
• In 1686, Sir Edmund Halley explained the monsoon as resulting from thermal contrasts between continents and oceans due to their differential heating.

Monsoon Mechanism

Onset of the South-West Monsoon

1. Movement of the Inter-Tropical Convergence Zone (ITCZ):

• The ITCZ shifts north and south of the equator in response to the apparent movement of the Sun.
• During June, when the sun is directly over the Tropic of Cancer, the ITCZ moves northwards.

2. Influence of Southeast Trade Winds:

• The southeast trade winds from the southern hemisphere cross the equator and change direction, blowing from southwest to northeast due to the Coriolis force.
• These winds gather moisture as they traverse the warm Indian Ocean.

3. Arrival of Southwest Monsoons:

• In July, the ITCZ relocates to approximately 20°-25° N latitude, situated in the Indo-Gangetic Plain, known as the Monsoon Trough.
• Southwest monsoons, originating from the Arabian Sea and the Bay of Bengal, bring rainfall to the region during this time.

4. Relationship with Jet Streams:

• Jet streams are wide bands of the upper air, high-speed winds that blow in the troposphere consistently in one direction in a meandering course.
• In winter westerly Jet stream blows over the Indian sub-continent in two parts North of the Himalayas and South of the Himalayas.
• In summers towards the end of May the Jet Stream blowing south Himalayas suddenly weakens and moves to new location north of Himalayas.
• Its movement towards north is main feature associated with onset of monsoon over India
• The shift in the position of the ITCZ is linked to the withdrawal of the westerly jet stream from its position over the north Indian plain, south of the Himalayas.
• The easterly Jet Stream (Somali Jet) establishes itself along 15°N latitude only after the western jet stream withdraws, contributing to the onset of the monsoon in India.

5. Modification of Winds over Land:

• As these monsoon winds approach the land, their direction is altered by relief features and thermal low pressure over northwest India.

6. Branches of Monsoon Approaching Indian Landmass:

• The monsoon approaches the Indian landmass through two branches:
  • Arabian Sea branch – Arabian Sea Branch advances northwards to Bombay by 10th June. By mid-June the Arabian Sea branch of monsoon spreads over Saurashtra- Kutch and central parts of country. It reaches Rajasthan by July 15th and the whole India is covered.
  • Bay of Bengal branch – The Bay of Bengal branch strikes the Myanmar coast and gets deflected northward entering India through the Khasi hills, It spreads over most of Assam by first week of June. On reaching southern periphery of Himalayan barriers the Bay of Bengal branch is deflected westward and progresses towards Gangetic plains. It arrives over Kolkata by June 7. The Arakan Hills along the coast of Myanmar deflect a significant portion of the Bay of Bengal branch towards West Bengal and Bangladesh.
  • The deflected current from Bay of Bengal and Arabian Sea branch of monsoon then merge into single current in central India. And cover remaining parts of west U.P., Haryana, Punjab and eastern Rajasthan by first week of July.

7. Phenomenon of ‘Breaks’ in Rainfall:

• The monsoon exhibits breaks in rainfall, with intermittent periods of rainless intervals, often related to the movement of the monsoon trough.

8. Regional Variations in Climatic Conditions:

• Despite a general unity in the overall pattern, there are noticeable regional variations in climatic conditions within the country.

Withdrawal of Monsoons

Monsoons begin to withdraw from North-western India – Punjab and Rajasthan by mid September and from the country by early December. Withdrawal of monsoon is more gradual process.

Retreating Monsoon Season

1. Retreating Southwest Monsoon Season:
   • Characterized by clear skies and rising temperatures.
   • Despite the season’s end, the land retains moisture, leading to high humidity and oppressive weather, known as the ‘October heat.’
2. Temperature Decline in Late October:
   • Towards the latter half of October, temperatures start dropping rapidly, especially in northern India.
3. Weather Patterns in Retreating Monsoon:
   • North India experiences dry weather during the retreating monsoon.
   • However, the eastern part of the Peninsula, particularly in October and November, encounters rainy conditions, making it the rainiest period of the year.
4. Impact of Cyclonic Depressions:
   • Widespread rainfall in the eastern Peninsula results from the passage of cyclonic depressions originating over the Andaman Sea.
   • These depressions often cross the eastern coast of the southern Peninsula, bringing destructive tropical cyclones.
5. Significance for the Coromandel Coast:
   • The Coromandel Coast receives a significant portion of its rainfall from these depressions and cyclones.
6. Importance of Northeast Monsoon:
   • While the rest of the country receives rainfall during the southwest monsoon season (June to September), the northeast monsoon is vital for farming and water security in the southern regions.

Factors Influencing South-West Monsoon Formation

1. Differential Heating and Cooling: Landmass of India experiences low pressure due to differential heating and cooling compared to the comparatively high pressure in the surrounding seas.
2. Inter Tropical Convergence Zone (ITCZ) Shift: During summer, the ITCZ shifts over the Ganga plain, normally positioned about 5°N of the equator, influencing the monsoon season.
3. High-Pressure Area East of Madagascar: An area of high pressure east of Madagascar, approximately at 20°S over the Indian Ocean, impacts the intensity and position of the Indian Monsoon.
4. Tibetan Plateau Heating: Intense heating of the Tibetan plateau during summer creates strong vertical air currents and forms low pressure over the plateau at about 9 km above sea level.
5. Westerly Jet Stream Movement: The movement of the westerly jet stream to the north of the Himalayas plays a role in the monsoon system’s dynamics.
6. Tropical Easterly Jet: The presence of the tropical easterly jet stream over the Indian peninsula during summer, also known as the African Easterly Jet, influences weather patterns.
7. Southern Oscillation (SO): In normal conditions, high pressure in the tropical eastern south Pacific Ocean contrasts with low pressure in the tropical eastern Indian Ocean. However, during certain years, there is a reversal in pressure conditions known as the Southern Oscillation.

Inter-Tropical Convergence Zone (ITCZ)

• The southeast trade winds in the southern hemisphere and the northeast trade winds in the northern hemisphere meet each other near the equator.
• The meeting place of these winds is known as the Inter-Tropical Convergence Zone (ITCZ).
• This is the region of ascending air, maximum clouds and heavy rainfall.
• The location of ITCZ shifts north and south of equator with the change of season.
• In the summer season, the sun shines vertically over the Tropic of Cancer and the ITCZ shifts northwards.
• The southeast trade winds of the southern hemisphere cross the equator and start blowing in southwest to northeast direction under the influence of Coriolis force.
• These displaced trade winds are called south-west monsoons when they blow over the Indian sub-continent.
• The front where the south-west monsoons meet the north-east trade winds is known as the Monsoon Front (ITCZ). Rainfall occurs along this front.
• In the month of July the ITCZ shifts to 20°- 25° N latitude and is located in the Indo-Gangetic Plain and the south-west monsoons blow from the Arabian Sea and the Bay of Bengal. The ITCZ in this position is often called the Monsoon Trough [maximum rainfall].
• The seasonal shift of the ITCZ has given the concept of Northern Inter-Tropical Convergence Zone (NITCZ) in summer (July – rainy season) and Southern Inter-Tropical Convergence Zone (SITCZ) in winter (Jan – dry season).
• NITCZ is the zone of clouds and heavy rainfall that effect India.