Why is the Bay of Bengal Monsoon Called 'Purvaiya' in Bihar? The Himalayan Geography Behind India's Monsoon

on
0

Bay of Bengal Branch of the Southwest Monsoon: Complete Mechanism, River Valley Corridors & Himalayan Advancement

on


 

👉ALL PAGES- LINKS INDEX👀


Shifting of the ITCZ and the Origin of the Bay of Bengal Monsoon Branch

  1. the inclination of the Earth's axis, 
  2. the geoid shape of the Earth, and 
  3. the apparent northward and southward migration of the Sun create seasonal changes in the global distribution of heat. 
  • This differential heating generates-
  1. seasonal variations in atmospheric pressure, 
  2. causing the major pressure belts, including the Inter-Tropical Convergence Zone (ITCZ), to migrate northward during the Northern Hemisphere summer and southward during the Southern Hemisphere summer.
  • The details of how all these factors played out to generate the monsoon have been discussed in the following blog link. So, click over there for a deeper understanding.



The First Surge of the Bay of Bengal Branch: Inland Penetration through River Valley Corridors


  • Unlike the Arabian Sea Branch, which is primarily guided by the Western Ghats, the Bay of Bengal Branch penetrates deep into the Indian Peninsula through broad river valleys that cut across the discontinuous Eastern Ghats. These valleys act as "natural wind corridors," allowing moisture-laden monsoon winds to advance far into the interior of peninsular India.
  • The first major corridor is formed by the "Cauvery River Valley"; here, after making landfall over the Cauvery Delta, the Bay of Bengal branch advances inland through this valley toward the "Mysore Plateau". As the winds approach the higher terrain of the Western Ghats, they interact with the Arabian Sea Branch, producing enhanced orographic rainfall over southern Karnataka.
  • Further north, the Krishna River Valley provides another major pathway. Moist monsoon winds travel through the Krishna Basin into the Deccan Plateau and eventually reach the windward slopes of the Western Ghats near Satara and Mahabaleshwar, where uplift significantly intensifies rainfall.
  • The Godavari River Valley, the largest river basin in peninsular India, serves as one of the most effective inland corridors. Moisture-laden winds advance through coastal Andhra Pradesh and Telangana into Marathwada and Vidarbha. Over central Maharashtra, the Bay of Bengal Branch frequently interacts with the Arabian Sea Branch, enhancing precipitation across the region.
  • The Mahanadi Valley forms another important penetration route. The monsoon enters the "Chhattisgarh Plain", a broad lowland enclosed by-
  1. the Maikal Hills, 
  2. Satpura Range, 
  3. Bastar Plateau, and 
  4. Eastern Ghats. 
  • The surrounding uplands restrict the rapid escape of moisture, allowing prolonged uplift and widespread rainfall. This abundant precipitation has made Chhattisgarh one of India's leading rice-producing regions, popularly known as the "Rice Bowl of Central India".
  • Finally, with respect to peninsular India up to the Vindhyachal range, the Damodar River Valley enables the Bay of Bengal Branch to penetrate into-
  • the Chotanagpur Plateau and 
  • the Hazaribagh Plateau, where plateau relief further enhances rainfall before the monsoon continues its journey toward the Indo-Ganga Plain.



The Second Surge: Division of the Bay of Bengal Branch into the Northeastern and Northwestern Branches

  • In the next stage of its advance, the Bay of Bengal Branch of the Southwest Monsoon reaches the "Ganga–Brahmaputra Delta", encompassing the Sundarbans and the coastal regions of Bangladesh. At this point, the monsoon current develops into two principal branches: 
  1. the Northeastern Branch and 
  2. the Northwestern Branch.
  • The first branch, the Northeastern Branch, advances across the low-lying plains of Bangladesh. Since much of Bangladesh consists of an extensive alluvial plain with very little topographic obstruction, only limited orographic uplift occurs over the country itself.

NOTE: "Bangladesh receives heavy monsoon rainfall, but its most devastating floods are primarily a consequence of the upstream geography of the "Ganga–Brahmaputra–Meghna Basin". This illustrates a fundamental principle of physical geography: landforms and drainage systems often exert a greater influence on hydrological hazards than local climatic conditions alone. Geography, therefore, remains the first and most fundamental factor governing the interaction between climate, rivers, and human society.

  • Consequently, the moisture-laden monsoon winds continue with minimal topographic obstruction toward the northeastern margin of the Indian subcontinent, due to the funnel-shaped topography of the Bangladesh Plain, where these winds encounter the steep and nearly vertical mountain walls of the Khasi Hills, Jaintia Hills, Garo Hills, and the adjoining Eastern Himalayas, and are forced to ascend rapidly.




  • This sudden uplift causes intense cooling and condensation, resulting in exceptionally heavy orographic rainfall. As a result, Mawsynram and Cherrapunji, located on the southern slopes of the Khasi Hills, receive the highest average annual rainfall in the world. 
  • The exceptionally heavy rainfall over the Himalayan foothills, northeastern India, and the upstream river basins generates enormous volumes of runoff, which ultimately converge in the low-lying deltaic plains of Bangladesh. Combined with its low elevation and numerous distributaries, this makes Bangladesh one of the most flood-prone countries in the world.
  • Thus-
  1. The northeastern branch of the Bay of Bengal monsoon not only produces the world's heaviest orographic rainfall over the Khasi Hills, but 
  2. Also plays a crucial role in sustaining the hydrology and flood dynamics of the entire Ganga–Brahmaputra–Meghna Delta.



The Third Surge: Westward Advance of the Bay of Bengal Branch along the Himalayan Foothills  

  • In the final stage of its advancement, the Bay of Bengal Branch of the Southwest Monsoon, after producing intense orographic rainfall over the Khasi Hills, Jaintia Hills, Garo Hills, and the Eastern Himalayas, encounters the nearly continuous east-west Himalayan barrier of the Himalayan mountain system.
  • Now, since the Himalayas extend in an almost continuous east–west direction and rise abruptly to great elevations, the moisture-laden monsoon winds are unable to cross them into Central Asia. Consequently, the winds are deflected westward and begin to flow parallel to the Himalayan foothills, following the Ganga Plain from Assam through North Bengal, Bihar, Uttar Pradesh, Uttarakhand, Haryana, and Punjab, as shown by a black dotted line parallel to the southern foothills of the Himalaya in the above infographics.
  • As the Bay of Bengal branch progresses westward, it continues to release moisture through repeated episodes of convection and orographic uplift along the Himalayan foothills. Consequently, its moisture content gradually diminishes, resulting in a progressive decrease in rainfall from east to west. 
  • This explains why northeastern India and the eastern Ganga Plain receive substantially higher monsoon rainfall than the northwestern parts of the Indo-Ganga Plain. The rainfall gradient from Assam towards northwestern India is therefore one of the most distinctive characteristics of the Southwest Monsoon.
  • Eventually, over the northwestern part of India, particularly around the Shimla Hills and adjoining regions of the Shivalik Range, the Bay of Bengal Branch converges with the Arabian Sea Branch, and this interaction of
  1. two moisture-laden air masses, 
  2. combined with the orographic influence of the Himalayan foothills, 
  3. often enhances precipitation over Himachal Pradesh and the adjoining Himalayan region. 
NOTE: - It is more scientifically accurate to describe this as enhanced orographic rainfall or intensified monsoon precipitation rather than a "cloudburst," because cloudbursts are localised extreme weather events that occur only under specific atmospheric conditions.
  • With this convergence, the inland journey of the Bay of Bengal Branch of the Southwest Monsoon across the Indian subcontinent is completed. 
  1. From its landfall over the eastern coast, 
  2. penetration through the river valleys of Peninsular India, 
  3. development into the northeastern and northwestern branches, 
  4. intense orographic rainfall over the Khasi Hills and Eastern Himalayas, and 
  5. finally, its westward movement along the Himalayan foothills, the Bay of Bengal branch demonstrates the profound influence of India's physiography on the spatial 
  6. distribution of monsoon rainfall.




Branches Advancement- A concluding remark 
  • The journey of the Southwest Monsoon across India clearly demonstrates that India's physiography is the true architect of its rainfall pattern. The Western Ghats channel and uplift the Arabian Sea Branch, while the river valleys, Eastern Himalayas, the Bay of Bengal cyclonic conditions and Himalayan foothills guide and transform the Bay of Bengal Branch. Together, these two branches create one of the world's most complex and efficient monsoon systems, supplying nearly 70–75% of India's annual rainfall and supporting the livelihoods of nearly two billion people across the Indian Subcontinent. Understanding these two branches is therefore fundamental to understanding-
  1. India's climate, 
  2. agriculture patterns, 
  3. economy- how agriculture is a driving force? 
  4. water security,
  5. Foresight vision of planned settlement,
  6. Cultural peculiarity- Because with "Sawan month- a month devoted to lord Shiv. For detailed study, click here. Indian cultural festive season is getting started and will continue till "Sharad Purnima", and 
  7. Disaster management: When landslides are very frequent in hilly regions, particularly in the Himalayan region- a recent one in Wayanad in Kerala.



Thanks
Critical comments are welcome.





 


Comments

Post a Comment