Southwest Monsoon: Lifeline of India’s Climate and Agriculture:

The southwest monsoon season, a vital meteorological phenomenon in the Indian subcontinent, marks the arrival of seasonal winds laden with moisture from the Indian Ocean. Characterized by its onset in June and withdrawal by September, this annual weather pattern plays a pivotal role in shaping the region’s climate, influencing agriculture, water resources, and overall livelihoods. The southwest monsoon brings widespread rainfall, crucial for sustaining ecosystems and supporting the agrarian economies of countries like India.

Southwest Monsoon: Birth, Journey, and Impact on India:

• Genesis of the Southwest Monsoon: Birth of Seasonal Winds: By May, the intensifying low-pressure conditions over the northwestern plains of India had become pronounced enough to attract the southeast trade winds from the Southern Hemisphere. 
  • Originating from the Indian Ocean, these winds are laden with moisture from the warm equatorial currents. 
• Southwest Monsoons: Equatorial Southwesterly turn: Upon crossing the equator, they take a southwesterly turn, earning them the name “southwest monsoons”.

Pressures and Surface wind (July)

What Defines the Southwest Monsoons Arrival?

• Abrupt Monsoon Onset: The commencement of the rainy season in the southwest monsoon period is marked by a sharp and abrupt onset. 
• Monsoon Burst with Thunder and Lightning: This sudden introduction of moisture-rich winds, accompanied by intense thunder and lightning, is termed the “burst” of the monsoons. 
• Regional Variation in Onset: Coastal areas like Kerala, Karnataka, Goa, and Maharashtra typically experience this burst in the first week of June, while interior regions might witness it by the first week of July.
• Temperature Drop: A significant drop in daytime temperature, ranging from 5°C to 8°C, is observed between mid-June and mid-July .

Monsoon Branches: Journey via the Arabian Sea and Bay of Bengal Branches

• The incoming monsoon winds are bifurcated by India’s topography and the thermal low-pressure system over northwestern India into two primary branches: 
  • The Arabian Sea branch and,
  • The Bay of Bengal branch.

Monsoon Tales: Arabian Sea Winds, Western Ghats, and Rain-shadow Wonders

The monsoon winds originating over the Arabian Sea further split into three branches:

• Western Ghats and Rain-shadow Effect: The monsoon winds from the Arabian Sea, upon encountering the Western Ghats, ascend its slopes, cooling down in the process. 

• • • This leads to heavy rainfall (between 250 cm and 400 cm) on the windward side of the Sahyadris (Coastal Maharashtra)  and the Western Coastal Plain. 
    • After crossing the Ghats, the winds descend, warm up, and lose their moisture. This results in minimal rainfall to the east of the Western Ghats, creating a rain-shadow region. 
    • Notable differences in rainfall can be observed across places like Kozhikode, Mangalore, Pune, and Bengaluru.

• Central India’s Monsoon Impact:  Another branch of the Arabian Sea monsoon impacts the region north of Mumbai.

• • It moves through the Narmada and Tapi river valleys, bringing rainfall to vast areas of central India. 

• The Chotanagpur plateau receives about 15 cm of rainfall from this branch. Eventually, these winds reach the Ganga plains and merge with the monsoon winds of the Bay of Bengal.

• Himalayan Rainfall Ballet: Monsoon Symphony Across Northwest India and Saurashtra: The third branch hits the Saurashtra Peninsula and Kachchh, then moves over west Rajasthan and along the Aravallis, producing limited rainfall. 

• It continues to Punjab and Haryana, where it converges with the Bay of Bengal monsoon branch. Together, they deliver rain to the western Himalayas.

• Bay of Bengal Ballet: Tracing the Monsoon Winds journey from Arakan Hills to the Himalayas: The monsoon winds of the Bay of Bengal initially hit the Myanmar coast and parts of southeast Bangladesh. 

• However, due to the Arakan Hills along Myanmar’s coast, a significant portion of this branch is redirected towards the Indian subcontinent.

• • • As a result, the monsoon enters West Bengal and Bangladesh from the south and southeast, rather than the typical southwesterly direction.
    • Influenced by the Himalayas and the thermal low in northwest India, this monsoon branch divides into two major streams. 
      • One stream moves westward along the Ganga plains, extending to the Punjab plains. 

• The other heads north, travelling up the Brahmaputra valley, leading to extensive rainfall in the north and northeast.

• • A sub-branch of the Bay of Bengal monsoon targets the Garo and Khasi hills in Meghalaya. 

• Mawsynram, situated on the Khasi Hills’ crest, holds the distinction of receiving the highest average annual rainfall globally.

Retreating Southwest Monsoon Season: October-November Transition

• The Period of Transition into Autumn in October-November: During October-November, with the apparent movement of the sun towards the south, the monsoon trough or the low-pressure trough over the northern plains becomes weaker.
  • This is gradually replaced by a high-pressure system. 
  • The months of October and November mark the phase of retreating monsoons,
• Timeline of Retreat to Southern India from Western Rajasthan: By September’s end, the southwest monsoon weakens due to the southward shift of the low-pressure trough from the Ganga plain, 
  • The monsoon begins its retreat from western Rajasthan in early September,
  • Bt September  month-end it leave areas like Rajasthan, Gujarat, and the Western Ganga plain 
  • By early November, this low-pressure influence reaches Karnataka and Tamil Nadu, and by mid-December, it completely exits the Peninsula.
• October Heat and Cool Nights with effects: The months of October-November form a period of transition from hot rainy season todry winter conditions.
• While day temperatures are high, nights are cool and pleasant. The land is still moist. 
• Owing to the conditions of high temperature and humidity, the weather becomes rather oppressive during the day. This is commonly known as ‘October heat’.
• In the second half of October, northern India experiences a rapid temperature decline.
• However, by late October, north India remains dry, eastern parts of the Peninsula experience rain.
• Cyclonic Influences: Impact on India’s Coasts and River Deltas: The low-pressure conditions, over northwestern India, get transferred to the Bay of Bengal by early November. This results in cyclonic depressions in the Andman sea.
• These cyclones, especially potent when crossing the eastern coast of the southern Peninsula, frequently affect the deltas of rivers like the Godavari, Krishna, and Kaveri, causing significant damage.
• While these cyclonic storms also impact West Bengal, Bangladesh, and Myanmar, they are less prevalent in the Arabian Sea. 
• The Coromondal coast mainly gets its rain from these cyclonic events.