SOUTHWEST MONSOON -
GEOGRAPHY
News: Southwest monsoon makes onset
over Nicobar Islands: IMD
What's in the news?
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Southwest monsoon, the lifeline of India's
farm-based economy, on Sunday made its onset over the Nicobar Islands, the
southernmost region of the country, the India Meteorological Department (IMD)
said here.
South West Monsoon:
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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.
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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.
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The southwest monsoon brings widespread rainfall,
crucial for sustaining ecosystems and supporting the agrarian economies of
countries like India.
Origin of Seasonal
Winds:
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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.
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Originating from the Indian Ocean, these winds are
laden with moisture from the warm
equatorial currents.
Equatorial Southwesterly
Turn:
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Upon crossing the equator, they take a
southwesterly turn, earning them the name “southwest monsoons”.
Features:
1. Abrupt Monsoon Onset:
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The commencement of the rainy season in the
southwest monsoon period is marked by a sharp and abrupt onset.
2. Monsoon Burst with
Thunder and Lightning:
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This sudden introduction of moisture-rich winds,
accompanied by intense thunder and lightning, is termed the “burst” of the
monsoons.
3. Regional Variation in
Onset:
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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.
4. Temperature Drop:
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A significant drop in daytime temperature, ranging
from 5°C to 8°C, is observed between mid-June and mid-July.
Monsoon Branches:
The incoming monsoon winds are bifurcated by India’s topography and the
thermal low-pressure system over northwestern India into two primary branches:
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Arabian Sea branch
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Bay of Bengal branch
1. Arabian Sea Branch:
The monsoon winds originating over the Arabian Sea further split into three
branches:
a. Western Ghats and
Rain-shadow Effect:
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The monsoon winds from the Arabian Sea, upon
encountering the Western Ghats, ascend its slopes, cooling down in the process.
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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.
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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.
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Notable differences in rainfall can be observed
across places like Kozhikode, Mangalore, Pune, and Bengaluru.
b. Central India’s
Monsoon Impact:
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Another branch of the Arabian Sea monsoon impacts
the region north of Mumbai.
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It moves through the Narmada and Tapi river
valleys, bringing rainfall to vast areas of central India.
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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.
c. Himalayan Rainfall
Ballet and Monsoon Symphony Across Northwest India and Saurashtra:
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The third branch hits the Saurashtra Peninsula and
Kachchh, then moves over west Rajasthan and along the Aravallis, producing
limited rainfall.
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It continues to Punjab and Haryana, where it
converges with the Bay of Bengal monsoon branch. Together, they deliver rain to
the western Himalayas.
2. Bay of Bengal Branch:
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The monsoon winds of the Bay of Bengal initially
hit the Myanmar coast and parts of southeast Bangladesh.
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However, due to the Arakan Hills along Myanmar’s
coast, a significant portion of this branch is redirected towards the Indian
subcontinent.
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As a result, the monsoon enters West Bengal and
Bangladesh from the south and southeast, rather than the typical southwesterly
direction.
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Influenced by the Himalayas and the thermal low in
northwest India, this monsoon branch divides into two major streams.
a. One stream moves westward
along the Ganga plains, extending to
the Punjab plains.
b. The other heads north,
travelling up the Brahmaputra valley,
leading to extensive rainfall in the north and northeast.
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A sub-branch of the Bay of Bengal monsoon targets
the Garo and Khasi hills in Meghalaya.
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Mawsynram, situated on the Khasi Hills’ crest,
holds the distinction of receiving the highest average annual rainfall
globally.
Go back to basics:
Tamil Nadu and South
West Monsoon:
The Tamil Nadu coast remains dry
during the southwest monsoon season due to two main factors:
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It is situated parallel
to the Bay of Bengal branch of the southwest monsoon, which brings less
rainfall to this area.
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The Tamil Nadu coast lies in the rain shadow area of the Arabian Sea
branch of the southwest monsoon, further reducing its rainfall.)