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Home / UPSC / Monsoon marvels: Unravelling the science behind India’s seasonal rains

Monsoon marvels: Unravelling the science behind India’s seasonal rains

Monsoon system in India — UPSC explainer
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Ravneet Kaur
TNS
Chandigarh, Updated At : 08:30 PM Jul 03, 2025 IST
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What is a monsoon?
The term “monsoon” is derived from the Arabic word “mausim”, meaning season. In meteorology, it refers to a seasonal reversal of winds accompanied by changes in precipitation, particularly over South and Southeast Asia.
A monsoon climate is characterised by wet summers and dry winters, caused by differential heating of land and sea.
What is the mechanism of formation of the Indian monsoon?
The Indian Monsoon is a complex climatic phenomenon resulting from several interrelated factors:
Mechanism of Indian monsoon formation:
1. Differential heating of land and sea
  • During summer, the Indian subcontinent heats up faster than the surrounding oceans, creating a low-pressure area over northwest India.
  • The Indian Ocean remains comparatively cooler, creating a high-pressure zone.
2. Shift of Inter-Tropical Convergence Zone (ITCZ)
  • The ITCZ moves northward (near the Tropic of Cancer) in summer, pulling moist air masses toward the subcontinent.
3. Trade winds and Coriolis effect
  • The southeast trade winds from the southern hemisphere cross the equator, get deflected to the right due to the Coriolis force and enter India as southwest monsoon winds.
4. Presence of Tibetan plateau and Jet streams
  • The Tibetan plateau acts as an elevated heat source, further intensifying the low pressure.
  • The Tropical Easterly Jet helps pull monsoonal winds into the Indian subcontinent.
5. Somali Jet and Subtropical Jet Stream withdrawal
  • The Somali Jet, a strong low-level jet over the Arabian Sea, feeds moisture into the monsoon system.
  • The retreat of the westerly subtropical jet stream is a precursor to monsoon onset.
What is the nature of the Indian monsoon?
The Indian monsoon is seasonal, pulsating and irregular in nature. It is influenced by multiple global and regional factors.
Features:
  • Bimodal: Southwest monsoon (June-September), Northeast monsoon (October-December)
  • Unreliable/erratic: Onset and withdrawal dates vary; rainfall is uneven
  • Orographic: Heavily influenced by topography (e.g., Western Ghats, Himalayas)
  • Cyclonic: Carries moisture-laden cyclonic winds from the ocean
  • Influenced by global phenomena: ENSO, IOD, Madden-Julian Oscillation (MJO)
What are the characteristics of monsoonal rainfall in India?
  1. Seasonal concentration: Rs 75% of annual rainfall occurs between June and September.
  2. Uneven distribution: Some regions receive over 1,000 cm (e.g., Mawsynram), others <50 cm (e.g., Rajasthan).
  3. Orographic and convective: Most rainfall is due to orographic lifting and convection.
  4. Breaks in monsoon: Periods of dry spells during monsoon due to shifting pressure systems.
  5. Sudden onset and withdrawal: Marked by rapid development and fading of rainfall zones.
  6. Monsoon trough: An important low-pressure zone affecting rainfall variability.
    7. What is the distribution pattern of annual rainfall in India?
    1. High rainfall areas (>200 cm)
    • Western coast (Konkan, Goa, Kerala)
    • Northeast India (Assam, Meghalaya, Tripura)
    2. Moderate rainfall areas (100-200 cm)
    • Gangetic plains, parts of eastern MP, Odisha
    3. Low rainfall areas (50-100 cm)
    • Punjab, western UP, Rajasthan (east)
    4. Scanty Rainfall Areas (<50 cm)
    • Thar Desert, Ladakh, Saurashtra
    Reason for variation: Orographic barriers, distance from the sea, monsoon path and presence of cyclonic systems.
    What is the impact of climate change on the Indian monsoon?
    Climate change has amplified the variability and intensity of the Indian monsoon in recent decades.
    Key Impacts
    • Increased frequency of extreme rainfall events: Localised floods, urban flooding (e.g., Mumbai, Delhi)
    • Longer dry spells & sudden bursts: Adverse for agriculture dependent on steady rainfall
    • Shifts in onset and withdrawal: Affects sowing and harvesting cycles
    • Warming of Indian Ocean and Arabian Sea: Alters moisture-carrying capacity and jet stream patterns
    • Melting of Himalayan glaciers: Long-term changes in river discharge and monsoon stability
    • Monsoon dependence on ENSO weakening: Traditional predictability models are now less reliable
    What is ENSO (El Niño-Southern Oscillation)?
    ENSO is a climatic phenomenon that involves cyclical changes in the temperature of the equatorial Pacific Ocean and associated atmospheric pressure patterns, which significantly affect global and Indian weather patterns — especially the Indian monsoon.
    🔹 ENSO has two main phases:
    1. El Niño (“The Little Boy”)
    • Characterised by unusual warming of central and eastern Pacific Ocean waters near Peru.
    • Leads to weakening of the Indian monsoon (dry spells, drought-like conditions).
    • Causes shifting of the Walker circulation and suppression of convection over the Indian Ocean.
    2. La Niña (“The Little Girl”)
    • Characterised by unusually cold Pacific Ocean waters.
    • Usually leads to enhanced Indian monsoon rains.
    • 3. Southern Oscillation
    • Refers to the seesaw pattern of atmospheric pressure between Tahiti (eastern Pacific) and Darwin (Australia).
    • Measured using the Southern Oscillation Index (SOI).
    🔸 ENSO’s Impact on India:
    • A strong El Niño is often associated with droughts in India (e.g., 2002, 2015).
    • La Niña years generally bring good monsoons (e.g., 2010, 2020).
    • Predicting ENSO events is crucial for agriculture, disaster planning, and water management.
    History of monsoon understanding & Hippalus’ contribution
    🔹 Ancient knowledge of monsoon:
    • Indian texts like the Rigveda and Arthashastra mention seasonal rains and agricultural cycles linked to monsoons.
    • Monsoon was historically observed but not scientifically understood.
    🔹 Who was Hippalus?
    • Hippalus was a Greek navigator and sailor of the 1st century BCE, often credited with discovering the direct sea route from Arabia to India via the southwest monsoon winds.
    • His understanding of seasonal wind reversal enabled open-sea sailing across the Arabian Sea, revolutionising trade between India, the Middle East and Europe.
    🔸 Importance of Hippalus’ contribution
    • Marked the beginning of systematic trade using monsoonal winds.
    • Led to the flourishing of maritime trade in ports like Muziris (Kerala) and Barbarikon (Pakistan).
    • Ancient traders timed their voyages with monsoon onset (June) to reach India and returned with retreating monsoon winds (October–November).
    What are Western Disturbances and their role in northern India’s climate?
    Western Disturbances (WDs) are extratropical storms originating in the Mediterranean region that bring rainfall and snowfall to northwest India during winter months (December-February).
    Features:
    • Travel eastward via westerly jet streams
    • Affect Punjab, Haryana, Rajasthan, western UP and the Himalayas
    • Cause winter rainfall crucial for rabi crops like wheat
    • Intensify cold wave conditions
    • Can interact with easterlies to trigger hailstorms
    SUMMARY CHART
    Topic
    Key Details
    ENSO
    Ocean-atmosphere phenomenon affecting Indian monsoon
    El Niño
    Warmer Pacific waters → weak monsoon
    La Niña
    Cooler Pacific waters → strong monsoon
    SOI
    Pressure difference indicator (Tahiti–Darwin)
    Hippalus
    Greek sailor who discovered southwest monsoon winds
    Impact of Hippalus
    Enabled maritime trade between India and the Mediterranean world

    UPSC-ORIENTED PRACTICE QUESTIONS

    🔹 Short answer type (150 words)
    1. Define monsoon. Discuss the role of the Inter-Tropical Convergence Zone (ITCZ) in the Indian monsoon.
    2. What are the main characteristics of monsoonal rainfall in India?
    3. Write a short note on the impact of Western Disturbances on Indian agriculture.
      4. 🔹 Long answer type (250 words)
      1. Examine the mechanisms involved in the onset and progression of the southwest monsoon over India.
      2. Assess the spatial distribution of annual rainfall in India and explain the factors responsible for regional variations.
      3. How is climate change affecting the Indian monsoon? Substantiate with recent trends and examples.
        4. 🔹 Analytical/essay-level questions
        1. “The Indian monsoon is both a boon and a bane.” Critically evaluate this statement in the context of agriculture and disaster vulnerability.
        2. Discuss how the irregular nature of the Indian monsoon affects economic planning and disaster preparedness.
        3. Analyse the extent to which global climatic phenomena such as ENSO and IOD influence the Indian monsoon system.
          4. Advertisement
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