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Subject: Geography

  • Naying Hydroelectric Project

    Why in the News?

    The Naying Hydroelectric Project (1000 MW), proposed on the Siyom (Yomgo) River in Shi-Yomi district, Arunachal Pradesh, represents a major addition to India’s clean energy expansion under the Decade of Hydro Power (2025–35).

    About Siyom (Yomgo) River:

    • Geography: A right-bank tributary of the Brahmaputra, flowing entirely within Arunachal Pradesh.
    • Origin & Course: Arises in West Siang, travels ~170 km, and joins the Brahmaputra near Assam.
    • Ecology: Basin supports rich biodiversity, agro-pastoral livelihoods, and lies within the Eastern Himalaya Biodiversity Hotspot.
    • Protected Areas: Mouling National Park lies on its eastern bank, part of the Dibang–Siang biosphere landscape.
    • Hydrological Role: Ensures irrigation, microclimate regulation, and provides run-of-the-river potential for clean energy, though demanding careful ecosystem balance.

    About Naying Hydroelectric Project:

    • Overview: A proposed 1,000 MW (4×250 MW) run-of-the-river project located in Shi-Yomi district, Arunachal Pradesh.
    • Developers: Jointly undertaken by North Eastern Electric Power Corporation (NEEPCO) and Arunachal Pradesh Hydropower Corporation Ltd (APHCL).
    • Design & Output: Features a concrete dam, underground powerhouse, and diversion tunnels, expected to generate 4,966.77 GWh annually.
    • Regulatory Approval: Received Central Electricity Authority (CEA) concurrence in 2013; progress slowed by environmental and social concerns.
    • Public Consultation: Environmental hearing scheduled for 12 November 2025 at Yapik Community Hall to assess ecological and community impacts.
    • Timeline: Construction targeted to start by 2028, with commissioning by 2032.
    • Policy Context: Forms part of the state’s Decade of Hydro Power (2025–2035), aiming for 19 GW capacity addition to support India’s net-zero goals.
    • Regional Linkages: Among five key hydel projects in the region – Heo (240 MW), Hirong (500 MW), Tato-I (186 MW), and Tato-II (700 MW).
    [UPSC 2022] Consider the following pairs:

    Reservoirs – States

    1. Ghataprabha — Telangana

    2. Gandhi Sagar — Madhya Pradesh

    3. Indira Sagar — Andhra Pradesh

    4. Maithon —Chhattisgarh

    Options:

    (a) Only one pair (b) Only two pairs (c) Only three pairs (d) All four pairs”

     

  • Delhi Morphological Ridge

    Why in the News?

    The Delhi government has decided to declare 41 sq. km of the Southern Ridge as a reserved forest under the Indian Forest Act, 1927, following long-pending directions from the National Green Tribunal (NGT).

    Delhi Morphological Ridge

    About Delhi Morphological Ridge:

    • The Delhi Ridge is the northern extension of the ancient Aravalli Range, stretching approximately 35 km from Tughlaqabad to Wazirabad, along the Yamuna River.
    • It is composed mainly of quartzite rock, is over 1.5 billion years old, and significantly older than the Himalayas.
    • It functions as Delhi’s green lungs, aiding in carbon sequestration, temperature regulation, and air pollution reduction.
    • It acts as a natural barrier against desert winds from Rajasthan and supports rich biodiversity, making Delhi one of the world’s most bird-rich capitals.
    • It is divided into four zones: Northern Ridge, Central Ridge, South-Central Ridge, and Southern Ridge.
    • Key conservation areas include the Northern Ridge Biodiversity Park and the Asola Bhatti Wildlife Sanctuary.

    Land Use Regulation in the Ridge:

    • Although the area shares ecological features with the Delhi Ridge, it is NOT officially notified as forest land, but it enjoys judicial protection.
    • A 1966 directive prohibits any NON-forest use or encroachment without court approval.
    • Any change in land use must be cleared by the Ridge Management Board (RMB) and the Supreme Court-appointed Central Empowered Committee (CEC).
    • The area is mapped using data from the Delhi Forest Department and the 2006 Seismic Zonation Map.
    • Formal notification as a Reserved Forest under the Indian Forest Act, 1927, is pending due to the absence of ground-truthing.
    • In revenue records, it is often marked as “gair mumkin pahad”, meaning uncultivable rocky hill.
    • The terrain is ecologically fragile, with shallow soil and rocky outcrops, making it unsuitable for construction.
    [UPSC 2001] The approximate age of the Aravalli range is-

    Options: (a) 370 million years (b) 470 million years (c) 570 million years (d) 670 million years

     

  • Volga River

    Why in the News?

    This newscard is an excerpt from the original article published in the Indian Express.

    Volga River

    About the Volga River:

    • Overview: The longest river in Europe (about 3,500 km), originating in the Valdai Hills northwest of Moscow and flowing southeast to the Caspian Sea at Astrakhan.
    • Drainage Basin: Covers around 1.35 million sq. km, among Europe’s largest river systems, with major tributaries, Kama, Oka, Vetluga, and Sura.
    • Historical Role: Served as a critical front during the Battle of Stalingrad (World War II) and remains central to Russian historical and strategic narratives.
    • Cultural Significance: Revered as “Mother Volga”, symbolising Russian unity, resilience, and identity, deeply embedded in folklore and national consciousness.
    • Economic Importance: It contributes one-fourth of Russia’s agricultural output, supports industrial fishing, and sustains key industries, oil refining, shipbuilding, hydroelectric power.
    • Navigation & Connectivity: Linked to the Baltic, Black, and Caspian Seas through an extensive network of canals and reservoirs, forming the backbone of Russia’s inland transport system.
    • Urban & Industrial Corridor: Major cities like Kazan, Samara, Nizhny Novgorod, and Volgograd lie along its course, forming Russia’s industrial-agricultural heartland.
    • Ecological Richness: Supports about 260 bird species and 70 fish species, making it a key biodiversity hotspot within Eurasia.
    [UPSC 2020] Consider the following pairs: River Flows into

    1. Mekong: Andaman Sea

    2. Thames: Irish Sea

    3. Volga: Caspian Sea

    4. Zambezi: Indian Ocean

    Which of the pairs given above is/are correctly matched?

    Options: (a) 1 and 2 only (b) 3 only (c) 3 and 4 only * (d) 1, 2 and 4 only

     

  • World’s Highest Motorable Road constructed at Mig La Pass in Ladakh

    Why in the News?

    The Border Roads Organisation (BRO) has achieved a historic milestone by constructing the world’s highest motorable road at Mig La Pass, situated 19,400 feet above sea level in Ladakh.

    • Strategic Value: Enables rapid troop movement and logistics support in high-altitude sectors; promotes eco-tourism and local trade.

    What is Project Himank?

    • Overview: A flagship Border Roads Organisation (BRO) initiative launched in December 1985 to build and maintain roads in Ladakh’s high-altitude regions.
    • Key Achievements: Built Umling La Road, Chisumle–Demchok, Darbuk–Shyok–DBO, Kargil–Zanskar, and now the Mig La Road (19,400 ft) under severe climatic stress.

    About Mig La Pass:

    • Importance: Crucial for India’s border logistics network, enabling swift troop deployment, supply transport, and surveillance near LAC and LoC.
    • Location: Situated on the Changthang Plateau, eastern Ladakh, near the Line of Actual Control (LAC) with China.
    • Elevation: Stands at 19,400 ft (5,913 m), now the highest motorable road in the world (2025), overtaking Umling La (19,024 ft).
    • Alignment: Lies along the Likaru–Mig La–Fukche route, forming a third strategic link from Hanle to Fukche near the Indo-China border.
    • Connectivity Role: Provides access to remote frontier villages—Hanle, Rongo, Kuyul, and Demchok—improving healthcare, communication, and supply access.
    • Geography: Part of the Changthang cold desert, with thin air, permafrost, and extreme cold, posing major engineering challenges.
    • Historical Link: Follows ancient Indo-Tibetan trade routes, reflecting Ladakh’s role in trans-Himalayan Silk Route exchanges.
    [UPSC 2007] Which one of the following Himalayan passes was reopened around in the middle of the year 2006 to facilitate trade between India and China?

    Options: (a) Chang La (b) Jara La (c) Nathu La* (d) Shipki La

     

  • Cyclone Shakhti forms over Arabian Sea

    Why in the News?

    The India Meteorological Department (IMD) confirmed the formation of Cyclone Shakthi (named by Sri Lanka) over the northeast Arabian Sea.

    About Cyclogenesis in the Arabian Sea:

    • Overview: Cyclogenesis is the formation and intensification of tropical cyclones under favourable oceanic and atmospheric conditions.
    • Seasonality: Most active during pre-monsoon (Apr–Jun) and post-monsoon (Oct–Dec) periods, when sea surface temperatures (SSTs) exceed 27 °C, moist convection intensifies, and the Coriolis effect induces rotation.
    • Formation Process: Warm moist air rises forming low pressure; latent heat of condensation deepens the system; upper-level outflow and low vertical wind shear sustain vertical growth, producing a warm eye with spiral rainbands.
    • Historical Pattern: The Arabian Sea was once less cyclone-prone than the Bay of Bengal due to cooler waters, dry winds, and high wind shear. Limited basin size and monsoon winds restricted cyclone growth.
    • Recent Change: Ocean warming and climate change have sharply increased cyclonic activity, making the region far more active in the last decade.
    • Rapid Intensification Trend: Short-term surges in wind speed (< 24 hrs) are now common, linked to warmer SSTs, Indian Ocean Dipole (IOD) shifts, and monsoon wind variability.
    • Oceanic–Climatic Drivers:
      • Indonesian Throughflow imports warm Pacific waters, raising SSTs.
      • Southern Ocean inflow brings cooler deep water, stabilising lower layers.
      • Dual cyclone seasons arise from monsoon wind reversal unique to the region.
    • Climate Change Impact:
      • IMD data show a 52 % rise in Arabian Sea cyclones in two decades, while Bay of Bengal activity slightly declined.
      • The Indian Ocean is among the fastest-warming oceans, increasing heat-moisture availability, altering global weather, and heightening coastal risks to life and infrastructure.

    Recent Examples:

    • Tauktae (2021) – winds > 185 km/h, heavy damage along Gujarat–Konkan.
    • Biparjoy (2023) – lasted 13 days, fed by SSTs ~31 °C.
    • Tej (2023) – hit Oman & Yemen, showing cross-basin movement.
    • Shakthi (2025) – latest late-season, fast-intensifying cyclone.

    Back2Basics: Tropical Cyclones

    • What is it: Large low-pressure systems over warm oceans, marked by rotating winds, heavy rain, and storm surges.
    • Conditions: Form when ocean temps >27°C, with moist rising air releasing latent heat to fuel convection.
    • Rotation: Driven by the Coriolis force – anticlockwise in Northern Hemisphere, clockwise in Southern.
    • Structure: Eye (calm), Eyewall (violent winds/rains), Rainbands (widespread showers).
    • Regional Names: Typhoons (Pacific), Hurricanes (Atlantic/Caribbean), Cyclones (Indian Ocean).
    • Drivers & Frequency: Common in Southeast Asia due to warm Pacific waters, El Niño/La Niña cycles, and climate change.
    • Impacts: Loss of life, property damage, flooding, soil salinisation, displacement, and disease outbreaks.
    • Climate Change Link: Global warming is making tropical cyclones stronger, less predictable, and more frequent, raising risks for coastal populations.

     

    [UPSC 2020] Consider the following statements:

    1. Jet streams occur in the Northern Hemisphere only.

    2. Only some cyclones develop an eye.

    3. The temperature inside the eye of a cyclone is nearly 10°C lesser than that of the surroundings.

    Which of the statements given above is/are correct?

    (a) 1 only (b) 2 and 3 only (c) 2 only* (d) 1 and 3 only

     

  • India’s only Mud Volcano erupts after 20-years in Andamans

    Why in the News?

    India’s only mud volcano at Baratang Island in the Andaman and Nicobar Islands erupted after remaining dormant for over two decades.

    India's only Mud Volcano erupts after 20-years in Andamans

    Note: The Barren Island has erupted recently.

    • India’s only active lava volcano, located about 140 km from Port Blair.
    • Lies at the junction of the Indian and Burmese tectonic plates.
    • Eruption history: 1787 (first recorded), followed by episodes in 1991, 2005, 2017, November 2022, and September 2025.

    About the Baratang Mud Volcano:

    • Location: Baratang Island, around 100–150 km north of Port Blair, situated in the North and Middle Andaman district.
    • Uniqueness: It is India’s only collection of mud volcanoes — 11 in total across the archipelago, 8 of which are on Baratang and Middle Andaman.
    • Eruptions: Significant eruptions were last reported in 2005; the 2025 eruption marks the first major event in 20 years.
    • Composition & Nature:
      • Emits cool mud, water, and gases (methane, hydrogen sulfide) rather than lava or fire.
      • Creates mud cones, bubbling pools, or dried crater-like formations.
      • Eruptions are low in intensity, involving slow oozing and gas bubbling rather than violent explosions.
    • Accessibility: A short 160-metre walk from the nearest road; the site lies near the Jarawa Tribal Reserve, where photography is prohibited for ethical and legal reasons.

    Geological Formation and Features:

    • Tectonic Setting: Formed due to subduction of the Indian Plate beneath the Burmese Plate, leading to gas and fluid release from deep layers.
    • Mechanism:
      • Decomposition of organic matter underground produces gas pressure that pushes mud upwards.
      • These gases, along with water and sediments, escape to the surface, creating muddy eruptions and bubbling vents.
    • Temperature & Composition:
      • The expelled material is cool, unlike magmatic volcanoes.
      • Contains saline water, organic sediments, and gases, giving it a distinctive odour and appearance.
    • Earth Processes: The phenomenon helps scientists study fluid migration, methane emissions, and crustal deformation in active subduction zones.
    [UPSC 2018] Consider the following statements:

    1.The Barren Island volcano is an active volcano located in the Indian territory.

    2.Barren Island lies about 140 km east of Great Nicobar.

    3.The last time the Barren Island volcano erupted was in 1991 and it has remained inactive since then.

    Which of the statements given above is/are correct?

    Options: (a) 1 only * (b) 2 and 3 (c) 3 only (d) 1 and 3

     

  • [pib] First Commercial Coal Mine in Arunachal Pradesh at Namchik-Namphuk

    Why in the News?

    Arunachal Pradesh has launched its first commercial coal mine at the Namchik-Namphuk coal block in Changlang district.

    About the Namchik–Namphuk Coal Mine:

    • Overview: Located in Changlang district, Arunachal Pradesh, is the state’s first commercial coal mine, situated near the Indo-Myanmar border.
    • Reserves & Quality: Holds ~15 million tonnes of lignite/sub-bituminous coal, primarily for thermal power and industrial use.
    • Operator & Allocation: Operated by Coal Pulz Private Limited (CPPL), allotted through a transparent auction in 2022, project first allocated in 2003 but stalled due to environmental and administrative delays.
    • Production & Revenue: Initial capacity of 0.2 million tonnes per annum, expected to generate ₹100 crore annually for the state government.
    • National Context: Marks Arunachal Pradesh’s entry into India’s coal-producing map as the country crosses 1 billion tonnes output (FY 2024-25).
    • Policy Alignment: Supports the EAST Vision (Empower, Act, Strengthen, Transform) for North-Eastern development.

    Significance:

    • Legal Mining: Ends decades of illegal mining through regulated, community-driven extraction.
    • Sustainable Development: Part of Mission Green Coal Regions, targeting 73,000 ha of land reclamation by 2030, embedding ecological restoration into mining.
    [UPSC 2008] In which one of the following states are Namchik-Namphuk Coalfields located?

    Options: (a) Arunachal Pradesh* (b) Meghalaya (c) Manipur (d) Mizoram

     

  • What are Flying Rivers/ Atmospheric Rivers?

    Why in the News?

    Droughts and fires in South America highlight the importance of “flying rivers” — rain-bearing vapor streams disrupted by Amazon deforestation.

    What are Atmospheric Rivers?

    • Overview: Long, narrow bands of concentrated water vapour in the lower atmosphere, often termed “rivers in the sky.”
    • Dimensions: Typically 2,000–5,000 km long, 400–500 km wide, and about 3 km deep.
    • Water Transport: Carry nearly 90% of water vapour across Earth’s mid-latitudes — almost double the Amazon River’s flow.
    • Formation: Warm tropical seawater evaporates, and winds transport this moisture; upon encountering land or mountains, vapour condenses into heavy rainfall or snow.
    • Role: Unlike short-term weather systems, Atmospheric Rivers (ARs) shape long-term hydrological cycles and trigger extreme precipitation events.

    Global Impacts of Atmospheric Rivers:

    • Flooding & Extreme Weather: Cause 80% of flood-related damages along the US West Coast; also linked to devastating floods in Europe, Africa, South America, and Australia.
    • South America: Amazon’s “flying rivers” disrupted by deforestation, leading to droughts in Peru, Bolivia, and Ecuador; threatens Amazon rainforest’s survival and risks savannisation.
    • East Asia: Up to 80% of heavy rainfall events in China, Korea, and Japan during early monsoon linked to ARs.
    • Climate Connection: Warming oceans are making ARs longer, wider, and more intense, increasing risks of catastrophic floods and landslides.
    • Positive Role: Contribute 30–50% of annual precipitation in some regions (e.g., US West Coast) and help end 33–74% of droughts.

    Atmospheric Rivers in India’s Context:

    • Interaction: ARs combine with cyclonic circulations and the Himalayan ranges, causing extreme rainfall and flash floods.
    • Case Studies:
      • 2010 Leh cloudburst (Ladakh) – flash floods and mudslides.
      • 2011 Kupwara floods (J&K) – severe AR-driven rainfall.
    • Study (1951–2020): Identified 574 AR events during the monsoon season in India.
    • Recent Trends: Nearly 80% of India’s most severe floods (1985–2020) linked to AR activity.
    • Cause: Rapid Indian Ocean warming intensifies evaporation, moisture transport, and AR-driven floods.
    • Impact: Leads to short, intense rainfall spells, landslides, flash floods, crop loss, and mass displacement of communities.
    [UPSC 2024] With reference to “water vapour,” which of the following statements is/are correct?

    1. It is a gas, the amount of which decreases with altitude.

    2. Its percentage is maximum at the poles.

    Select the answer using the code given below:

    Options: (a) 1 only* (b) 2 only (c) Both 1 and 2 (d) Neither 1 and 2

     

  • Climate Models and Their Accuracy

    Why in the News?

    The US President Donald Trump called climate change the “greatest con job ever,” disgusted with the predictions based on climate models central to climate science.

    Climate Models and Their Accuracy

    What are Climate Models?

    • Overview: Climate models are computer simulations using mathematical equations to represent the Earth’s climate system, including the atmosphere, oceans, land surface, and ice.
    • Basis: Built on physics, chemistry, and biology, they simulate interactions among Earth’s components.
    • Purpose: Forecast temperature, rainfall, humidity, sea-level rise, and extreme weather under scenarios like high greenhouse gas emissions.
    • Difference from Weather Models: Weather models predict short-term local events, while climate models analyze long-term regional and global patterns.

    How do Climate Models work?

    • Grid System: Earth divided into a 3D grid of cells across land, atmosphere, and oceans.
    • Equations: Each cell governed by equations on energy movement, air, ice, and land processes.
    • Data Input: Observational data (greenhouse gases, ocean conditions, land use) fed into the model.
    • Interactions: Equations simulate changes in each cell and their effects on neighboring cells.
    • Outputs: Provide projections for temperature, precipitation, sea levels, ice cover, and extreme climate events.

    Evolution of Climate Models:

    Model Type What is it? Strengths Limitations
    Energy Balance Models (EBMs) 

    (1960s)
    • The earliest climate models.
    • They treat Earth like a single box system, calculating surface temperature by balancing incoming solar radiation vs outgoing infrared radiation.
    • Essentially, they answer: “How warm should Earth be if X amount of energy comes in and Y amount goes out?”
    • Very simple; first to link CO₂ emissions with global warming.
    • Computationally inexpensive.
    • Oversimplified — ignores atmosphere, oceans, and circulation.
    • Cannot simulate rainfall, winds, or regional climate.
    Radiative Convective Models (RCMs) 

    (1960s–70s)
    • Introduced the vertical structure of the atmosphere.
    • They divide the atmosphere into layers and simulate how radiation (solar + infrared) and convection move heat upward and downward.
    • Show how greenhouse gases trap heat and alter temperatures at different heights.
    • Capture greenhouse effect more realistically;
    • Explain vertical temperature profiles;
    • Useful for studying stratospheric cooling.
    • Still ignore oceans and global circulation;
    • Cannot project regional variations or weather patterns.
    General Circulation Models (GCMs) (Global Climate Models)

    (1970s onwards)
    • The first 3D models of Earth’s climate.
    • Divide the planet into grid cells (100–250 km), each with equations for atmosphere, oceans, ice, and land.
    • Simulate winds, currents, rainfall, temperature, and pressure by solving physical equations of motion, energy, and mass.
    • Comprehensive representation of climate;
    • Simulate monsoon, El Niño, ocean currents; reproduce past climate trends.
    • Very resource-intensive; grid too coarse to capture local detail (cities, villages);
    • Uncertainty in clouds and aerosols.
    Earth System Models (ESMs)

    (1990s–present)
    • Advanced GCMs that integrate biogeochemical cycles (carbon cycle, vegetation, ocean chemistry, aerosols, land-use changes).
    • Show how human activities (deforestation, fossil fuels, pollution) interact with natural systems, feedback loops, and long-term climate.
    • Holistic view of climate–biosphere interactions;
    • Essential for IPCC reports and policy projections.
    • Extremely complex;
    • Uncertainties in carbon feedbacks, aerosols, and long-term ecological processes.
    Regional Climate Models (RCMs)

    (1990s–present)
    • High-resolution versions of GCMs, zoomed into specific regions (25–50 km grids).
    • Use downscaling techniques to provide localised forecasts of rainfall, temperature, droughts, and monsoons.
    • Useful for city- or country-level policy (flood risk, agriculture, urban heat);
    • Capture Indian monsoon and Himalayan glaciers better.
    • Dependent on GCM input;
    • Projections limited to chosen region;
    • Computationally intensive.

    How accurate are Climate Models?

    • Strengths: Modern models predict sea-level rise, polar ice loss, temperature increases, and rainfall trends with high accuracy.
    • Validation: Predictions are compared with historical climate records to confirm reliability.

    • Limitations:

      • Lack of precise data on clouds, volcanic activity, El Niño events.
      • Limited accuracy for regional variations (e.g., urban floods, Indian monsoon extremes).
      • Less accuracy in Global South due to data scarcity and complex climate systems.
      • Grid resolution (100–250 km per cell) causes oversimplification of land–atmosphere interactions.
    [UPSC 2025] The World Bank warned that India could become one of the first places where wet-bulb temperatures routinely exceed 35°C. Which of the following statements best reflect(s) the implication of the above-said report?

    I. Peninsular India will most likely suffer from flooding, tropical cyclones and droughts.

    II. The survival of animals including humans will be affected as shedding of their body heat through perspiration becomes difficult.

    Select the correct answer using the code given below:

    (a) I only      (b) II only      (c) Both I and II      (d) Neither I nor II

     

  • South-west Monsoon: Its Onset and Retreat

    Why in the News?

    According to the India Meteorological Department (IMD), the southwest monsoon began its earliest withdrawal in a decade on September 14 from western Rajasthan, three days before the usual date of September 17.

    What is Monsoon? 

    • Overview: A seasonal reversal of winds, southwest winds (wet) in summer and northeast winds (dry) in winter.
    • Onset Date: Officially begins June 1 over Kerala, advances northwards, covering entire India by mid-July.
    • Importance: Accounts for ~75% of India’s annual rainfall, critical for Kharif crops, water storage, and ecosystems.

    Mechanism of Monsoon Onset:

    • Differential Heating: Indian landmass heats faster than the Indian Ocean, creating low pressure over NW India that pulls in moist winds.
    • Mascarene Highs:
      • Located near Mascarene Islands (east of Madagascar).
      • Strengthen during May–June winter in Southern Hemisphere.
      • Push strong cross-equatorial winds from the SE Indian Ocean towards India.
    • Cross-Equatorial Flow:
      • Southeast trades from Mascarene High cross the equator.
      • Deflected by Coriolis, they become southwest monsoon winds, feeding both Arabian Sea branch and Bay of Bengal branch.
    • ITCZ (Intertropical Convergence Zone) Shift: Moves northwards over Ganga plains, anchoring the monsoon trough.
    • Tibetan Plateau Heating: Acts as an elevated heat source, deepening low pressure.
    • Jet Stream Influence:
      • Subtropical Westerly Jet (STWJ) shifts north of Himalayas, allowing the monsoon trough.
      • Tropical Easterly Jet (TEJ) develops, enhancing moisture flow.
    • Local Triggers: Orographic uplift along Western Ghats, NE Hills, and Indo-Gangetic plains causes heavy rains.

    What is Retreat/Withdrawal of Monsoon?

    • Earliest Withdrawal (2025): Began Sept 14 from western Rajasthan — earliest in a decade (normal = Sept 17).
    • Withdrawal Process: Gradual, completes by mid-October.
    • IMD Withdrawal Criteria:
      1. Development of anti-cyclonic circulation at lower troposphere.
      2. No rainfall for 5 consecutive days over the region.
      3. Prevalence of dry weather conditions.
    • Seasonal Marker: IMD fixes Sept 30 as the official end of SW monsoon.
    • Agricultural Role: Retreat moisture crucial for Rabi crop sowing.

    Influencing Factors for Monsoon Retreat:

    • Seasonal Cooling: Reduced solar heating over land in September weakens low pressure.
    • Pressure Gradient Reversal: High pressure redevelops over NW India, collapsing SW winds.
    • ITCZ Shift: Moves back southwards towards the equator, reversing wind patterns.
    • Jet Stream Role: TEJ weakens, westerlies return, pushing out moist winds.
    • Topography & Seas: Coastal and mountainous regions (e.g., SE peninsula, Bay of Bengal) may still receive residual/post-monsoon showers.
    • Mascarene Highs: As SH winter ends, Mascarene highs weaken, cross-equatorial inflow diminishes, aiding withdrawal.

    Climatic Phenomena affecting the Indian Monsoon:

    1. ENSO (El Niño–Southern Oscillation):

    • ENSO originates in the equatorial Pacific Ocean and strongly influences the Pacific Walker Circulation (PWC).
      1. El Niño years: The eastern and central Pacific waters warm up. This weakens the Walker circulation and reduces the flow of moisture-laden winds from the Mascarene Highs towards India. As a result, the monsoon becomes weak or deficient, often leading to droughts.
      2. La Niña years: The opposite happens; Pacific waters cool, the Walker circulation strengthens, and strong cross-equatorial winds from the Mascarene Highs bring more moisture into India. Monsoon rainfall is usually above normal, sometimes leading to floods.
    • Key point: ENSO acts like a “remote controller” sitting in the Pacific but directly influencing the strength of the Indian monsoon winds.

    2. Indian Ocean Dipole (IOD):

    • The Indian Ocean itself has its own seesaw pattern of sea surface temperatures.
      • Positive IOD: Western Indian Ocean (near Africa) is warmer, and eastern Indian Ocean (near Indonesia) is cooler. This strengthens cross-equatorial winds from the Mascarene Highs, feeding more moisture into India. Result: Good rainfall, strong monsoon, even if El Niño is present.
      • Negative IOD: Western Indian Ocean is cooler, eastern side is warmer. This pulls away monsoon winds from India and weakens the rainfall.
    • Key point: IOD is a “local driver” sitting in the Indian Ocean, which can either amplify or cancel out ENSO’s effect.

    3. ENSO–IOD Interaction:

    • Monsoon outcome is not decided by ENSO or IOD alone, but by how they combine:
      • El Niño + Positive IOD: IOD can cancel El Niño’s bad effect (1997 monsoon was near normal).
      • El Niño + Negative IOD: Worst-case combo, often brings severe droughts.
      • La Niña + Positive IOD: Both reinforce each other, leading to very heavy rainfall and flood risk.
      • ENSO Neutral + Positive/Negative IOD: IOD becomes the deciding factor.

    The Big Picture:

    • Pacific Walker Circulation is the “conveyor belt” moving rising and sinking air across the Pacific and Indian Oceans.
      • When it shifts east (El Niño): India gets less rain.
      • When it strengthens west (La Niña): India gets more rain.
    • IOD modifies this system locally in the Indian Ocean-  it can either buffer or worsen ENSO’s impact.
    • The Mascarene Highs act as the main “engine room” for cross-equatorial winds, but the Walker circulation and IOD decide how strong that engine runs.
    • Madden–Julian Oscillation (MJO): An eastward-moving pulse of cloud and rainfall that travels around the equator every 30–60 days.
      • MJO decides the intra-seasonal variability: when it rains heavily (active phase) and when dry breaks occur.

     

    [UPSC 2012] Consider the following statements:

    1. The duration of the monsoon decreases from southern India to northern India.

    2. The amount of annual rainfall in the northern plains of India decreases from east to west.

    Which of the statements given above is/are correct?

    Options: (a) 1 Only (b) 2 Only (c) Both 1 and 2* (d) Neither 1 nor 2

     

    [UPSC 2017] With reference to ‘Indian Ocean Dipole (IOD)’ sometimes mentioned in the news while forecasting Indian monsoon, which of the following statements is/are correct?

    1. The IOD phenomenon is characterized by a difference in sea surface temperature between tropical Western Indian Ocean and tropical Eastern Pacific Ocean.

    2. An IOD phenomenon can influence an El Nino’s impact on the monsoon.

    Select the correct answer using the code given below:

    (a) 1 only  (b) 2 only * (c) Both 1 and 2  (d) Neither 1 nor 2