Description
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Source: DowntoEarth
Context
According to a study conducted by the India Meteorological Department, the southwest monsoon season in India has prolonged by 1.6 days per decade over the last 50 years, boosting its contribution to annual rainfall from 75% to 79%. This prolongation is associated with delayed monsoon withdrawal, which affects agricultural yields and alters rainfall measurements important for climate research.
What is the Southwest Monsoon?
- The southwest monsoon season in India brings much-needed rainfall, which is essential for agricultural activity and replenishing water sources.
- It is triggered by seasonal wind reversals and the shifting of the Inter-Tropical Convergence Zone (ITCZ); this monsoon system provides relief from the blistering summer heat.
Factors Influencing South-West Monsoon Formation
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The South-West Monsoon is influenced by multiple interrelated geophysical, oceanic, and atmospheric factors.
Differential Heating and Cooling
- Land-Sea Thermal Contrast:
- In summer, Indian landmass heats up rapidly, creating a low-pressure zone over the subcontinent.
- Surrounding Indian Ocean waters remain relatively cooler, maintaining a comparatively high-pressure zone.
- This contrast drives moisture-laden winds from the sea to the land.
Shift of Intertropical Convergence Zone (ITCZ)
- ITCZ Definition:
- A broad trough of low pressure in equatorial latitudes where northeast and southeast trade winds converge.
- Normally located about 5°N of the equator, it shifts northward in summer.
- Monsoon Trough:
- During the monsoon season, ITCZ shifts over the Ganga Plain, acting as a monsoon trough that channels monsoon winds into India.
High-Pressure Area over the Indian Ocean
- East of Madagascar (20°S):
- A subtropical high-pressure cell forms here.
- Its intensity and position influence the strength and onset of the monsoon over India.
Heating of the Tibetan Plateau
- Summer Heating Effect:
- Intense summer heating of the Tibetan Plateau generates strong vertical air currents.
- Leads to low-pressure formation at around 9 km above sea level, enhancing monsoon circulation.
Jet Streams Influence
Easterly Jet Stream & Tropical Cyclones
- Guides tropical depressions into India → high rainfall zones.
- Frequency, direction, intensity decide rainfall pattern.
Tibetan Plateau
- High altitude + more insolation → drives Tropical Easterly Jet Stream (Tibet → Mascarene High).
Somali Jet Stream
- Low-altitude summer current off Somalia.
Strong Somali Current → better monsoon performance.
Southern Oscillation (SO)
- Normal Condition:
- High pressure over the tropical eastern Pacific and low pressure over the tropical eastern Indian Ocean.
- SO Reversal:
- In some years, low pressure develops over the eastern Pacific and high pressure over the eastern Indian Ocean.
- This reversal affects monsoon rainfall distribution over India.
El Niño Effect
- Definition:
- A warm ocean current that periodically replaces the cold Peruvian current along the coast of Peru.
- Named “El Niño” meaning the Christ child, as it often begins near Christmas.
- Impact:
- Raises sea surface temperatures in the eastern Pacific.
- Weakens trade winds, disrupting monsoon circulation and often causing droughts in India.
Mechanism of South West Monsoon
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Seasonal Reversals
- Characteristic Feature: Monsoon is marked by seasonal reversals of the wind system between summer and winter.
- Summer: Winds blow from ocean to land (southwest monsoon).
- Winter: Winds blow from land to ocean (northeast monsoon).
Northward Shift of ITCZ
- Heating of Landmass: During April–May, intense heating over land north of the Indian Ocean creates low-pressure over NW India.
- Northward Shift: This results in the Inter-Tropical Convergence Zone (ITCZ) shifting northwards.
- ITCZ in July: Located around 20°–25° N (Gangetic plains), forming the monsoon trough.
- Trade Wind Reversal: Southern Hemisphere trade winds cross the equator, deflected to blow SW to NE due to the Coriolis force.
Shifting of the ITCZ
- Linked to withdrawal of the Westerly Jet Stream from south of the Himalayas.
- Low Pressure Cell: Attracts SE trade winds across the equator between 40°E–60°E.
- Easterly Jet Stream: Forms along 15°N only after Westerly Jet withdrawal → triggers the burst of monsoon.
Onset of Monsoon in India
Coastal Areas
- First week of June: Kerala, Karnataka, Goa, Maharashtra.
Interior Regions
- First week of July: Central & Northern India.
Southwest Monsoon Branches
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Arabian Sea Branch
- Western Ghats: Orographic rainfall on windward side; rain-shadow effect on leeward side.
- Central India: Through Narmada & Tapi valleys; merges with Bay of Bengal branch in Ganga plains.
- Saurashtra–Kachchh: Parallel to Aravallis; limited rainfall; converges with Bay of Bengal branch over Punjab & Haryana.
Bay of Bengal Branch
- Myanmar Coast: Deflected by Arakan Hills towards India.
- Two Streams:
- Westward → Ganga plains → Punjab.
- Northward → Brahmaputra valley → Meghalaya hills (Mawsynram = highest rainfall).
- Tamil Nadu: Lies in rain-shadow; gets NE monsoon in retreat phase.
Breaks in Monsoon
- Definition: Period of 1+ weeks without rain during SW monsoon.
- Causes:
- Low storm frequency in monsoon trough/ITCZ.
- Winds parallel to west coast.
- Re-establishment of Subtropical Westerly Jet → high pressure blocks monsoon flow.
Southwest Monsoon Extension
- Southwest Monsoon (SWM) contributes ~75% of India’s annual rainfall.
- Duration has extended in the past 50 years due to delayed withdrawal.
- Study published in journal Mausam by India Meteorological Department (IMD) scientists.
Duration and Changes
- Normal SWM season: June 1 – September 30.
- Extension rate: Added 1.6 days per decade between 1970–2020.
- Reason: Delayed withdrawal of monsoon winds from the subcontinent.
- Onset over Kerala: Year-to-year variability but no significant change in dates in past 50 years.
- Extended SWM has increased rainfall contribution from 75% to 79% of annual total.
- Impact: Distorts IMD’s monsoon rainfall metrics (June 1 – September 30), affecting long-term climate studies.
Delayed Withdrawal Trends
- Normal withdrawal start: Around September 17 from western Rajasthan.
- Last 14 years: Delay of at least one day on average.
- Extreme delays:
- 2019: Withdrawal on October 9 (22 days late; 40 days late if old norm of Sept 1 considered).
- 2021: Withdrawal on October 6 (19 days late).
- Normal complete withdrawal date: October 15; past decade saw dates post-normal.
- In April 2020, IMD (Indian Meteorological Department) revised withdrawal dates for different regions.
- Cited climate change as a primary factor.
Factors Causing Widespread Monsoonal Rainfall Across India in Recent Times
Continuous Westerly Winds
- Moisture-laden westerlies from the Arabian Sea have been continuously moving inland.
- These strong westerly winds enhance moisture transport and lead to heavy rainfall in western, central, and peninsular India.
Monsoon Trough Dynamics
- Definition: A semi-permanent low-pressure area extending between Pakistan and the Bay of Bengal during the monsoon season.
- Oscillation: The trough shifts north–south within the season, influencing rainfall patterns.
a. Trough in the South
- Leads to more rainfall over central, eastern, and peninsular India.
b. Trough in the North
- Causes Himalayan foothills to get more rainfall, but rest of India experiences reduced rainfall.
Persistence of an Off-shore Trough
- A shallow low-pressure trough developed along the west coast between south Gujarat and north Kerala.
- Persisted for over a week, causing continuous rainfall in coastal and adjacent inland regions.
Intermittent Wind Shear Zone Development
- Formation of a wind shear zone along 20° N latitude from central to peninsular India.
- Different wind speeds and directions along this zone create atmospheric instability, enhancing rainfall activity.
Low-Pressure System over Bay of Bengal
- A low-pressure area developed over the west-central Bay of Bengal, off the Odisha coast.
- Acts as a rain-bearing system, feeding moisture and causing widespread rains in eastern and central India.
Impact of Climate Change on Rainfall Patterns in India
Altered Cyclonic Activity
- Increased frequency & intensity of cyclones in the Indian Ocean due to rising sea surface temperatures.
- Coastal states such as Odisha, West Bengal, and Andhra Pradesh experience intense rainfall and storm surges linked to cyclonic events.
Increased Variability in Rainfall
- Greater inter-annual and intra-seasonal variability in rainfall patterns.
- Unpredictable monsoon onset and withdrawal, making water resource management and agricultural planning difficult.
Intensified Monsoon Events
- Monsoon seasons now characterized by heavier rainfall in shorter durations.
- Leads to flash floods, urban waterlogging, and soil erosion in vulnerable regions.
Increase in Northeast Monsoon Rainfall
- Retreating monsoon rainfall increased by:
- Over 10% in ~80% of tehsils in Tamil Nadu.
- 44% in Telangana and 39% in Andhra Pradesh in the last decade.
- Odisha, West Bengal, Maharashtra, and Goa also recorded notable increases during this period.
Extended Droughts and Dry Spells
- Shift in monsoon patterns has led to prolonged dry periods.
- Adversely affects agriculture, groundwater recharge, and rural livelihoods.
Regional Disparities in Rainfall
- Since 1981–2011 baseline, studies show:
- 30% increase in southwest monsoon rainfall in traditionally dry areas (Rajasthan, Gujarat, Konkan, central Maharashtra, Tamil Nadu).
- 30% decrease in rainfall in traditionally high-rainfall states like Assam and Meghalaya.
Impact of Monsoons on Life in India
Positive Impacts
- Agricultural Dependence
- About 64% of the Indian population depends on agriculture for livelihood.
- Agriculture in India is largely monsoon-dependent, especially in regions lacking developed irrigation facilities.
- Agricultural Prosperity
- Timely and adequately distributed rainfall is crucial for crop productivity.
- Failure of monsoon adversely affects crop yields, particularly in rainfed areas.
- Crop Diversity
- Regional variations in monsoon climate allow cultivation of a wide variety of crops across different regions.
- Cultural and Lifestyle Influence
- Differences in monsoon patterns are reflected in food habits, clothing styles, and housing designs across India.
- Hydroelectric Power Generation
- Monsoon rains recharge dams and reservoirs, ensuring water availability for hydropower generation and irrigation.
- Winter Rainfall Benefits
- Temperate cyclones bring winter rainfall to North India, which is highly beneficial for Rabi crops such as wheat and mustard.
Negative Impacts
- Rainfall Variability
- Uneven distribution of rainfall causes droughts in some areas and floods in others, affecting livelihoods and infrastructure.
- Soil Erosion
- Sudden monsoon bursts lead to severe soil erosion, reducing soil fertility and affecting agricultural productivity.
- Landslides in Hilly Areas
- Heavy monsoon rains in hilly terrain trigger landslides, damaging natural ecosystems and human-made infrastructure.
- Such events disrupt transport, communication, and economic activities, impacting human life socially and economically.
Challenges of Southwest Monsoon Extension in India
Agricultural Disruptions
- Delayed harvests of Kharif crops due to extended rainfall.
- Sowing delays for Rabi crops like wheat and mustard.
- Increased crop damage from unseasonal rains and waterlogging.
Flooding & Waterlogging
- Urban flooding in cities with poor drainage systems.
- Rural waterlogging leading to soil degradation.
Economic Losses
- Post-harvest losses due to moisture-related spoilage.
- Disruption in supply chains for perishable goods.
Public Health Concerns
- Increase in vector-borne diseases like malaria and dengue due to stagnant water.
- Higher cases of water-borne diseases like cholera.
Hydrological Imbalance
- Excessive rainfall in certain regions vs deficient rains elsewhere.
- Impact on reservoir management and hydroelectric power scheduling.
Climate Change Linkages
- Changing monsoon patterns attributed to Indian Ocean warming & ENSO variability.
- Greater uncertainty in seasonal predictions.
Way Forward
Improved Forecasting Systems
- Strengthen IMD’s monsoon prediction models with AI and satellite data.
- Regional forecasts for state-specific planning.
Climate-Resilient Agriculture
- Promote short-duration crop varieties to avoid harvest overlaps.
- Encourage crop diversification to reduce risk.
Water Management Strategies
- Enhance reservoir operation protocols to handle unseasonal inflows.
- Promote rainwater harvesting and groundwater recharge.
Urban Flood Mitigation
- Upgrade stormwater drainage networks.
- Implement flood zoning regulations.
Disaster Preparedness
- Pre-position relief materials in vulnerable districts.
- Improve early warning systems at the local level.
Climate Adaptation Policies
- Integrate monsoon variability into agricultural and infrastructure planning.
- Strengthen National Monsoon Mission initiatives.
Recent Indian Initiatives for Monsoon Prediction
Accurate and timely monsoon prediction is vital for farmers (for sowing, harvesting, irrigation, and pesticide application) and policymakers (for payment of compensation, deciding Minimum Support Price, and disaster preparedness).
Monsoon Mission of India
- Launched by: Ministry of Earth Sciences in 2012.
- Approach:
- Adoption of high-resolution models, super parameterizations, and data assimilation techniques.
- Quantitative improvement in forecasting skill for the India Meteorological Department (IMD).
- Milestone:
- First operational use of Monsoon Mission Dynamical Model for the 2017 monsoon seasonal forecast.
Objectives:
- Improve Seasonal and Intra-seasonal Monsoon Forecasts.
- Enhance Medium Range Forecasting capabilities.
Agro-Meteorological Advisory Services (AAS)
- Collaboration: IMD + Indian Council of Agricultural Research (ICAR).
- Coverage: 130 agro-met field units across districts.
- Delivery: Advisories issued in vernacular languages for farmers.
- Applications in Farm Operations:
- Sowing management during delayed onset of rains.
- Changing crop variety in case of rainfall delays.
- Pesticide spraying for disease control based on rainfall occurrence.
- Irrigation management during heavy rainfall forecasts.
Flood Forecasting Support
- IMD provides meteorological data to the Central Water Commission (CWC) for issuing flood warnings.
Indo-US Monsoon Research Expedition (2018)
- Vessel: Indian Ocean Research Vessel 'Sagar Nidhi'.
- Objective: Study the Bay of Bengal’s role in the Southwest Monsoon.
- Method: Collection of oceanographic and meteorological data to refine prediction models.
National Supercomputing Mission
- Purpose: Bridge computational gaps for timely and accurate monsoon forecasts.
- Benefit: Enhanced data processing and model simulation capabilities for IMD.
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Source: Down to earth
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Practice Question
Q. Discuss the causes and impacts of the prolonged Southwest monsoon in India.
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Frequently Asked Questions (FAQs)
The seasonal wind system that brings the majority of India’s rainfall between June and September, caused by differential heating of land and sea.
It generally hits Kerala around June 1, advancing northwards to cover the entire country by mid-July.
- Arabian Sea Branch – brings heavy rain to the west coast.
- Bay of Bengal Branch – moves towards northeast and central India.
