Definition of Monsoon
A monsoon is a seasonal reversing wind system accompanied by corresponding changes in precipitation patterns. The term derives from the Arabic word "Mausim," meaning "season."
Technically, monsoons represent a cyclic reversal of pressure and wind systems occurring annually, characterized by a shift in atmospheric circulation patterns. Modern meteorological understanding defines monsoons as large-scale seasonal wind systems blowing persistently in one direction over vast areas of the globe, which reverse direction with the change of season.
At their fundamental level, monsoons operate as land and sea breezes on a planetary scale, driven by the differential heating of land and oceanic surfaces in response to incoming solar radiation.
Characteristics of Indian Monsoon
Temporal and Spatial Distribution
The Indian monsoon operates in two distinct phases:
- Southwest Monsoon (June to September) - Accounts for approximately three-fourths (75%) of India's total annual rainfall
- Northeast Monsoon (October to December) - Significant for southern peninsular India
Monsoon Progression Timeline
June 1st: Arrival on Kerala coast
Mid-July: Covers most of India
September onwards: Begins retreat
December: Complete withdrawal
Rainfall Variability and Distribution
The coefficient of variation (CV) demonstrates clear regional patterns:
- CV < 25%: Areas receiving over 100 cm annual rainfall (highly reliable)
- CV > 50%: Regions receiving less than 50 cm annual rainfall (highly variable)
World's Highest Rainfall: Mawsynram in Meghalaya's Khasi Hills records the world's highest average annual rainfall due to intense orographic precipitation.
Wind Patterns and Atmospheric Circulation
During the southwest monsoon, winds blow from southwest to northeast due to the pressure gradient between the cooler Indian Ocean and heated Indian landmass. The monsoon manifests through two primary branches:
- Arabian Sea Branch: Travels westward along the Gangetic Plains toward Punjab
- Bay of Bengal Branch: Moves up the Brahmaputra valley in northeastern India
Temperature and Pressure Changes
The arrival of southwest monsoons brings a significant temperature drop of 3°C to 6°C, occasionally reaching 5°C to 10°C in some regions. This sudden temperature change accompanies the arrival of warm, moisture-laden winds that replace the hot, dry pre-monsoon conditions.
Key Monsoon Features
- Sudden burst behavior rather than gradual onset
- Normal arrival date: June 1st in Kerala
- Creates low-pressure system over heated Tibetan Plateau
- 87% of plains' rainfall occurs during monsoon period
- Characteristic break periods during active monsoon season
Theories of Indian Monsoon
1. Thermal Concept (Halley's Theory)
Primary Cause: Differential heating of land and sea
Mechanism:
- Winter: Landmass cools rapidly → high pressure → outflow of cold, dry air
- Summer: Landmass heats rapidly → low pressure → inflow of moisture-laden oceanic air
Limitations:
- Cannot explain sudden burst of monsoons
- Fails to explain monsoon breaks
- Cannot account for spatial/temporal distribution
- Oversimplifies complex circulation patterns
2. Dynamic Concept (Flohn's Theory)
Proposed by: German meteorologist Flohn (1951)
Core Principle: Monsoons result from seasonal migration of planetary winds and pressure belts
Key Mechanism:
- ITCZ formation near equator (March-September)
- Southeast trades cross equator and deflect (Coriolis force)
- Transform into southwest monsoon winds
- Moisture-laden after traveling over warm Indian Ocean
Advantages: Successfully explains monsoon onset, breaks through ITCZ position changes, and provides realistic representation of monsoon variability
3. Jet Stream Theory
Key Jets:
- Subtropical Westerly Jet (STJ): 25-35°N, 12-14 km height, 150-300 km/h
- Tropical Easterly Jet (TEJ): 15°N over peninsular India
Monsoon Burst Mechanism:
- Northward shift of STJ by end of May
- Weakening of high pressure over NW India
- ITCZ pushes northward
- Emergence of easterly tropical jet
Critical Relationship: Monsoon strength directly determined by easterly tropical jet strength over central India
4. Tibetan Plateau Effect
Role: Elevated heat source profoundly influencing monsoon dynamics
Mechanism:
- Plateau heats rapidly in summer (2-3°C warmer than surroundings)
- Creates strong low-pressure system
- Attracts moisture-laden Arabian Sea and Bay of Bengal winds
Snow Cover Impact: Thick, widespread snow coverage over Tibet typically precedes weak monsoon years with reduced rainfall
Critical Finding: Intensity and duration of Tibetan Plateau heating directly correlates with Indian monsoon rainfall amounts
5. ITCZ Shifting and Monsoon Trough
Normal Position: Near equator
Monsoon Position: 20-25°N latitude (Indo-Gangetic Plain)
Characteristics:
- Area of ascending air
- Maximum cloud formation
- Heaviest rainfall in India
- Exceptionally active during July
Formation: Results from convergence of deflected southeast trades and air from subtropical high-pressure zone
6. Walker & Hadley Cell Dynamics
Hadley Cell:
- Ascending air above Tibetan Plateau
- Spreads southward to descending limb near Mascarene High
- Southwesterly return currents at surface = SW monsoon
Walker Cell: East-west circulation with ascending branch over semi-arid NW India/Pakistan
Critical Relationship:
- Good monsoons: Intense Hadley + Weak Walker
- Poor monsoons: Weak Hadley + Strong Walker
Role of El-Niño on Indian Monsoon
El-Niño represents a major oceanic-atmospheric phenomenon in the Pacific Ocean that significantly impacts Indian monsoon rainfall through teleconnections.
Mechanism of El-Niño Impact
El-Niño conditions involve warming of the eastern Pacific Ocean, which weakens the trade winds responsible for transporting moisture toward the Indian subcontinent.
The reduction in moisture transport and altered atmospheric circulation associated with El-Niño result in deficient rainfall across various parts of India. The relationship is inverse: El-Niño years typically coincide with below-average Indian monsoon rainfall and drought conditions.
Impacts on Monsoon Characteristics
- Weakened Monsoon Winds and Delayed Onset: Trade winds carrying moisture weaken substantially; monsoon onset may be delayed beyond June
- Deficient Rainfall: Below-average rainfall during monsoon season, particularly affecting central and northern India
- Regional Variations: Some southern Indian regions may receive near-normal or above-normal rainfall due to local circulation patterns
- Agricultural and Economic Consequences: Reduced summer crop production (rice, sugarcane, cotton, oilseeds); contributes to inflation and reduced GDP growth
- Temperature Extremes: Reduced cloud cover leads to increased temperatures and potential heatwave conditions
Historical Examples of El-Niño Impact
One of the strongest on record, yet India experienced normal monsoon rainfall—an apparent anomaly that led to the discovery of the Indian Ocean Dipole's moderating role. This demonstrated that El-Niño impacts can be modulated by other climate factors.
Caused severe droughts across Australia, Indonesia, India, and southern Africa. However, in 1983, a positive Indian Ocean Dipole simultaneously developed, which prevented the expected drought in India despite the strong El-Niño.
A relatively moderate event resulted in one of the worst droughts in India, demonstrating that El-Niño intensity does not always correlate directly with monsoon impact severity. This variability highlights the importance of understanding modulating factors like IOD.
Key Insight: El-Niño's impact on Indian monsoon is not deterministic. The presence of other climate phenomena like the Indian Ocean Dipole can significantly modify or even neutralize El-Niño's expected effects on rainfall.
Role of Indian Ocean Dipole (IOD) on Indian Monsoon
The Indian Ocean Dipole (IOD) represents a seesaw ocean-atmosphere system in the Indian Ocean analogous to El-Niño in the Pacific, discovered in 1999.
IOD Definition: Characterized by continuous changes in sea-surface temperature (SST) between the western Indian Ocean (off the African coast) and the eastern Indian Ocean (around Indonesia).
Positive IOD Phase Impacts
Enhanced Rainfall and Convergence Patterns
A positive IOD enhances rainfall along the African coastline and over the Indian subcontinent, particularly over central India. This enhancement occurs through anomalous convergence patterns strengthened over the Bay of Bengal.
Key Characteristics of Positive IOD:
- Modulation of Atmospheric Circulation: Anomalous warm conditions in western Indian Ocean create zones of convergence
- Moisture Transport: Positive tropospheric moisture anomalies with easterly transports over eastern Indian Ocean
- Spatial Rainfall Pattern: Meridional tripolar pattern with above-normal rainfall in central India and below-normal to the north and south
Negative IOD Phase Impacts
Suppressed Rainfall
A negative IOD suppresses rainfall over affected Indian regions as high temperatures and rainfall patterns reverse from positive IOD conditions.
Key Characteristics of Negative IOD:
- Zonal Dipolar Pattern: Positive rainfall anomalies in central/western parts and negative in eastern regions
- Moisture Transport Alterations: Negative tropospheric moisture anomalies with westerly transport anomalies over eastern Indian Ocean
- Convergence on Western Side: Different from positive IOD's Bay of Bengal convergence
Historical Examples of IOD Impact
India experienced near-normal monsoon rainfall (2% above normal) despite the record-breaking 1997-1998 El-Niño. This unexpected outcome was explained by the simultaneous positive IOD phase. The positive IOD's anomalous convergence over the Bay of Bengal neutralized the ENSO-induced anomalous subsidence, allowing normal moisture influx despite the powerful El-Niño.
Similar to 1997, the 1983 El-Niño coincided with a positive IOD that facilitated normal or excess rainfall over India despite unfavorable ENSO conditions.
Positive IOD in 1994 again produced good rainfall despite simultaneous El-Niño conditions, further confirming the IOD's buffering capacity against El-Niño drought impacts.
The combined effects of negative IOD and El-Niño in 1992 cooperatively produced deficient rainfall, demonstrating that when both factors align negatively, monsoon suppression is particularly severe.
June experienced 30% rainfall deficiency due to developing El-Niño effects, but a strong positive IOD that developed during late monsoon was so powerful that it compensated for the deficit rainfall, resulting in normal seasonal totals. This is one of the most striking recent examples of IOD's growing importance.
Interplay Between El-Niño and IOD
| Climate Condition | Monsoon Impact | Mechanism |
|---|---|---|
| El-Niño + Positive IOD | Near-normal to above-normal rainfall | Constructive interference - IOD moisture enhancement counteracts El-Niño suppression |
| El-Niño + Negative IOD | Severe drought | Destructive interference - both phenomena amplify monsoon suppression |
| Normal + Positive IOD | Above-normal rainfall | IOD enhances moisture convergence over India |
| Normal + Negative IOD | Below-normal rainfall | IOD suppresses moisture transport to India |
Evolving Relationship: Recent decades have witnessed a weakening of the traditional ENSO-monsoon relationship. The understanding of IOD as a moderating factor has enhanced monsoon rainfall forecasting accuracy. For instance, in 2023, while El-Niño was established in the Pacific, IMD anticipated approximately 80% probability for positive IOD conditions during June-August, suggesting potential for monsoon rainfall close to normal despite El-Niño's presence.
Spatial Distribution and Variability of Monsoon Rains
The Indian subcontinent exhibits enormous spatial variability in monsoon rainfall, ranging from some of the wettest regions on Earth to some of the driest. India receives an average annual rainfall of approximately 125 cm, with the southwest monsoon accounting for about 78% during June to September.
Classification of Rainfall Zones
| Rainfall Zone | Annual Rainfall | Regions | Characteristics |
|---|---|---|---|
| Very High Rainfall | >400 cm | Assam, Meghalaya, Arunachal Pradesh, Western Ghats (Kerala, Karnataka, Goa) | Mawsynram: 1,187 cm annually (world's highest); Cherrapunji: ~1,100 cm; Intense orographic precipitation |
| High Rainfall | 200-400 cm | West Bengal, Tripura, Nagaland, Manipur, Odisha, Konkan coast | Tropical rainforests; wet deciduous vegetation; Bay of Bengal and Arabian Sea branch influence |
| Moderate Rainfall | 100-200 cm | Indo-Gangetic Plains, Bihar, Jharkhand, eastern Madhya Pradesh, parts of Tamil Nadu/Andhra Pradesh | Influenced by monsoon trough location and depression paths |
| Low Rainfall | 50-100 cm | Parts of Maharashtra, Gujarat, Karnataka, Madhya Pradesh, Haryana, Punjab, western UP | Arid/semi-arid regions; tropical grasslands and dry deciduous forests |
| Very Low Rainfall | <50 cm | Thar Desert (Rajasthan) - Jaisalmer | Driest zone in India; extreme aridity |
Spatial Patterns and Controlling Mechanisms
1. Coastal-Interior Gradient
Rainfall decreases dramatically with increasing distance from the sea. Example: Western coast receives 250-400 cm annually, while the interior Deccan Plateau (100-150 km eastward) receives only 50-100 cm.
2. Orographic Barriers and Rain Shadow Effects
Western Ghats Effect:
- Windward side (western): 400-500 cm annually due to orographic precipitation
- Leeward side (eastern - Deccan Plateau): Minimal rainfall due to rain shadow effect
Himalayan Effect: Bay of Bengal branch deflected westward by Himalayan barrier, spreading moisture across Ganga plains with steady decline from east to west.
3. Monsoon Trough and Depression Tracks
The monsoon trough—a semi-permanent low-pressure feature at 20-25°N latitude—concentrates intense rainfall along its axis. Monsoon depressions (3-4 per month during June-September) originate in head Bay of Bengal and move northwestward, generating 80-100+ cm during depression days.
4. Eastern and Western Coastal Asymmetry
- Western coast: Exceptionally heavy rainfall due to Arabian Sea branch
- Eastern coast: More moderate rainfall distribution
- Tamil Nadu coast: Relatively dry during SW monsoon due to rain shadow; receives significant rainfall during NE monsoon (October-December)
5. Bay of Bengal vs Arabian Sea Branch Differences
Arabian Sea Branch: More powerful; entire current advances toward India; generates concentrated, intense rainfall along western coast
Bay of Bengal Branch: Only portion enters India (remainder goes to Myanmar/Thailand); provides steady but less concentrated rainfall over broader area
Coefficient of Variation and Rainfall Reliability
| Variability Category | CV Range | Regions | Rainfall Reliability |
|---|---|---|---|
| Low Variability | < 25-30% | Northeastern states (Tripura, Mizoram, Assam, Meghalaya), Western coastal belt, Peninsular India south of 17°N | Highly predictable and consistent year-to-year |
| Intermediate Variability | 30-60% | Central plains, Bihar, West Bengal, Odisha, peninsular regions | Moderate year-to-year fluctuations |
| High Variability | > 100% | Northwestern and central India (Rajasthan, Gujarat, interior Deccan Plateau) | Extremely unreliable; alternates between abundant rainfall and severe drought |
Temporal Distribution Within Monsoon Season
Monthly Concentration
Monsoon rainfall is highly concentrated within specific months:
- June: 20-25% of seasonal total (lower contribution)
- July: Maximum rainfall across most of India
- August: Remains substantial
- September: Declining rainfall
June: 556 mm | July: 659 mm (peak) | August: 427 mm | September: 252 mm
Active and Break Cycles
Active spells: Normalized rainfall anomalies exceed +1.0 standard deviations
Break spells: Anomalies fall below -1.0 standard deviations, persisting for at least 3 consecutive days
Recent trend: Increase in short break spells (3 days) and moderate active spells (4-7 days) since 1977; frequency and spatial extent of breaks has expanded
Intraseasonal Oscillations
Monsoon rainfall is modulated by 10-20 day and 30-60 day intraseasonal oscillations (ISO), creating alternating sequences of active and weak conditions due to northward propagation of tropical convergence zone.
Recent Changes in Spatial Rainfall Patterns (2012-2022)
- 55% of Indian tehsils witnessed an increase and 11% witnessed a decrease in SW monsoon rainfall
- Nearly 80% of Tamil Nadu tehsils experienced increased NE monsoon rainfall by more than 10%
- Significant increasing trends in OND rainfall along west coast tehsils (Maharashtra, Goa) and east coast (Odisha, West Bengal), attributed to increased cyclonic activity
- Approximately 64% of Indian tehsils experienced increase in frequency of heavy rainfall days by 1-15 days per year
- Important finding: Increases are concentrated in heavy rainfall events rather than steady precipitation, creating greater extremes
Conclusion: The spatial distribution of monsoon rainfall across the Indian subcontinent reflects the complex interplay of atmospheric circulation, topographic barriers, distance from moisture sources, and synoptic-scale weather systems. The variation spans from over 1,000 cm annually in Meghalaya to less than 50 cm in Rajasthan, with coefficients of variation exceeding 100% in northwestern regions versus less than 15% in northeastern regions. Recent trends indicate increasing rainfall concentration in extreme daily events, shifting seasonal timing, and regional reorganization of monsoon impacts.
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