Characteristics of the Indian Monsoon
When we talk about the Indian monsoon, we’re not just talking about rain — we’re talking about the lifeline of our agriculture, water resources, and even the rhythm of our rural economy. In India, monsoon is both a climatic phenomenon and an economic driver.
Seasonal Reversal – The Identity of Monsoon
The term monsoon itself comes from the Arabic word mausim (meaning “season”), and its core characteristic is seasonal reversal of winds.
- South-West Monsoon (June–September):
During summer, the land heats up faster than the ocean. A low-pressure zone forms over the Indian subcontinent, drawing in moisture-laden winds from the Indian Ocean. These winds blow from the south-west direction, bringing heavy rainfall. - North-East Monsoon (October–December):
After September, the pattern flips. Now, the land cools faster than the ocean, reversing the pressure gradient. Winds blow from the north-east, picking up moisture from the Bay of Bengal, especially benefiting Tamil Nadu and parts of coastal Andhra Pradesh.
Variability – Why Monsoon is Never Uniform
Monsoon rains are unpredictable — both spatially (place to place) and temporally (time to time).
- Some regions drown in rain, others face droughts, sometimes even within the same season.
- Example: Rajasthan and Gujarat remain largely arid despite the monsoon season.
This variability is why farmers in one district may celebrate bumper harvests while neighbouring districts suffer crop loss.
Long Period Average (LPA) – The Benchmark
The IMD measures each year’s rainfall against a benchmark called the Long Period Average — the average monsoon rainfall over 50 years.
- Current LPA (1951–2000) = 89 cm
- Deficient: < 90% of LPA
- Normal: 96–104% of LPA
- Excess: > 110% of LPA
| Category | Rainfall (% of LPA) |
|---|---|
| Deficient | < 90 |
| Below Normal | 90–96 |
| Normal | 96–104 |
| Above Normal | 104–110 |
| Excess | > 110 |
This statistical classification is key for crop planning, reservoir management, and even GDP predictions.
Spatial Distribution – Uneven Rain Map
- High rainfall zones: Western Ghats, North-East India, Meghalaya (250–400 cm).
- Moderate rainfall: Central India, Gangetic Plains (100–200 cm).
- Low rainfall: Western Rajasthan, Ladakh (< 100 mm).
The Himalayas play a critical role through orographic uplift — winds hitting the slopes rise, cool, and release moisture.
Monsoon Break – The Pause Button
The monsoon trough is a belt of low pressure formed due to the northward shift of ITCZ.
- Normal: Brings widespread rain to central and northern India.
- Break: When the trough shifts northwards to Himalayan foothills → heavy rain in the hills but reduced rainfall in most of India.
- Breaks can last from days to weeks.
- Resumption happens when the trough shifts southward.
Key causes of breaks:
- El Niño: Weakens monsoon.
- Negative IOD: Cooler western Indian Ocean → reduced moisture inflow.
- Cyclones in Arabian Sea: Divert moisture westwards.
- Madden-Julian Oscillation (MJO): Suppressed phase reduces rainfall.
Complexity – Why Prediction is Hard
The Indian monsoon is influenced by multiple systems:
- ENSO (El Niño, La Niña)
- Indian Ocean Dipole
- Jet Streams (Sub-Tropical, Somali Jet)
- Atlantic Zonal Mode (Atlantic Niño)
- Madden-Julian Oscillation
- Dust, aerosols, and pollution
Even minor changes in any of these can alter rainfall patterns dramatically.
Monsoon Prediction – IMD’s Models
Forecast Types:
- Nowcast (< 24 hrs)
- Short Range (up to 3 days)
- Medium Range (3–10 days)
- Extended Range (10–30 days)
- Long Range (seasonal scale)
Approaches:
- Statistical Models: Link historic climate parameters (like SST gradients, Eurasian snow cover) with rainfall records. Reliable for broad trends but often miss extreme events.
- Dynamic Models: Simulate weather using real-time data. More realistic but need dense data networks (a current weakness in India).
- High-Resolution Computer Models: Provide localised forecasts, useful for farmers and disaster planning.
Even today, statistical models remain IMD’s backbone, making seasonal forecasts prone to errors.
Impact on Agriculture – Why Monsoon is a Lifeline
- Over half of India’s farmland is rain-fed.
- Adequate rainfall = good soil moisture, healthy crops like rice, wheat, sugarcane, cotton.
- Deficient rainfall = droughts, crop failure, food shortage, rural distress.
In essence, the monsoon decides not just the harvest but also rural employment, inflation, and even political stability.
