Monsoon System of India

Subject: Geography | Unit: Climate & Weather | Topic: Monsoon System Exam: AP Group 2 (APPSC)

Introduction

The monsoon is the defining climatic feature of India — the country receives 75-90% of its annual rainfall during the Southwest monsoon (June-September). The word "monsoon" comes from Arabic "mausim" meaning season, referring to the seasonal reversal of wind direction. India's agriculture, water resources, hydropower, and economy are fundamentally dependent on the monsoon. Understanding its mechanism, onset/withdrawal patterns, branches, and phenomena like El Nino is critical for APPSC exams.

Context

The monsoon unifies India's diverse climate. Despite spanning 30 degrees of latitude and featuring deserts, rainforests, plains, and mountains, India's climate is broadly classified as "monsoon type." Modern understanding combines the classical thermal theory (differential heating of land and sea) with dynamic factors — the ITCZ migration, jet stream behaviour, and Somali Jet — to explain monsoon mechanics.

Core Content

Four Seasons of India (IMD Classification)

Monsoon Mechanism — Key Theories

Classical/Thermal Theory: Differential heating of land and sea — summer: land heats > low pressure > onshore winds (sea to land); winter: land cools > high pressure > offshore winds (land to sea).

Modern/Dynamic Theory:

• ITCZ (Inter-Tropical Convergence Zone) shifts to 20-25 deg N over Gangetic Plain in summer = Monsoon Trough
• Position of trough determines rainfall distribution
• Active monsoon: trough near foot of Himalayas
• Monsoon break: trough shifts north (less rain over plains)

Jet Stream Theory:

• Sub-Tropical Westerly Jet (STJ) present over India in winter acts as stabilizing "lid"
• By late May: Tibetan Plateau heats up > STJ shifts north of Himalayas
• Tropical Easterly Jet (TEJ) develops at ~14 km altitude, acts as "exhaust fan"
• Burst of monsoon = withdrawal of STJ + arrival of TEJ

Somali/Low-Level Jet: Cross-equatorial flow deflected by East African highlands; brings moisture from Indian Ocean; crucial for Arabian Sea branch.

Monsoon Onset Timeline

Monsoon Withdrawal Timeline

Two Monsoon Branches

Arabian Sea Branch (Stronger):

• Hits Western Ghats: heavy orographic rainfall on windward side
• Rain shadow on leeward side (Mumbai ~2,400 mm vs Pune ~700 mm)
• One arm enters Narmada-Tapi trough > Central India
• Another arm parallel to Aravallis > Punjab (little rain in Rajasthan)

Bay of Bengal Branch:

• Strikes Myanmar coast and NE India first
• Deflected westward by Himalayas along Ganga Plain
• Meghalaya Hills: Mawsynram (>11,000 mm, world's wettest place)

Rainfall Distribution

El Nino, La Nina, and IOD

ENSO: El Nino = warming of eastern Pacific; weakens Walker Cell; SOI negative = El Nino.

IOD: Temperature difference between western and eastern Indian Ocean. Positive IOD can partly counter El Nino (e.g., 2019: El Nino + positive IOD = 110% of LPA).

Special Phenomena

• Western Disturbances: Temperate cyclones from Mediterranean; bring winter rainfall to NW India; important for wheat (Rabi)
• Retreating Monsoon (NE Monsoon): Winds reverse Oct-Nov; Tamil Nadu gets ~50% rainfall from NE monsoon
• October Heat: Post-monsoon high temp + humidity in northern India
• Nor'westers (Kalbaisakhi): Thunderstorms in NE India, West Bengal (April-May)
• Mango Showers: Pre-monsoon showers in Kerala, Karnataka
• Cherry Blossoms: Pre-monsoon showers helping coffee blossoming in Karnataka

AP Connection

• AP gets rainfall from both SW monsoon (June-Sep) and NE monsoon (Oct-Dec)
• Coastal AP: SW monsoon brings bulk of rainfall (June-September)
• Rayalaseema: semi-arid, often drought-prone; rain shadow region (~550-750 mm)
• NE monsoon important for southern coastal districts (Nellore, Prakasam)
• AP coastline highly vulnerable to Bay of Bengal cyclones during retreating monsoon
• Average annual rainfall of AP: ~900-1,000 mm
• Anantapur district is one of India's most drought-prone (~550 mm)

Key Points Summary

1. Monsoon from "mausim" (Arabic) = season; seasonal reversal of wind direction
2. India receives 75-90% of annual rainfall during SW monsoon (June-September)
3. ITCZ shifts to 20-25 deg N in summer = Monsoon Trough
4. STJ withdrawal + TEJ arrival = burst of monsoon
5. June 1: Kerala onset; October 20: Kerala last to withdraw
6. Arabian Sea branch is STRONGER; Bay of Bengal branch hits NE India first
7. Mawsynram (>11,000 mm) > Cherrapunji = world's wettest
8. El Nino = weak monsoon; La Nina = strong monsoon
9. Positive IOD boosts monsoon; can counter El Nino effect
10. Tamil Nadu gets ~50% rainfall from RETREATING monsoon (NE winds over Bay of Bengal)
11. Western Disturbances from Mediterranean bring winter rain to NW India (important for wheat)
12. Monsoon break = trough shifts to Himalayan foothills; dry spell over plains
13. Rain shadow: Pune (700 mm) on leeward vs Mumbai (2,400 mm) on windward of Western Ghats
14. AP gets rainfall from both SW and NE monsoons
15. Rayalaseema is semi-arid and drought-prone; Anantapur driest in AP

Exam Strategy

• Onset/withdrawal dates are frequently asked — June 1 (Kerala), July 15 (full India), Sep 1 (NW withdraws first)
• El Nino vs La Nina: El Nino = warm Pacific = weak monsoon; La Nina = cool Pacific = strong monsoon
• Two branches: Arabian Sea (stronger, hits Western Ghats) vs Bay of Bengal (hits NE first, deflected by Himalayas)
• AP-specific: Both SW and NE monsoons; Rayalaseema drought-prone; cyclone vulnerability on coast
• Trick: Arabian Sea branch is STRONGER (not Bay of Bengal, even though BoB gets more cyclones)

Key Terms Glossary