Chennai vs Mangalore rainfall and the Arctic’s role in the monsoon – OPSC OAS Mains GS Paper I PYQ

📝Model Answer

Introduction

Chennai (east coast, Tamil Nadu) and Mangalore (west coast, Karnataka) lie at nearly the same latitude, yet their rainfall regimes differ sharply — Mangalore receives the bulk of its rain during the South-West monsoon (June-September), while Chennai receives most of its rainfall during the North-East monsoon (October-December) — a contrast rooted in relief, wind direction, and the Western Ghats.

Why Chennai and Mangalore Receive Different Rainfall Patterns

1. Position relative to the Western Ghats: Mangalore lies on the windward side of the Western Ghats, directly facing the moisture-laden South-West monsoon winds from the Arabian Sea, resulting in heavy orographic rainfall.
2. Rain-shadow effect on Chennai: Chennai lies on the leeward side of the Western Ghats during the South-West monsoon, so the winds lose most of their moisture on the western slopes before reaching the east coast, leaving Chennai relatively dry in June-September.
3. Role of the retreating North-East monsoon: After withdrawing from North India, the monsoon winds blow from the north-east, picking up moisture over the Bay of Bengal and striking Tamil Nadu’s coast directly, giving Chennai its main rainfall season (October-December).
4. Direct wind-coast alignment: The Coromandel Coast (Chennai) is oriented to directly face the Bay of Bengal branch of the retreating monsoon, whereas Mangalore, facing the Arabian Sea, does not benefit significantly from this NE monsoon phase.
5. Influence of tropical cyclones and depressions: Chennai’s October-December rainfall is also augmented by cyclonic depressions forming in the Bay of Bengal during the retreating monsoon period, a feature largely absent on the west coast at that time.

Role of the Arctic Sea in India’s Monsoon System

1. Emerging teleconnection: Recent research (NCPOR and international studies) has established a teleconnection between Arctic sea ice extent and the Indian Summer Monsoon Rainfall (ISMR), linking polar climate change to tropical monsoon variability.
2. Declining Arctic sea ice and monsoon variability: Reduced Arctic sea ice, especially in the Barents-Kara Sea region, is associated with delayed monsoon onset and greater unpredictability in rainfall timing.
3. Regionally differentiated impact: Studies indicate that reduced Central Arctic sea ice correlates with decreased rainfall over western and peninsular India but increased rainfall over central and northern India, altering the traditional spatial distribution of monsoon rains.
4. Mechanism via Rossby waves: Melting Arctic ice enhances ocean-to-atmosphere heat transfer, triggering atmospheric Rossby waves that alter pressure systems over Europe and Central Asia, which in turn influence the subtropical jet stream steering the monsoon.
5. Two-way relationship: Research also shows the Indian monsoon itself influences Arctic sea ice through heat flux propagation via tropical wave trains, indicating a bidirectional Arctic-monsoon linkage rather than a one-way effect.
6. Implications for forecasting: As the Arctic warms nearly four times faster than the global average, its growing influence is likely to make monsoon prediction more complex, necessitating integration of Arctic sea-ice data into India’s monsoon forecasting models.

Conclusion