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Context:
The India Meteorological Department (IMD) declared the monsoon onset over Kerala on May 24, 2025, eight days ahead of its normal date schedule of June 1. Earliest monsoon onset in over a decade (last such early arrival in 2009)
What is the Southwest Monsoon?
- Seasonal wind system that brings ~70% of India’s annual rainfall.
- Active during June to September.
- Crucial for agriculture, drinking water, hydropower, and overall economy.
Criteria for Monsoon Onset Over Kerala (IMD)
- Rainfall Criterion:
- ≥2.5 mm rainfall for 2 consecutive days at 60% of 14 designated stations in Kerala and Lakshadweep.
- Wind Field Criterion:
- Westerly winds up to 600 hPa with speeds of 15–20 knots at 925 hPa.
- OLR Criterion:
- Outgoing Longwave Radiation (OLR) < 200 W/m², indicating active convection/cloud formation.
Causes of Early Monsoon in 2025
| Meteorological Factor | Impact |
|---|---|
| Madden-Julian Oscillation (MJO) | Boosted convection; observed from May 13 over south Andaman Sea |
| Mascarene High Intensification | Directed strong moist winds from southern Indian Ocean |
| Convection Surge | Enhanced cloud buildup and early rainfall |
| Somali Jet Strengthening | Strengthened cross-equatorial flow towards Kerala |
| Heat Low Formation | Persistent low pressure over NW India drew in moist air |
| Monsoon Trough Activation | Triggered rainfall across central and northeast India |
Implications of Early Monsoon Onset
Positive Outcomes:
- Early sowing of Kharif crops (e.g., paddy, pulses) in key states.
- Reservoirs replenished earlier in water-scarce regions like Karnataka, Tamil Nadu.
- Cooler summer trends in South India due to early rains.
Challenges/Risks:
- Urban Flooding Risk: Cities like Bengaluru may be unprepared for early heavy rainfall.
- Forecast Uncertainty: Early onset ≠ longer or stronger monsoon; water planning challenges.
- Weather Pattern Shifts: May alter seasonal temperature and rainfall distribution.
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