Context: The India Meteorological Department (IMD) declared the onset of the southwest monsoon over Kerala on May 24, 2025, eight days ahead of the usual June 1 date—one of the earliest arrivals since 2009.
About Early Arrival of Monsoon
The Southwest Monsoon is a seasonal wind system that brings over 70% of India’s annual rainfall during June–September, crucial for agriculture, water resources, and economic activity.
When Is Monsoon Onset Declared?
The IMD confirms monsoon onset over Kerala using three criteria:
- Rainfall Trigger: 60% of 14 designated stations must record ≥2.5 mm rainfall for two consecutive days.
- Wind Field Criteria: Westerlies extend up to 600 hPa with 15–20 knots at 925 hPa.
- Outgoing Longwave Radiation (OLR): Must fall below 200 W/m², indicating active convection and cloud cover.
Factors Behind Early Monsoon Onset 2025
- Madden‑Julian Oscillation (MJO): Active phase over the Indian Ocean boosted convection and rainfall (impact noted from May 13 over the South Andaman Sea).
- Mascarene High Intensification: Strengthened high-pressure system over the southern Indian Ocean directed moist winds toward India’s coast.
- Convection Surge: Rising heat and moisture enhanced vertical cloud build-up, triggering early rains.
- Somali Jet Strengthening: Enhanced cross‑equatorial winds accelerated monsoon currents into Kerala and Karnataka.
- Heat Low Formation: Persistent low pressure over Pakistan and northwest India created suction for monsoonal air.
- Monsoon Trough Activation: Elongated low-pressure zone from the Arabian Sea to the Bay of Bengal activated widespread rainfall.
Consequences of Early Monsoon Onset
- Agriculture Boost: Early sowing of Kharif crops (paddy, pulses) can commence, aiding crop calendar adherence.
- Reservoir Recharge: Early rains replenish water levels in drought‑prone states like Tamil Nadu and Karnataka.
- Urban Flooding Risk: Cities such as Bengaluru may face flash floods if unprepared for early downpours.
- Altered Weather Patterns: Cooler summer trends observed in South India due to early cloud cover.
- Forecast Challenges: Early onset does not guarantee a longer or stronger season, complicating water resources planning.
Conclusion
The early southwest monsoon onset of 2025 resulted from synergistic ocean‑atmosphere dynamics including MJO and Somali Jet strengthening. While it offers advantages for agriculture and water security, it also demands urban preparedness and adaptive planning for variable rainfall patterns.
