Early Monsoon Onset in India
The India Meteorological Department (IMD) announced the early onset of the southwest monsoon over Kerala on May 24, 2025. This event occurred eight days ahead of the expected date of June 1. The early monsoon is as it marks the beginning of the four-month rainy season, which contributes over 70% of India’s annual rainfall. This early onset is rare, with the last occurrence in 2009.
Criteria for Monsoon Onset
The IMD uses specific criteria to declare the monsoon onset. These include:
- Rainfall Measurement: At least 60% of 14 key meteorological stations must report 2.5 mm or more of rainfall for two consecutive days.
- Wind Field Analysis: Westerly winds must be maintained at a depth of up to 600 hPa, with wind speeds between 15-20 knots.
- Outgoing Longwave Radiation (OLR): Satellite-derived OLR values must fall below 200 Watts per square metre to indicate sufficient atmospheric moisture.
Factors Influencing Early Onset
Several atmospheric and oceanic factors contributed to the early monsoon this year:
- Madden-Julian Oscillation (MJO): This ocean-atmospheric phenomenon enhances rainfall when in a favourable phase, influencing weather patterns across India.
- Mascarene High: A high-pressure area in the south Indian Ocean affects rainfall intensity along India’s west coast.
- Increased Convection: Enhanced vertical transport of heat and moisture has led to increased rainfall, particularly in northern regions.
- Somali Jet: This low-level wind band strengthens monsoon winds as it crosses the Arabian Sea.
- Heat-Low Development: A low-pressure zone over the Arabian Sea draws moist air, facilitating rainfall.
- Monsoon Trough: This elongated low-pressure area influences rainfall distribution during the monsoon season.
Regions Affected by Monsoon Onset
The southwest monsoon made impact on various regions on its onset day. It covered:
- The southwest and east-central Bay of Bengal
- Southern and central Arabian Sea
- Kerala, Lakshadweep, and Mahe
- Parts of Karnataka and Tamil Nadu
The monsoon’s early arrival in Karnataka was particularly notable, occurring over ten days ahead of the usual schedule.
Monsoon Progression
Following the initial onset, the monsoon advanced into additional regions. By May 25, it extended into parts of west-central and east-central Arabian Sea, Goa, Maharashtra, and further into northeast India, including Mizoram, Manipur, and Nagaland. The Northern Limit of Monsoon now marks its progress through various states.
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