Early Onset of the Southwest Monsoon
What caused an ‘early’ monsoon onset in India this year?
Context: In a significant development for India’s agricultural and economic calendar, the India Meteorological Department (IMD) announced the early onset of the southwest monsoon over Kerala on May 24, marking its earliest arrival in 15 years.
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- The monsoon’s arrival, typically expected around June 1, heralds the start of the crucial four-month rainy season that brings over 70% of India’s annual rainfall.
- The last time the monsoon hit Kerala this early was in 2009, when it arrived on May 23.
- The early onset this year is attributed to a combination of favorable large-scale atmospheric and oceanic factors.
How IMD Declares Monsoon Onset Over Kerala
The IMD uses a specific set of meteorological criteria to confirm the onset of the southwest monsoon:
- Rainfall: At least 60% of 14 designated stations across southern India, including Thiruvananthapuram, Kochi, Kozhikode, and Mangalore, must report 2.5 mm or more rainfall for two consecutive days.
- Wind Field: Westerly winds should be present up to 600 hPa pressure levels, with wind speeds ranging between 15-20 knots (27-37 km/hr).
- Outgoing Longwave Radiation (OLR): OLR levels must fall below 200 W/m², indicating increased cloudiness and atmospheric convection.
- Once these parameters are met for two consecutive days, IMD declares the monsoon’s onset.
Monsoon Covers Broad Regions on Onset Day
- This year, the southwest monsoon covered a wide area on the day of its onset. According to IMD, the monsoon reached:
- Kerala, Lakshadweep, and Mahe
- Southern and coastal Karnataka
- Tamil Nadu (except northern regions)
- Parts of Mizoram and northeast India
- South and central Arabian Sea
- East-central and southwest Bay of Bengal
- By Sunday (May 25), it had further advanced into:
- Goa and parts of Maharashtra
- Manipur, Nagaland, and more of Mizoram
- North and west-central Bay of Bengal
- The Northern Limit of Monsoon (NLM), which tracks its progress, now extends through Devgad, Belagavi, Haveri, Mandya, Dharmapuri, Chennai, Aizawl, and Kohima.
What Caused the Early Monsoon in 2025?
Several favourable weather systems and oceanic conditions led to the unusually early monsoon onset:
- Madden-Julian Oscillation (MJO): A dominant intra-seasonal atmospheric phenomenon, MJO’s favourable phase this year enhanced convection and cloud formation over the Indian Ocean, boosting monsoon activity.
- Mascarene High: A high-pressure zone near the Mascarene Islands in the South Indian Ocean intensified, channelling moisture-laden winds towards the Indian subcontinent.
- Strong Somali Jet: The low-level Somali Jet Stream, a cross-equatorial wind current, brought strong westerly winds to the Arabian Sea, accelerating monsoon circulation.
- Increased Convection: A surge in convective activity—vertical transport of heat and moisture—helped in the formation of rain-bearing systems across southern India.
- Heat-Low Formation: A low-pressure area over Pakistan and adjoining regions acted as a suction zone, drawing in moist air from the Indian Ocean and strengthening the monsoon flow.
- Monsoon Trough and Onset Vortex: A well-established monsoon trough and favourable onset vortex over the Arabian Sea further supported the early rainfall over Kerala and adjoining regions.
Significance of Early Monsoon Onset for India
- An early and widespread monsoon onset has major implications for agriculture, especially in southern and central India.
- It allows farmers to begin sowing operations early and boosts rural economic activity.
- Additionally, early rainfall replenishes reservoirs, improves groundwater levels, and reduces heat stress.
The early arrival of the 2025 southwest monsoon over Kerala, declared on May 24, is a promising start to the season. Backed by strong atmospheric and oceanic signals like the MJO, Somali Jet, and Mascarene High, the monsoon has already made substantial progress into multiple regions. As it continues its advance, the early onset is expected to bring positive momentum to India’s agriculture and water resources.
