Early Monsoon Onset in India 2025

Context

The southwest monsoon arrived in India on May 24, 2025, marking an early onset by about eight days compared to the usual June 1 schedule. This was the earliest arrival since 2009. The early onset is significant as the monsoon accounts for over 70% of India’s annual rainfall, playing a vital role in agriculture, water resources, and the overall economy.

Criteria for Declaring Monsoon Onset by IMD

The India Meteorological Department (IMD) declares the monsoon onset based on a set of meteorological criteria, which must be met for two consecutive days after May 10:

• Rainfall: At least 60% of 14 designated weather stations in Kerala and nearby areas must record rainfall of 2.5 mm or more.
• Wind Field: Westerly winds (blowing from west to east) must prevail up to the 600 hPa pressure level with speeds between 15 to 20 knots (27–37 km/h) at 925 hPa.
• Outgoing Longwave Radiation (OLR): OLR values, which indicate atmospheric heat and moisture conducive to rainfall, should be below 200 watts per square meter in the region between 5°N to 10°N latitude and 70°E to 75°E longitude.

Once these conditions are satisfied for two consecutive days, IMD officially declares the monsoon onset.

Factors Causing the Early Monsoon Onset in 2025

Several large-scale atmospheric and oceanic factors contributed to the early arrival of the monsoon this year:

1. Madden-Julian Oscillation (MJO)

• The MJO is a large-scale, eastward-moving tropical atmospheric disturbance involving clouds, winds, rainfall, and pressure changes.
• It completes a global cycle every 30–60 days and has two phases: an active phase with enhanced convection and rainfall, and a suppressed phase with reduced rainfall.
• In 2025, the MJO was in a favorable active phase over the Indian Ocean, enhancing convection and moisture convergence, which accelerated the onset of the monsoon over Kerala and adjoining regions.

2. Mascarene High

• The Mascarene High is a high-pressure system over the Mascarene Islands in the southern Indian Ocean.
• Its intensification during the monsoon period strengthens the pressure gradient between the Indian Ocean and the Indian landmass, facilitating the flow of moist westerly winds towards India.
• This enhanced pressure system helped direct moist winds to the west coast of India, supporting early monsoon rains.

3. Somali Jet

• The Somali Jet is a low-level, cross-equatorial wind originating near Mauritius and Madagascar that strengthens during May.
• A strong Somali Jet accelerates the transport of moisture-laden winds from the southern hemisphere to the Arabian Sea and the west coast of India.
• In 2025, the Somali Jet was stronger than usual, aiding the early advance of monsoon currents over Kerala and Karnataka.

4. Convection and Low-Pressure Systems

• Increased convective activity, which involves the vertical movement of heat and moisture, led to cloud formation and rainfall over southern India.
• A low-pressure area over the Arabian Sea and a trough over Vidarbha enhanced moisture inflow and atmospheric instability, facilitating the monsoon’s early progress.
• An elongated low-pressure zone stretching from the Arabian Sea to the Bay of Bengal activated rainfall over central and northeast India.

5. Neutral ENSO Conditions

• The absence of El Niño or La Niña (neutral ENSO phase) created a favorable environment for the monsoon’s timely or early onset.
• Neutral ENSO conditions generally support normal or above-normal monsoon rainfall.

Implications of Early Monsoon Onset

• Agriculture: Early monsoon allows for timely sowing of Kharif crops such as paddy and pulses, potentially improving crop yields and agricultural productivity.
• Water Resources: Early rains help replenish reservoirs and groundwater levels, which is beneficial for drought-prone states like Tamil Nadu and Karnataka.
• Urban Challenges: Cities like Bengaluru may face increased risks of urban flooding due to early and intense rainfall.
• Weather Patterns: Early onset may lead to cooler summer temperatures in southern India and can alter the usual monsoon progression, posing challenges for weather forecasting and water management.

Conclusion

The early onset of the southwest monsoon in India in 2025 was triggered by a combination of favorable atmospheric and oceanic conditions. The Madden-Julian Oscillation’s active phase, intensification of the Mascarene High, strengthening of the Somali Jet, and conducive low-pressure systems collectively accelerated the arrival of monsoon rains. This early arrival has both positive and challenging implications for agriculture, water resources, and urban management in India.

Source: The Indian Express

UPSC Mains Practice Question

Q. Discuss the implications of early monsoon onset on India’s agriculture, water resource management, and overall economy. How should policymakers and farmers prepare for such variability in monsoon patterns?

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