Context:

The India Meteorological Department (IMD) announced the onset of the southwest monsoon over Kerala on May 24, 2025, arriving eight days earlier than the typical date of June 1. This early onset marks one of the earliest monsoon arrivals in over a decade, a phenomenon last observed in 2009.

About the Southwest Monsoon:

The Southwest Monsoon is a vital seasonal wind system that delivers over 70% of India’s annual rainfall between June and September. It is crucial for agricultural productivity, water resource management, and has a significant influence on the country's overall economic activity.

When is Monsoon Onset Declared?

IMD Criteria for Declaring Monsoon Onset over Kerala

To officially declare the onset of the southwest monsoon over Kerala, the India Meteorological Department (IMD) evaluates three key parameters:

1. Rainfall Threshold: At least 60% of 14 designated weather stations must record 2.5 mm or more rainfall for two consecutive days.
2. Wind Pattern: Westerly winds must be established up to the 600 hPa level, with wind speeds ranging between 15–20 knots at the 925 hPa level.
3. Outgoing Longwave Radiation (OLR): OLR values should be below 200 W/m², signaling active convection and significant cloud cover.

Drivers of the Early Monsoon Onset – 2025

Several meteorological and oceanic factors contributed to the unusually early arrival of the southwest monsoon over Kerala in 2025:

1. Madden-Julian Oscillation (MJO):
   An active phase of the MJO—an eastward-moving tropical disturbance—enhanced convection and rainfall over the Indian Ocean. Its influence was notably observed from May 13 over the south Andaman Sea, as reported by the IMD.
2. Intensification of the Mascarene High:
   A stronger-than-usual high-pressure system over the southern Indian Ocean helped steer moist southwesterly winds toward the Indian subcontinent, aiding monsoon advancement.
3. Surge in Convection:
   Increased surface heating and moisture transport led to intensified vertical cloud development, triggering early and widespread rainfall.
4. Strengthening of the Somali Jet:
   Enhanced cross-equatorial low-level winds, known as the Somali Jet, contributed to the rapid movement of monsoon currents over Kerala and Karnataka.
5. Formation of a Heat Low:
   A persistent low-pressure zone over Pakistan and northwest India created a suction effect, drawing moist monsoonal air inland more quickly.
6. Activation of the Monsoon Trough:
   An elongated low-pressure area stretching from the Arabian Sea to the Bay of Bengal supported early rainfall activity across central and northeastern India.

Impacts of the Early Monsoon Onset – 2025

The early arrival of the southwest monsoon in 2025 has brought both promising opportunities and critical challenges:

1. Agricultural Advantage:
   The early rains enable timely sowing of Kharif crops such as paddy and pulses, helping farmers align better with the agricultural calendar and potentially boosting yields.
2. Reservoir Replenishment:
   Early monsoon showers support the recharge of reservoirs, particularly benefiting drought-prone regions like Tamil Nadu and Karnataka.
3. Urban Flooding Concerns:
   Cities unprepared for premature rainfall—such as Bengaluru—face an increased risk of waterlogging and flash floods due to inadequate drainage readiness.
4. Disruption of Seasonal Weather Patterns:
   The early onset has contributed to cooler-than-normal summer temperatures across southern India, potentially altering regional climate dynamics.
5. Forecasting and Planning Challenges:
   An early start does not guarantee a longer or stronger monsoon. This uncertainty complicates water resource management and agricultural planning in the months ahead.

Conclusion:
The early monsoon onset in 2025 reflects the influence of favorable atmospheric and oceanic factors, including the Madden-Julian Oscillation and the strengthened Somali Jet. While it offers hope for agriculture and water security, it also highlights the need for urban preparedness and adaptive planning to address emerging weather anomalies and infrastructure challenges.