Early Monsoon Onset in India, Factors and Impact

Learn about Monsoon Onset in India—its timing, significance, impact on agriculture, and how the IMD forecasts the arrival of the southwest monsoon each year.

Context: The southwest monsoon arrived in Kerala on May 24, 2025, eight days ahead of its normal date (June 1), making it the earliest onset since 2009, as per the India Meteorological Department (IMD).

What is Monsoon Onset?

The Monsoon Onset refers to the official beginning of the southwest monsoon season over the Indian subcontinent.
It marks the first significant and sustained rainfall over southern Kerala, driven by changes in atmospheric and oceanic conditions.
Declared by: The India Meteorological Department (IMD) officially declares the monsoon onset based on specific meteorological criteria.

Scientific Criteria for Monsoon Onset

Rainfall: 60% of key stations in Kerala must record ≥2.5 mm rainfall for two consecutive days.
Wind field: Depth and speed of westerly winds at the lower troposphere must meet criteria.
OLR (Heat/Radiation): Outgoing Longwave Radiation must be <200 W/m² in a specific box region.

Factors for Early Onset (2025)

Higher sea surface temperatures in the Arabian Sea & Bay of Bengal.
Active Madden-Julian Oscillation (MJO).
Stronger Somali Jet winds are bringing in moisture early.

Low-pressure systems accelerate monsoon flow.

Does Early Monsoon = Good Monsoon?

Onset only marks arrival, not intensity or distribution of rainfall.
Early onset years can still witness droughts or skewed rainfall.

Also Read: Monsoon in India

Madden-Julian Oscillation (MJO)

What it is: A large-scale tropical atmospheric disturbance that moves eastward around the globe every 30–60 days.

Madden-Julian Oscillation (MJO)

Key Features:
- Involves coupled patterns of rainfall and winds.
- Originates over the Indian Ocean and moves toward the Pacific.
- Affects weather globally, including monsoons, cyclones, and El Niño/La Niña patterns.
Impact on India:
- Strong MJO phases can enhance monsoon rainfall.
- Weak or absent MJO phases may suppress rainfall, causing dry spells.
Why it matters: MJO is crucial for intra-seasonal weather forecasting, especially for monsoon prediction and cyclogenesis in the Indian Ocean region.