What caused an ‘early’ monsoon onset in India this year?

IMD Confirms Early Monsoon Onset in Kerala, Marking One of the Earliest Since 2009

In a weather event that has stirred both scientific interest and economic optimism, the India Meteorological Department (IMD) declared the early monsoon onset in Kerala on Saturday, May 24 — a full eight days ahead of its typical June 1 arrival. This significant development marks one of the earliest monsoon arrivals in the last decade, and the first since 2009 when the rains came on May 23.

The declaration signals the start of the June-to-September southwest monsoon season, which delivers over 70% of India’s annual rainfall, critical for agriculture, water reservoirs, and the broader economy. Given its widespread impact, the early monsoon onset in Kerala is not just a meteorological milestone but a noteworthy event in India’s economic and environmental calendar.

What Triggers the Monsoon Declaration?

The IMD follows a rigorous methodology to officially declare the onset of monsoon over Kerala. After May 10 each year, the agency begins close monitoring of the region using three core indicators:

  1. Rainfall Consistency: At least 60% of 14 designated weather stations across southern India must record a minimum of 2.5 mm rainfall for two consecutive days.

  2. Wind Patterns: Persistent westerly winds should flow at a depth up to 600 hectoPascals (hPa), with speeds ranging between 15-20 knots (~27–37 km/h).

  3. Outgoing Longwave Radiation (OLR): OLR values, monitored via satellites, must fall below 200 Watts/m², signaling active convection and cloud cover essential for monsoon development.

When these criteria are met over two continuous days, the IMD formally announces the monsoon’s arrival.

This year, all three parameters aligned perfectly, prompting the IMD to declare the early monsoon onset in Kerala on May 24. In a rare occurrence, this onset also coincided with the simultaneous arrival in the Lakshadweep Islands, parts of the Arabian Sea, the Bay of Bengal, southern Karnataka, and as far northeast as Mizoram.

What Caused This Exceptionally Early Monsoon Onset?

A confluence of favorable meteorological and oceanic conditions expedited the 2025 monsoon onset’s progress. Experts at the IMD cited the following key contributors:

1. The Madden-Julian Oscillation (MJO):

A traveling pulse of cloud and rainfall in the tropical atmosphere, the MJO intensified in the Indian Ocean earlier this month. Its presence accelerated cloud formation and atmospheric circulation, making conditions ideal for rainfall over southern India.

2. Mascarene High:

The high-pressure zone near the Mascarene Islands was exceptionally strong this year, pushing monsoon winds toward the Indian subcontinent with greater force.

3. Convective Activity:

Increased vertical heat and moisture transport, known as convection, was observed over southern India and even as far as Haryana. These systems moved southeastward, enhancing rainfall and aiding monsoon development.

4. The Somali Jet:

This cross-equatorial wind band intensified earlier than usual and moved through the Arabian Sea, enhancing wind strength and consistency along India’s western coast.

5. Heat-Low Formation:

As temperatures soared across Pakistan and northwest India, a low-pressure zone formed, pulling in moist air from the Arabian Sea and amplifying monsoon circulation.

6. Monsoon Trough and Vortex:

A robust monsoon trough and an accompanying cyclonic vortex over the Arabian Sea added further momentum, creating a strong pressure gradient essential for the rains to advance.

Geographical Spread of the Monsoon

The early monsoon onset in Kerala has been unusually expansive. On day one, the monsoon covered:

  • Entire Kerala

  • Lakshadweep

  • Mahe (Puducherry)

  • Parts of southern and coastal Karnataka

  • Northeast states like Mizoram

By Sunday, May 25, it had spread to:

  • Goa

  • South Maharashtra

  • Parts of Manipur and Nagaland

  • Further areas in the central and eastern Arabian Sea and the Bay of Bengal

The Northern Limit of Monsoon (NLM) — a notional line tracking monsoon onset advancement — now stretches through Devgad, Belagavi, Haveri, Mandya, Dharmapuri, Chennai, Aizawl, and Kohima.

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