Kerala,25 May 2026: The southwest monsoon onset in Kerala is increasingly occurring in May rather than the traditional June 1 date, according to a study analyzing 124 years of rainfall data from the India Meteorological Department (IMD). Researchers say global warming is a primary reason for the earlier onset, and the trend is expected to continue in coming decades.
The study, titled Analysis of Temporal and Spatial Variations in Extreme Precipitation over Kerala, was published in the Journal of Agrometeorology. It was led by P.S. Biju, a senior scientist at IMD Thiruvananthapuram, along with researchers from Amrita Vishwa Vidyapeetham, the National Institute of Technology Patna, and the Indian Institute of Space Science and Technology.
Monsoon Arriving Earlier More Often
According to IMD records from 1901 to 2025, the southwest monsoon onset occurred in May on 57 occasions and in June on 68 occasions. When the data is extended further back to 1870, covering 156 years, May onset was recorded 70 times against 86 June occurrences.
Since the year 2000, the monsoon arrived in May on 14 separate occasions, pointing to a clear acceleration of the early-onset trend in recent decades. The study’s average onset date remains around June 1, with a standard deviation of roughly six days.
Why the Monsoon Is Coming Earlier
Senior scientist P.S. Biju explained that rising summer temperatures linked to global warming are causing low-pressure systems over land to develop earlier than usual. Monsoon winds travel from high-pressure zones near Madagascar and move toward these land-based low-pressure systems after crossing the equator.
When global warming triggers the earlier formation of these low-pressure systems, the monsoon winds arrive sooner. Changes in the monsoon trough and heat lows also contribute to the shift, according to Biju.
Rainfall Declining but Extreme Events Rising
Kerala is facing a contradictory pattern: while overall southwest monsoon rainfall is declining at a rate of 26 cm per decade, the total number of rainy days is going up. At the same time, severe weather events are becoming more frequent in central Kerala and the Western Ghats regions of northern Kerala.
Coastal Kerala, however, is showing a decrease in extreme rainfall events. The northeast monsoon also recorded a declining trend, while summer rainfall showed a slight increase. Average annual rainfall across the state stands at approximately 2,920 mm, with significant variation by region.
Spatial Differences Across Kerala
Northern Kerala receives the highest southwest monsoon rainfall, ranging between 2,300 mm and 3,000 mm, while rainfall decreases progressively toward central and southern districts. Some southern regions record less than 1,500 mm during the southwest monsoon season. In contrast, northeast monsoon rainfall is more concentrated in southern Kerala.
What Needs to Happen Next
The study calls for district-level climate adaptation strategies across Kerala, along with improved flood management systems, better water storage infrastructure, and climate-resilient agricultural planning. Researchers say these measures are necessary to help the state cope with its rapidly changing rainfall patterns.
