Kolkata's Historic Rainfall: How Climate Change and Warming Oceans Are Reshaping Weather Patterns in South Asia

Kolkata's devastating deluge earlier this week claimed ten lives across the city and neighboring districts, highlighting a growing climate crisis.

Scientists are increasingly concerned that these extreme weather events are becoming commonplace rather than exceptions as climate patterns shift dramatically.

The catastrophic rainfall between Monday night and Tuesday morning delivered a remarkable 251.4 mm of precipitation within just 24 hours, according to India Meteorological Department (IMD) data. This marks Kolkata's wettest day since 1986 and ranks as the sixth-highest single-day rainfall recorded in the region throughout 137 years of meteorological history.

The disaster resulted in ten fatalities, with nine victims succumbing to electrocution in Kolkata and surrounding areas.

Rainfall intensity peaked dramatically between 3:00 am and 4:00 am on September 23, when 98 mm fell within a single hour—just barely below the 100 mm threshold that would officially classify the event as a cloudburst. Meteorologists characterized the rainfall as exceptionally intense and unusual, connecting it to broader climate system changes.

The IMD attributed the extreme precipitation to a low-pressure system that formed over the northwest Bay of Bengal on the morning of September 22, which subsequently moved inland toward Gangetic West Bengal and neighboring Odisha. This weather system created sustained moisture convergence over Kolkata and surrounding regions, triggering the extraordinary downpour.

Kolkata recorded a staggering 251.4 mm of rainfall within 24 hours, as per IMD data. Photo Credit: AFP

Climate experts have connected this extreme rainfall event to shifting climate patterns, particularly the accelerating warming trends in the Indian Ocean.

"This spell of rainfall is very rare. A persistent low-pressure system over Odisha allowed intense convective cloud formation. Continuous moisture incursion from the Bay of Bengal fed the clouds, keeping them energized for longer than usual," explained Mahesh Palawat, Vice President of Meteorology and Climate Change at Skymet Weather.

Palawat further highlighted the Bay of Bengal's warming trajectory, noting its unusually elevated sea surface temperatures caused by global warming, which leads to increased evaporation rates and more powerful weather systems.

Dr. Raghu Murtugudde, Emeritus Professor at the University of Maryland and retired professor from IIT-Mumbai, established connections between ocean warming trends and increasingly erratic monsoon patterns across India. He explained that global warming has disrupted traditional monsoon cycles through the formation of multiple typhoons in the Pacific Ocean.

Experts have attributed this extreme rainfall to changing climate patterns. Photo Credit: Reuters

"Multiple typhoons are forming in the Pacific due to warming, and these massive systems draw in moisture from the North Indian Ocean. This disrupts monsoon circulation and encourages the development of low-pressure systems in the Bay of Bengal," Dr. Murtugudde elaborated.

He cautioned that these climate interactions are delaying monsoon withdrawal, potentially leading to extended rainfall periods in major urban centers including Kolkata and Mumbai.

Recent scientific research identifies the Indian Ocean as a critical driver of global climate variability, with surface temperatures rising faster than any other major ocean since the 1950s. A study published in Nature revealed ocean temperatures have increased by more than 1.5°C since the industrial revolution began—significantly higher than previous estimates suggested.

Dr. Roxy Mathew Koll from the Indian Institute of Tropical Meteorology (IITM) in Pune warns that this accelerated ocean warming intensifies cyclones, monsoons, and extreme rainfall events across the region.

Marine heatwaves are expected to increase from 20 to 220-250 days per year. Photo Credit: Reuters

"By the end of this century, sea surface temperatures across the Indian Ocean are projected to remain above 28°C year-round, creating ideal conditions for deep convection and cyclogenesis. We've already seen a marked increase in heavy rainfall and severe cyclones since the 1950s, and this trend is only expected to accelerate," Dr. Koll stated.

He added that marine heatwaves—extended periods of abnormally high sea temperatures—are projected to increase from 20 days annually to between 220-250 days per year, potentially transforming the tropical Indian Ocean into a near-permanent heatwave zone. These marine heatwaves not only rapidly intensify cyclones but also damage vital marine ecosystems, resulting in coral bleaching and biodiversity loss with serious implications for fisheries and coastal communities.

Scientists also point to significant changes in the Indian Ocean Dipole (IOD), a key climate driver influencing monsoon variability and cyclone formation. Projections indicate extreme IOD events could increase by 66 percent while moderate events may decline by over 50 percent by 2100, further disrupting weather patterns throughout South Asia.

The recent catastrophic rainfall in Kolkata represents more than an isolated incident—it serves as a stark warning of how climate change is fundamentally reshaping weather systems across the subcontinent. Experts emphasize this event underscores the urgent need for climate-resilient urban infrastructure, enhanced disaster preparedness protocols, and aggressive climate action initiatives.