A mysterious patch of cooling water in the North Atlantic Ocean, known as the ‘Atlantic Cold Blob’, may be reshaping the Indian summer monsoon and influencing rainfall patterns across South Asia, according to a new study that links distant ocean changes to dramatic shifts in rainfall over India. The study, led by climate scientist Nimmakanti Mahendra of Purdue University and collaborators, provides evidence that cooling sea-surface temperatures south of Greenland are altering atmospheric circulation patterns across Eurasia, ultimately redistributing monsoon rainfall across India. The findings help explain why northwest India has become significantly wetter in recent decades while parts of the Indo-Gangetic Plains have experienced declining rainfall.
The researchers found that since 1999, monsoon rainfall over northwest India has increased by about 24.6 per cent, while rainfall over the Indo-Gangetic Plains has declined. This emerging rainfall dipole has become increasingly evident in observations but has largely been missed by leading climate models. The shift matters enormously because the Indian monsoon supports agriculture, water supplies and livelihoods for nearly one-fifth of humanity. Changes in where rainfall falls can trigger floods in some regions and droughts in others, affecting food production, groundwater recharge and economic stability.
The Indo-Gangetic Plains, one of the world’s most densely populated and agriculturally productive regions, have already witnessed drying trends. Previous studies cited by the researchers suggest the recent decline in rainfall in parts of the Ganga basin may be among the most severe arid conditions recorded in over a millennium.
Atlantic Cold Blob
The Atlantic Cold Blob is an area of unusually cool ocean water located in the sub-polar North Atlantic, south of Greenland. Scientists believe it is linked to a slowdown of the Atlantic Meridional Overturning Circulation (AMOC), a vast system of ocean currents that transports warm water northward and helps regulate global climate.
“What is happening in recent years is that Arctic, Greenland ice melt run-off and precipitation is putting a lot of freshwater into the North Atlantic,” Mahendra told TNIE. “That freshwater dilutes the salty ocean water and weakens the deep-water formation process. As a result, a cold blob develops south of Greenland while the Gulf Stream region becomes warmer, creating a stronger temperature gradient.” He explained that under normal conditions, the Gulf Stream transports heat and salt northwards.
However, increasing freshwater from melting Arctic ice, rainfall and Greenland ice-sheet discharge is disrupting that process, weakening ocean circulation and creating the distinctive cooling patch. While the cold blob has previously been associated with European heat waves and changes in Atlantic weather systems, the new research suggests its influence extends thousands of kilometres away into the heart of the Asian monsoon.
Greenland and Indian rainfall
The study proposes that the cold blob alters temperature contrasts across the North Atlantic, which then affects the behaviour of the mid-latitude jet stream, a fast-moving ribbon of air high in the atmosphere.
These changes trigger large-scale atmospheric waves known as Rossby waves, which travel eastward across Europe and Asia. The waves modify the strength and position of the Asian subtropical jet stream, particularly over Central Asia and the Tibetan Plateau region. The researchers identified a mechanism called the “barotropic governor effect”. In simple terms, this process redistributes atmospheric energy and momentum within the jet stream, strengthening its core while weakening adjacent regions. “This stronger temperature gradient creates an environment that amplifies the westerly jet stream,” Mahendra said. “The energy generated over the North Atlantic is transferred eastward through atmospheric waves and eventually reaches the Asian jet stream north of India, altering monsoon circulation.” As the jet stream changes, so does the monsoon circulation beneath it. Moisture-bearing winds are redirected westward, leading to enhanced rainfall over northwest India and Pakistan while reducing moisture transport into parts of the Indo-Gangetic Plains.
“The circulation changes create low-level convergence over northwest India, pulling moisture from the Arabian Sea and producing heavy rainfall in regions where we would not normally expect such intense monsoon precipitation,” he said. To test the theory, the team ran climate model experiments in which they imposed the observed North Atlantic sea-surface temperature pattern while keeping other oceans unchanged. The simulations successfully reproduced the observed rainfall increase over northwest India and decline over the Indo-Gangetic Plains, strengthening confidence in the proposed mechanism.
Why climate models missed it
One of the study’s most significant findings is that many state-of-the-art climate models fail to capture the observed Atlantic cooling pattern and the associated atmospheric response. As a result, they do not reproduce the rainfall redistribution seen across India over the past two decades. “The models are not picking this up because they are missing the ocean circulation changes occurring in the North Atlantic,” Mahendra said. “Our question was simple: if the models are not capturing these changes now, how can we fully trust their projections for the future? This study identifies where the models are going wrong.” According to the researchers, this limitation raises concerns about future monsoon projections, particularly if models continue to underestimate the influence of North Atlantic changes on Asian climate systems.
Mahendra said that climate modelling centres have increasingly begun incorporating North Atlantic variability into their assessments, though its implications for the Indian monsoon have received far less attention than its impacts on Europe and North America.
Implications for India
The findings arrive at a time when India is grappling with increasing weather extremes. Northwest India has experienced several devastating flood events in recent years, while concerns over rainfall deficits and groundwater stress persist in parts of the Indo-Gangetic region. “That is why we are seeing unusually heavy rainfall events in recent years,” Mahendra said. “In mountainous regions such as Uttarakhand, intense rainfall can saturate soils and trigger landslides. The same mechanism was evident during some of the recent extreme rainfall events.” The study also highlights how climate ‘tipping elements’ may be interconnected. A weakening AMOC in the North Atlantic - itself considered a potential tipping element - could influence another major climate system, the South Asian monsoon.
The research results were validated by Prof Nagajaru Chilukoti, Assistant Professor, Earth and Atmospheric Sciences, NIT Rourkela and Dr Jasti S Chowdary, a scientist at the Indian Institute of Tropical Meteorology (IITM) and Matthew Huber, a professor in the Department of Earth, Atmospheric, and Planetary Sciences at Purdue University in Indiana.