For centuries, climate changes in the Indian Ocean closely followed the Pacific, but that relationship is now weakening.
Over the past 400 years, climate conditions in the tropical Indian Ocean generally changed along with those in the Pacific. In other words, shifts in Pacific climate have historically had a strong influence on what happens in the Indian Ocean.
But since the 1980s, that long-standing relationship has weakened sharply. Researchers say human-caused warming is now driving an unusually large change in how closely the two oceans behave.
“A key finding is that global warming and human emissions are now overwhelming the Pacific’s natural influence on the Indian Ocean,” said Caroline Ummenhofer, a senior scientist at Woods Hole Oceanographic Institution (WHOI).
Scientists have struggled to determine just how unusual the recent change is because reliable instrumental records cover less than a century. That is a short window for judging a climate relationship that can vary over much longer timescales.
“The modern data we have is limited and doesn’t go back far enough. With climate models and paleo-records, we are now able to say with more confidence that the recent changes we are seeing are really quite exceptional,” said lead author Shawn Wang. Wang is a former WHOI graduate student and postdoctoral researcher, and he is currently a postdoc at the University of Colorado Boulder.
Four centuries establish the usual pattern
To look further into the past, Wang and his colleagues used natural records of climate preserved in corals, tree rings, and stalagmites from the tropics. These records allowed them to trace climate variability in the Indian and Pacific oceans back to the early 1600s.
For most of that period, the two oceans showed a close relationship, with Indian Ocean climate generally following changes in the Pacific. That longer record gave researchers a way to identify periods when the usual connection weakened.
“This is one of the first studies to examine the breakdown in the connection between the Pacific and Indian oceans using evidence from past climates, modern observations, and climate models,” said co-author Caroline Ummenhofer.
Volcanoes disrupted the connection
One period looked notably different. Between about 1810 and 1850, the usual relationship between the Indian and Pacific oceans weakened.
The researchers linked that disruption to a series of large tropical volcanic eruptions. Climate simulations spanning the past millennium supported that explanation, showing that volcanism could temporarily reduce the Pacific’s influence on Indian Ocean climate.
The size of the effect depended on both the strength of the eruptions and the climate conditions at the time.
That 19th-century episode gave researchers a historical example of the two oceans becoming less closely connected. It also provided a comparison for the much more recent breakdown associated with human-caused greenhouse gas emissions.
The findings are reported in the Nature Communications study “Coupling of Pacific and Indian Ocean variability disrupted by 19th century volcanism.”
The Indian Ocean can act independently
Understanding how the two oceans influence one another matters because their connection helps scientists predict future changes in rainfall and other aspects of climate.
Previous studies have often focused on individual ocean basins. By combining records of past climate, modern observations, and climate models, the new work provides a longer view of how the Indian and Pacific oceans influence one another.
“The Indian Ocean is a huge heat reservoir, and it can decouple from what the Pacific Ocean is doing. The results of this study underscore the independent behavior of the Indian Ocean,” said co-author Delia Oppo, an emeritus research scholar at WHOI.
Reference: “Coupling of Pacific and Indian Ocean variability disrupted by 19th century volcanism” by Shawn Wang, Delia W. Oppo and Caroline C. Ummenhofer, 26 August 2026, Nature Communications.
DOI: 10.1038/s41467-026-76705-y
This study was supported by the U.S. National Science Foundation, the WHOI Investment in Science Program, and the WHOI Academic Programs Office.
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