Study reveals volcanic and human-driven disruptions to Pacific–Indian Ocean climate connection
A new study published inNature Communications has found that a long-standing connection between Pacific atmospheric circulation and temperatures across the tropical Indian Ocean has broken down dramatically in recent decades. The research, conducted by scientists at the Woods Hole Oceanographic Institution (WHOI), combines paleoclimate data and climate models to show that while large volcanic eruptions temporarily disrupted this relationship in the early 19th century, human-caused greenhouse gas emissions are now driving an unprecedented and exceptional shift.
The study, titled "Coupling of Pacific and Indian Ocean variability disrupted by 19th century volcanism," extends the record of Indian and Pacific Ocean climate variability back to the early 1600s using natural archives such as corals, tree rings, and stalagmites. Lead author Shawn Wang, now a postdoctoral researcher at the University of Colorado Boulder and formerly at WHOI, and colleagues combined modern observations with these paleoclimate records to assess how unusual recent changes are.
Historical context: volcanic eruptions broke the pattern
Climate conditions in the Indian Ocean typically follow the behavior of the Pacific Ocean. For most of the past 400 years, the two ocean basins were largely coupled, consistent with modern observations. However, the study found that the Indian–Pacific relationship was notably different during the period 1810–1850. The researchers link this unusual interval to a series of large tropical volcanic eruptions that disrupted the usual connections between the two ocean basins, with computer simulations covering the past millennium supporting the volcanic explanation.
Caroline Ummenhofer, a climate scientist at WHOI and co-author of the study, said in a statement: "A key finding is that global warming and human emissions are now overwhelming the Pacific's natural influence on the Indian Ocean."
Modern breakdown is unprecedented
Since the 1950s, the tropical Indian Ocean has warmed by about 1 degrees Celsius per decade, driven by changes associated with rising greenhouse gas concentrations. Meanwhile, the Pacific Walker circulation—a vast system of moving air above the tropical Pacific—has strengthened since the 1980s. The result is a striking reversal in the sign of the correlation between the two ocean systems.
The researchers compared the modern relationship (1945–2025) with equivalent 80-year periods in both simulations and paleoclimate reconstructions spanning the past millennium. The modern correlation fell outside the 95 percent confidence ranges of both, even when compared with periods of intense volcanic activity. This indicates that the recent breakdown is unprecedented in the context of the past 400 years.
Implications and uncertainties
The study's authors note that the simulations did not reproduce every detail of the 19th-century volcanic disruption, and exactly how the Pacific Walker circulation will change as warming continues remains unclear. However, the bigger message, according to the researchers, is that climate change does not simply warm individual oceans—it can also alter the relationships between them, potentially changing some of the patterns scientists use to understand what comes next.
The Indian Ocean is a huge heat reservoir and can decouple from the Pacific, the study notes. The findings underscore the independent behavior of the Indian Ocean and suggest that the recent shift is due to human-caused greenhouse gas emissions.
The study is one of the first to examine the breakdown in the connection between the Pacific and Indian Oceans using past climates, observations, and models. It used 35 coral records, tree-ring data, and a stalagmite record to reconstruct the climate relationship back to 1631.