Global warming is breaking a 400-year climate link between two oceans
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A new study suggests that the climate connection between the tropical Indian and Pacific oceans can be disrupted by major volcanic eruptions, while human-caused greenhouse gas emissions may now be driving an even more unusual breakdown in that relationship.
Researchers at the Woods Hole Oceanographic Institution (WHOI) combined paleoclimate evidence with climate model simulations to examine how closely the two ocean basins have behaved over the past several centuries.
Their findings indicate that volcanism can temporarily weaken the climate link between the Indian and Pacific oceans. More recently, however, human-driven climate forcing has produced what the researchers describe as an “exceptional” shift in the way climate variability in the two regions is connected.
The study was published in Nature Communications under the title “Coupling of Pacific and Indian Ocean variability disrupted by 19th century volcanism.”
A Climate Connection That Usually Moves in Step
Climate conditions across the Indian Ocean often respond to changes occurring in the Pacific Ocean. This connection helps shape broad patterns of rainfall, temperature, and atmospheric circulation across the tropics.
Since the 1980s, however, scientists have noticed that this relationship has weakened. The Indian Ocean has increasingly behaved differently from what would normally be expected based on conditions in the Pacific, a change that researchers have linked to climate warming.
One challenge is that reliable instrumental climate records cover less than a century. That makes it difficult to determine whether the recent change is truly unusual or simply part of a longer natural cycle.
To look further back in time, the WHOI team turned to tropical paleoclimate records preserved in corals, tree rings, and stalagmites. These natural archives allowed the researchers to reconstruct Indian and Pacific Ocean climate conditions back to the early 1600s.
Volcanoes Disrupted the Ocean Link in the 1800s
The paleoclimate evidence shows that the Indian and Pacific oceans remained closely connected through most of the past 400 years.
One period stood out.
Between 1810 and 1850, the relationship between the two ocean basins changed significantly. Researchers linked that disruption to a series of major tropical volcanic eruptions, which appear to have weakened the Pacific Ocean’s usual influence over climate conditions in the Indian Ocean.
Computer simulations covering the past thousand years supported that interpretation.
The strength of the disruption depended on both the size of the volcanic eruptions and the background climate conditions that existed at the time.
“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, a senior scientist at WHOI.
Modern Changes Appear Far More Unusual
The volcanic connection was only part of what the researchers uncovered. By comparing the modern climate with several centuries of past conditions, they were also able to judge how unusual the current weakening of Indian and Pacific Ocean coupling really is.
“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.
The findings suggest that although volcanic eruptions can temporarily interrupt the connection between the two ocean basins, the modern breakdown appears to be driven by a different force.
“A key finding is that global warming and human emissions are now overwhelming the Pacific’s natural influence on the Indian Ocean,” Ummenhofer said.
Why the Indian Ocean Matters for Climate Predictions
Understanding how strongly the Indian and Pacific oceans influence each other is important for climate forecasting.
Connections between ocean basins can help scientists anticipate shifts in rainfall and other major climate patterns. If those relationships weaken or change, forecasts based on historical behavior may become less reliable.
Much past research has examined major ocean basins separately. This study instead focuses on the ways they interact, and on what happens when those connections begin to weaken.
“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.
The 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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