Global Warming Breaks a 400-Year Climate Link Between the Indian and Pacific Oceans

A new study from the Woods Hole Oceanographic Institution (WHOI) finds that major volcanic eruptions can temporarily weaken the climate link between the tropical Indian and Pacific oceans. Human-caused greenhouse gas emissions, however, appear to be driving an even more unusual breakdown in that relationship today. The study was published in Nature Communications under the title "Coupling of Pacific and Indian Ocean variability disrupted by 19th century volcanism."

Climate conditions in the Indian Ocean often respond to changes in the Pacific Ocean, shaping rainfall, temperature and atmospheric circulation across the tropics. Since the 1980s, scientists have noticed this relationship weakening, with the Indian Ocean behaving differently than Pacific conditions would predict. Reliable instrumental climate records cover less than a century, making it hard to tell whether this change is truly unusual.

To look further back, the WHOI team combined paleoclimate evidence from corals, tree rings and stalagmites with climate model simulations, reconstructing ocean conditions to the early 1600s. The two oceans stayed closely linked through most of the past 400 years, except between 1810 and 1850, when the relationship changed significantly. Researchers linked that disruption to a series of major tropical volcanic eruptions, and simulations covering the past thousand years supported the finding.

"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. Lead author Shawn Wang, a former WHOI postdoctoral researcher now at the University of Colorado Boulder, said the data show the recent weakening is "really quite exceptional."

"A key finding is that global warming and human emissions are now overwhelming the Pacific's natural influence on the Indian Ocean," Ummenhofer said. Co-author Delia Oppo, an emeritus research scholar at WHOI, said the Indian Ocean is a huge heat reservoir that can decouple from the Pacific. The study was supported by the US National Science Foundation and WHOI's own science and academic programs.