What happens in one ocean doesn’t necessarily stay there.
For centuries, changes over the tropical Pacific have traveled through the atmosphere to influence temperatures in the Indian Ocean thousands of kilometers away.
Think of it as a long-distance climate conversation between two enormous bodies of water.
And that conversation matters to us on land. Indian Ocean temperatures can influence rainfall and other climate conditions across densely populated parts of Africa, Asia, and Australia.
Relatively stable relationships between tropical oceans can also help scientists predict climate conditions from seasons to years ahead.
But one important part of that relationship is behaving very differently.
A new study published in Nature 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.
Looking back through centuries of climate history, the researchers found that the modern shift appears exceptional.
An important player is the Pacific Walker circulation, a vast system of moving air above the tropical Pacific.
The basic idea is simple. Differences in ocean temperatures help move air across the Pacific. Warm air rises over warmer waters, travels through the atmosphere, sinks elsewhere, and helps drive winds near the surface.
During events such as El Niño, this circulation can weaken. Through an atmospheric bridge, those changes can also influence temperatures across the Indian Ocean.
Historically, when the Pacific system changed, the Indian Ocean’s basin-wide temperature pattern – known as the Indian Ocean Basin Mode – tended to change with it.
But since the 1950s, the tropical Indian Ocean has warmed by about 0.1 degrees Celsius per decade, driven by changes associated with rising greenhouse gas concentrations. Meanwhile, the Pacific Walker circulation has strengthened since the 1980s.
The result is striking: the sign of the relationship between the two has reversed in recent decades.
In simple terms, one important part of the Indian Ocean climate system is no longer following the Pacific’s lead as reliably as it once did.
“A key finding is that global warming and human emissions are now overwhelming the Pacific’s natural influence on the Indian Ocean,” climate scientist Caroline Ummenhofer of the Woods Hole Oceanographic Institution (WHOI) said in a statement.
But modern ocean measurements only stretch back about a century. So how could researchers tell whether this was truly unusual rather than something the climate system had done naturally before?
Lead author Shawn Wang, now at the University of Colorado Boulder, and colleagues combined modern observations and climate models with natural climate archives: 35 coral records, three tree-ring records, and one stalagmite record.
These natural archives preserve clues to past environmental conditions, allowing the team to reconstruct Indo-Pacific climate patterns from 1631 to 1990.
For most of those four centuries, the Pacific and Indian Ocean systems largely moved together.
Then came a striking exception.
Between roughly 1810 and 1850, their relationship weakened considerably. That period coincided with several enormous tropical volcanic eruptions, including Indonesia’s Mount Tambora eruption in 1815.
frameborder=”0″ allow=”accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share” referrerpolicy=”strict-origin-when-cross-origin” allowfullscreen>Large eruptions can send sulfur-containing gases high into the atmosphere, where they form aerosols that reduce incoming sunlight and temporarily cool Earth’s surface.
Because that cooling isn’t uniform, it can alter ocean temperatures and winds. The simulations suggest these volcanic disturbances temporarily disrupted the usual connection between the Pacific and Indian Oceans.
So this climate relationship has broken down before. But there’s a crucial difference.
The 19th-century disruption was associated with powerful volcanic eruptions. Today’s shift is occurring alongside sustained human-caused warming.
The researchers compared the modern 1945–2025 relationship with equivalent 80-year periods in simulations and reconstructions spanning the past millennium. The modern correlation fell outside the 95 percent ranges represented by both – even compared with periods of intense volcanic activity.
That doesn’t mean every connection between the Pacific and Indian Oceans has collapsed. Another relationship involving the Indian Ocean Walker circulation did not undergo the same significant modern shift.
But the change researchers did detect could matter far beyond the oceans.
Stable interactions between tropical ocean basins are one source of climate predictability. If those relationships change, scientists may need to account for that when predicting future climate conditions. The authors say the findings have implications for climate-risk management and regional water-resource management.
Related: A Major Ocean Current Is On The Verge of Collapse. Scientists Say The Effects May Reach California
There are still uncertainties. The simulations didn’t reproduce every detail of the 19th-century volcanic disruption, and exactly how the Pacific Walker circulation will change as warming continues remains unclear.
The bigger message is that climate change doesn’t 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 study was published in Nature Communications.
This article was fact-checked by Fiona MacDonald and edited by Fiona MacDonald. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.
