Human Warming Is Disrupting the Pacific Walker Circulation and Indian Ocean Link

Changes in one ocean do not necessarily remain confined to it.

For hundreds of years, shifts in the tropical Pacific have travelled through the atmosphere, affecting Indian Ocean temperatures thousands of kilometres away.

It is, in effect, a far-reaching climate dialogue between two vast ocean basins.

That exchange has consequences for people on land. Temperatures in the Indian Ocean can shape rainfall and wider climate conditions across heavily populated regions of Africa, Asia and Australia.

Fairly consistent links between tropical oceans can also enable scientists to forecast climate conditions months or even years in advance.

However, a significant element of this relationship is now acting in a markedly different way.

Pacific Walker circulation and Indian Ocean temperatures

A new study in Nature Communications reports that a longstanding connection between atmospheric circulation over the Pacific and temperatures throughout the tropical Indian Ocean has weakened dramatically in recent decades.

After examining several centuries of climate history, the researchers concluded that the present-day change appears to be highly unusual.

Central to the story is the Pacific Walker circulation, an immense pattern of air movement above the tropical Pacific.

The mechanism itself is straightforward. Contrasts in sea temperatures help circulate air across the Pacific: air rises over warmer water, moves through the atmosphere, descends in another location and contributes to near-surface winds.

During phenomena including El Niño, this circulation may become weaker. Via an atmospheric bridge, such changes can affect temperatures throughout the Indian Ocean as well.

In the past, changes in the Pacific system were generally matched by changes in the Indian Ocean's basin-wide temperature pattern, called the Indian Ocean Basin Mode.

Since the 1950s, though, the tropical Indian Ocean has warmed by around 0.1 degrees Celsius each decade as a result of changes linked to increasing greenhouse gas concentrations. At the same time, the Pacific Walker circulation has intensified since the 1980s.

The outcome is notable: in recent decades, the direction of the relationship between these two systems has flipped.

Put simply, a key part of the Indian Ocean climate system is no longer responding to the Pacific as dependably 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.

Reconstructing Indo-Pacific climate history

Modern ocean observations extend back only around a century. The question, then, was how researchers could establish whether this was a genuinely unusual event rather than a natural climate pattern seen previously.

Lead author Shawn Wang, now at the University of Colorado Boulder, and his colleagues brought together current observations and climate models with natural climate archives: 35 coral records, three tree-ring records and one stalagmite record.

These archives hold evidence of earlier environmental conditions, enabling the researchers to reconstruct Indo-Pacific climate patterns between 1631 and 1990.

Across most of those four centuries, the Pacific and Indian Ocean systems broadly changed in step.

There was, however, one major exception.

From about 1810 to 1850, the relationship weakened substantially. This interval overlapped with several huge tropical volcanic eruptions, among them the 1815 eruption of Indonesia's Mount Tambora.

Major eruptions can propel sulphur-containing gases high into the atmosphere. There, they create aerosols that limit incoming sunlight and temporarily cool Earth's surface.

As this cooling is uneven, it can shift ocean temperatures and winds. The simulations indicate that these volcanic events briefly interrupted the usual link between the Pacific and Indian Oceans.

Why the modern shift is different

This climate relationship has therefore broken down in the past, but an important distinction remains.

The disruption during the 19th century was linked to exceptionally powerful volcanic eruptions. The current shift, by contrast, is happening alongside continuing human-caused warming.

The team compared the 1945–2025 relationship with matching 80-year periods from simulations and reconstructions covering the past millennium. The modern correlation lay beyond the 95 percent ranges shown by both, including those from times of intense volcanic activity.

This does not indicate that every Pacific–Indian Ocean connection has failed. A separate relationship involving the Indian Ocean Walker circulation did not experience the same significant contemporary change.

Yet the shift that the researchers identified may have impacts well beyond the oceans.

Stable relationships between tropical ocean basins provide one basis for climate predictability. Should those links alter, scientists may have to incorporate the changes when forecasting future climate conditions. According to the authors, the results carry implications for climate-risk management and regional water-resource management.

Uncertainties nevertheless remain. The simulations did not capture every aspect of the 19th-century volcanic disruption, while precisely how the Pacific Walker circulation will respond as warming continues is still uncertain.

The broader lesson is that climate change does more than warm oceans individually. It may also transform the connections between them, potentially reshaping some of the patterns scientists rely on to interpret what lies ahead.

The study appeared in Nature Communications.

This article was fact-checked by Fiona MacDonald and edited by Fiona MacDonald. Although we take pride in our process, we are only human. If you notice an error, please let us know.