A vast stretch of the Southern Indian Ocean off Western Australia is freshening at a rate scientists describe as “unprecedented” – and climate change is the driving force, according to new research published earlier this month in Nature Climate Change.

The study, led by researchers at the University of Colorado Boulder, finds that rising global temperatures over the past six decades have reshaped wind patterns and ocean currents, funnelling increasing volumes of freshwater into a region historically defined by high salinity.

Scientists have warned that the shift could disrupt major ocean circulation systems and alter the delicate balance between ocean and atmosphere that regulates climate worldwide.

“We’re seeing a large-scale shift of how freshwater moves through the ocean,” said Weiqing Han, professor in the Department of Atmospheric and Oceanic Sciences in the College of Arts and Sciences. “It’s happening in a region that plays a key role in global ocean circulation,” she said.

On average, seawater contains around 3.5% salt – roughly one and a half teaspoons dissolved in a cup of water. But salinity is far from uniform. Across the eastern Indian and western Pacific tropics lies the Indo-Pacific freshwater pool, a vast region shaped by heavy rainfall and relatively low evaporation.

This freshwater reservoir feeds into the planet’s thermohaline circulation – often described as a global ocean “conveyor belt” – which redistributes heat, salt and freshwater around the world. Warm, fresher surface waters move from the Indo-Pacific toward the Atlantic, helping to moderate western Europe’s climate. In the North Atlantic, waters cool, grow saltier and denser, sink, and return southward at depth toward the Indian and Pacific Oceans.

Off southwestern Australia, however, conditions have traditionally favoured high salinity. Evaporation typically exceeds rainfall, leaving surface waters comparatively salty. Yet observational records now show that this balance has shifted dramatically.

Han and colleagues calculated that the area of salty seawater in the region has shrunk by 30% over the past 60 years – marking the most rapid freshening observed anywhere in the Southern Hemisphere.

“This freshening is equivalent to adding about 60% of Lake Tahoe’s worth of freshwater to the region every year,” said first author Gengxin Chen, visiting scholar in the Department of Atmospheric and Oceanic Sciences and senior scientist at the Chinese Academy of Sciences’ South China Sea Institute of Oceanology.

“To put that into perspective, the amount of freshwater flowing into this ocean area is enough to supply the entire US population with drinking water for more than 380 years,” he said.

Crucially, the influx is not the result of increased local rainfall. By combining long-term observations with computer simulations, the researchers found that global warming is altering surface winds over the Indian and tropical Pacific Oceans. These wind shifts are redirecting currents, channelling more water from the Indo-Pacific freshwater pool into the Southern Indian Ocean.

As salinity declines, seawater becomes less dense. Fresher water tends to sit above saltier, heavier water, intensifying stratification between surface and deep layers. This layering weakens vertical mixing – the process by which heat and nutrients are exchanged between ocean depths and the surface.

Reduced mixing carries consequences. Nutrients trapped in deeper waters may no longer reach sunlit surface layers, limiting food availability for plankton and other foundational marine organisms. At the same time, excess heat absorbed at the surface becomes less able to disperse downward, compounding thermal stress in already warming waters.

The findings also add a new dimension to concerns about the stability of the thermohaline circulation. Previous studies have warned that freshwater from melting Arctic sea ice and the Greenland Ice Sheet could slow circulation in the North Atlantic. An expanding Indo-Pacific freshwater pool may further influence this system by delivering even more low-salinity water into the Atlantic basin.

“Salinity changes could affect plankton and sea grass. These are the foundation of the marine food web. Changes in them could have far-reaching impact on the biodiversity in our oceans,” Chen said.

As climate change reshapes wind, water and heat flows across ocean basins, the Southern Indian Ocean’s rapid freshening stands as a stark indicator of how interconnected – and vulnerable – Earth’s circulation systems have become.

Click here for more from the Oceanographic Newsroom.