The Southern Indian Ocean Is Losing Salt at an “Astonishing” Rate

A vast region of the Southern Indian Ocean is freshening at an unprecedented pace.

Ocean water is not just “wet.” Its saltiness helps determine how seawater stacks up in layers, how currents move heat around the planet, and how easily nutrients can reach the sunlit surface where much of marine life begins. That is why scientists are paying close attention to a startling shift now unfolding off Western Australia: the Southern Indian Ocean there is freshening fast.

According to a study published in Nature Climate Change, researchers from the University of Colorado Boulder report that rising global temperatures over the past 60 years have altered major wind patterns and ocean currents. These shifts are funneling increasing amounts of freshwater into the Southern Indian Ocean. The researchers warn that this trend could reshape how the ocean and atmosphere interact, interfere with large circulation systems that regulate climate worldwide, and place added stress on marine ecosystems.

“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. “It’s happening in a region that plays a key role in global ocean circulation.”

The Indo-Pacific Freshwater Pool

Much of the incoming freshwater can be traced back to a huge tropical region where surface waters are naturally diluted by frequent rain. This zone, stretching from the eastern Indian Ocean across to the western Pacific in the Northern Hemisphere tropics, stays relatively fresh because rainfall is high while evaporation is comparatively low. Scientists often call it the Indo Pacific freshwater pool.

That pool is not isolated. It connects to the thermohaline circulation, a global current system sometimes described as a conveyor belt because it moves heat, salt, and freshwater between ocean basins. Warm surface waters from the Indo Pacific feed into pathways that ultimately influence conditions in the Atlantic. In the North Atlantic, that transported water cools, becomes denser, sinks, and then returns southward at depth before eventually flowing back toward the Indian and Pacific Oceans. Small shifts in salinity can matter here because salt helps set seawater density, and density helps power the sinking and spreading that keep the system moving.

The waters off Australia’s southwest have typically been dry at the surface, with evaporation outpacing rainfall. That pattern has historically favored higher salinity. But long-term observations show that the balance is changing.

Han’s team estimates that the area covered by salty seawater in this Southern Indian Ocean region has shrunk by about 30% over the past 60 years. They describe it as the fastest freshening seen 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 U.S. population with drinking water for more than 380 years,” he said.

Climate-Driven Wind Shifts

The researchers found that the growing influx of freshwater cannot be explained by local rainfall. By analyzing observational records alongside computer simulations, they concluded that global warming is reshaping surface wind patterns across the Indian and tropical Pacific Oceans. These altered winds are steering ocean currents in ways that transport more freshwater from the Indo-Pacific freshwater pool into the Southern Indian Ocean.

As salt levels drop, seawater becomes less dense. Fresher water tends to remain above saltier, heavier water, increasing the separation between surface and deep layers. This enhanced layering limits vertical mixing, the process that normally allows surface water to sink and deeper water to rise, distributing heat and nutrients throughout the ocean.

Previous studies have suggested that climate change could slow part of the thermohaline circulation, as melting from the Greenland Ice Sheet and Arctic sea ice adds freshwater to the North Atlantic, disrupting the salinity balance needed for the conveyor belt to keep moving. The expansion of the freshwater pool could further influence this system by transporting fresher water into the Atlantic.

Weaker vertical mixing could also harm marine life. When nutrient-rich water from the depths does not reach the sunlit surface, organisms in upper layers have fewer resources to survive. At the same time, reduced mixing traps excess heat near the surface, raising temperatures further for species already coping with warming oceans.

“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.

Reference: “The expanding Indo-Pacific freshwater pool and changing freshwater pathway in the South Indian Ocean” by Gengxin Chen, Weiqing Han, Aixue Hu, Gerald A. Meehl, Arnold L. Gordon, Toshiaki Shinoda, Nan Rosenbloom, Lei Zhang and Yukio Masumoto, 3 February 2026, Nature Climate Change.
DOI: 10.1038/s41558-025-02553-1

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