One of the ocean's saltiest regions is freshening: What it means for circulation - Earth - Earth Sciences - Environment

Salinity, current and current trend. Credit: Nature Climate Change (2026). DOI: 10.1038/s41558-025-02553-1

The southern Indian Ocean off the west coast of Australia is becoming less salty at an astonishing rate, largely due to climate change, new research shows.

In a study published in Nature Climate Change, researchers at the University of Colorado Boulder and colleagues report that over the past six decades, rising temperatures have reshaped global wind patterns and ocean currents, bringing increasing amounts of fresh water into the southern Indian Ocean.

The changes could alter the interactions between the ocean and the atmosphere, disrupt major ocean circulation systems that help regulate climates around the world, and potentially affect 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."

On average, seawater has a salinity of about 3.5%, roughly equivalent to dissolving one and a half teaspoons of table salt in a cup of water. But across an expansive region stretching from the eastern Indian Ocean into the western Pacific Ocean in the Northern Hemisphere tropics, surface waters are naturally less salty. Frequent tropical rainfall brings large amounts of freshwater to the region, while evaporation is relatively low.

This area, known as the Indo-Pacific freshwater pool, is associated with a giant "conveyor belt" of ocean circulation that redistributes heat, salt and freshwater around the planet. Known as the thermohaline circulation, this system channels warm, fresh surface waters from the Indo-Pacific flow toward the Atlantic Ocean, contributing to the mild climate in western Europe.

In the northern Atlantic Ocean, the water cools, becomes saltier and denser, and eventually sinks before flowing southward in the deep ocean back to the Indian and Pacific oceans.

Rapid freshening off western Australia

Over the past six decades, observational data has detected changes in salinity in the southern Indian Ocean off the southwest coast of Australia. The area is typically dry, with evaporation largely exceeding precipitation. As a result, the seawater in the region has historically been salty.

Han and her team calculated that the area of salty seawater has decreased by 30% over the past six decades, representing the most rapid increase in fresh water 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 U.S. population with drinking water for more than 380 years," he said.

How climate change is driving the shift

The freshening is not a result of local precipitation changes. Using a combination of observations and computer simulations, the team found that global warming is altering surface winds over the Indian and tropical Pacific oceans. These wind shifts are pushing ocean currents to channel more water from the Indo-Pacific freshwater pool to the southern Indian Ocean.

As seawater becomes less salty, its density decreases. Because fresher water usually sits on top of saltier, denser water, the surface water and deep ocean water become more separated into layers. These stronger contrasts in salinity between layers reduce vertical mixing, an important process that normally allows surface waters to sink and deeper waters to rise, redistributing nutrients and heat throughout the ocean.

Risks for ocean circulation and life

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.

Reduced mixing could also impact marine ecosystems. When nutrients from deeper waters fail to reach the sunlit surface, organisms living in shallow waters have less food. Weaker mixing also prevents excess heat in the surface waters from dissipating into deeper layers, making shallow waters even hotter for organisms already under stress from rising temperatures.

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

Provided by University of Colorado at Boulder 

by University of Colorado at Boulder

edited by Sadie Harley, reviewed by Robert Egan 

Source: One of the ocean's saltiest regions is freshening: What it means for circulation