The Southern Indian Ocean off the west coast of Australia is undergoing a significant and accelerating change: it’s becoming less salty. New research, published in Nature Climate Change, reveals that this freshening has been happening steadily for approximately six decades, and is largely attributed to the effects of climate change.
The study, conducted by researchers at CU Boulder and collaborating institutions, points to shifts in global wind patterns and ocean currents as the primary drivers. These changes are channeling increasing amounts of freshwater into the Southern Indian Ocean, a region critical to global ocean circulation. “We’re seeing a large-scale shift of how freshwater moves through the ocean,” explains Weiqing Han, a professor in the Department of Atmospheric and Oceanic Sciences at CU Boulder. “It’s happening in a region that plays a key role in global ocean circulation.”
Understanding the significance of this requires a grasp of ocean salinity and its role in the planet’s climate system. Seawater typically has a salinity of around 3.5%, equivalent to about one and a half teaspoons of salt dissolved in a cup of water. However, the Indo-Pacific freshwater pool, stretching from the eastern Indian Ocean to the western Pacific Ocean, naturally exhibits lower salinity due to frequent rainfall and relatively low evaporation rates.
This Indo-Pacific freshwater pool is intimately connected to the thermohaline circulation – often described as a global “conveyor belt” – which redistributes heat, salt, and freshwater around the world. Warm, fresh surface waters from the Indo-Pacific flow towards the Atlantic Ocean, contributing to the relatively mild climate of Western Europe. In the North Atlantic, this water cools, becomes saltier and denser, and sinks, eventually returning southward in the deep ocean back to the Indian and Pacific Oceans.
The recent research focuses on a historically salty region off the southwest coast of Australia, an area where evaporation typically exceeds precipitation. Over the past six decades, however, the area of highly saline water has decreased by 30%, representing the most rapid increase in freshwater observed anywhere in the Southern Hemisphere. According to Gengxin Chen, a visiting scholar at CU Boulder and senior scientist at the Chinese Academy of Sciences’ South China Sea Institute of Oceanology, this freshening equates to adding roughly 60% of Lake Tahoe’s volume of freshwater to the region annually. “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,” Chen stated.
Crucially, this isn’t a result of increased rainfall in the Southern Indian Ocean itself. The researchers determined that global warming is altering surface winds over the Indian and tropical Pacific Oceans. These altered wind patterns are driving ocean currents to transport more water from the Indo-Pacific freshwater pool southward, into the Southern Indian Ocean.
The consequences of this freshening extend beyond simply changing the ocean’s salt content. As freshwater is less dense than saltwater, it tends to sit on top, creating stronger layers and reducing vertical mixing. Vertical mixing is a vital process that allows surface waters to sink and deeper waters to rise, redistributing nutrients and heat throughout the ocean. Reduced mixing could disrupt this crucial process.
This development builds on previous research suggesting that climate change could slow down portions of the thermohaline circulation. Melting from the Greenland Ice Sheet and Arctic sea ice already adds freshwater to the North Atlantic, disrupting the salinity balance necessary for the conveyor belt to function efficiently. The expansion of the Indo-Pacific freshwater pool could further exacerbate this issue by introducing more freshwater into the Atlantic system.
The impact on marine ecosystems is also a significant concern. Reduced vertical mixing can limit the supply of nutrients from deeper waters to the sunlit surface layers, potentially impacting plankton and sea grass – the foundation of the marine food web. “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 explained. Weaker mixing can trap excess heat in the surface waters, potentially stressing marine organisms already vulnerable to rising temperatures.
The rapid freshening of the Southern Indian Ocean serves as a stark reminder that climate change isn’t just about warming temperatures; it’s also fundamentally reorganizing the planet’s water systems, with potentially far-reaching and complex consequences for global climate and marine life.
Reference: Chen G, Han W, Hu A, et al. The expanding Indo-Pacific freshwater pool and changing freshwater pathway in the South Indian Ocean. Nat Clim Chang. 2026. Doi: 10.1038/s41558-025-02553-1
