Southern Indian Ocean Saltiest Zone Shrinks 30 Percent in Six Decades

The highly saline zone of the Southern Indian Ocean off southwest Australia has decreased in area by 30 per cent over the past six decades, representing the most rapid freshwater increase observed anywhere in the Southern Hemisphere, according to a study by the University of Colorado Boulder published in Nature Climate Change. Climate change-driven wind shifts are channelling more water from the Indo-Pacific freshwater pool into the region.

The study, led by Weiqing Han of CU Boulder with first author Gengxin Chen, found that freshening is not caused by local precipitation increases but by global warming altering surface winds over the Indian and tropical Pacific Oceans, pushing ocean currents to redirect freshwater from the Indo-Pacific freshwater pool toward the Southern Indian Ocean. The rate of freshwater addition is equivalent to approximately 60 per cent of Lake Tahoe's volume every year, or enough to supply the entire United States population with drinking water for more than 380 years, the researchers note.

The Southern Indian Ocean freshwater pool is a key node in thermohaline circulation, the global ocean conveyor belt that redistributes heat, salt and freshwater around the planet. Warm, fresh surface waters from the Indo-Pacific normally flow toward the Atlantic, contributing to mild climates in western Europe. As the affected zone becomes less saline, surface water density decreases, strengthening the separation between surface and deep ocean layers and reducing vertical mixing — a process that normally allows surface waters to sink and redistributes nutrients and heat throughout the ocean.

Reduced vertical mixing has cascading ecological implications. When nutrients from deeper waters fail to reach the sunlit surface zone, marine organisms dependent on upwelled nutrients face food scarcity. Weaker mixing also prevents excess heat in surface waters from dissipating into deeper layers, further warming shallow waters already under thermal stress and threatening marine biodiversity. The study links Southern Indian Ocean freshening to changes in global ocean circulation patterns that could affect climate regulation and marine ecosystem health far beyond the southwest Australian coast, including potential consequences for the Indian Ocean's role in monsoon dynamics.

Key figure — Southern Indian Ocean salty zone decreased by 30% in area over the past six decades