The change reverses decades of freshening in a layer that carries heat and carbon northward. Researchers propose a link to declining sea ice and call for more monitoring to track the changes.
Researchers have documented a sharp increase in salt content in Antarctic Intermediate Water, reversing decades of freshening. The study, published on October 28, examined a layer encircling Antarctica at depths of 2,300–4,000 feet.
That water spreads northward into the Indian, Pacific and Atlantic oceans, carrying heat, carbon, oxygen and nutrients toward the tropics and across the equator, co-author Bishakhdatta Gayen told Inside Climate News. Gayen is a researcher and professor affiliated with the University of Melbourne and the Centre for Atmospheric and Oceanic Sciences in Bengaluru.
Gayen said the study combined six independent datasets covering Southern Ocean changes from 2004 to 2024. The measurements came from drifting instruments used by Argo, an international ocean-research programme.
Standard Argo floats usually drift at about 1,000 metres. Every 10 days, they change their buoyancy, descend to about 2,000 metres and rise to the surface. Along the way, they measure temperature, pressure and electrical conductivity—how well the water conducts electricity. Scientists calculate salinity and density from those readings. At the surface, the floats transmit their data by satellite.
Antarctic Intermediate Water is a cold, relatively low-salinity body of water that forms as surface waters sink near the Antarctic Polar Front, generally between 50°S and 60°S. It occupies a shallower layer than the denser Antarctic Bottom Water, which fills ocean depths below 4,000 metres.
Antarctic sea ice reached its greatest extent in the satellite record in 2014, then abruptly fell to a record low in 2016. Another low followed in 2023, when its extent was about 1 million square miles below the 1981–2010 average.
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The researchers link the salinity reversal to that ice loss: less sea ice leaves less freshwater to enter the ocean during the melt season, making Antarctic Intermediate Water saltier.
Analysis
Gayen said changes in the layer’s density, thickness and salinity influence heat exchange between surface and deeper waters and could alter ocean heat storage and redistribution. He cited increased storm frequency and ocean heat loss identified in a 2024 study as possible effects, potentially within five years.
Michael Meredith, a British Antarctic Survey oceanographer offering an independent assessment, said the research shows that sea-ice loss since 2016 is already affecting ocean water properties and circulation. He said the Southern Ocean’s role in taking up human carbon dioxide emissions helps moderate climate change. Whether the salinity reversal is permanent remains unclear because the observational record is relatively short, he said.
If low sea-ice conditions persist, Meredith expects the changes to keep spreading through the oceans, with effects on climate and ecosystems. Upper-ocean circulation can affect climate within years to decades, he said. Both scientists called for more monitoring; Meredith also said climate models still struggle to reproduce recent sea-ice losses accurately, making further research necessary.
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