In a paper published Wednesday, researchers described the change in Antarctic Intermediate Water, a band that runs from about 2,300 to 4,000 feet deep and encircles the continent.
They link it to the rapid decline of Antarctic sea ice.
The team combined six independent datasets covering 2004 to 2024, including readings from Argo floats, robotic instruments that drift through the ocean, dive to different depths to measure properties such as salinity and density, then surface to transmit their data.
Antarctic sea ice hit a satellite-era high in 2014, fell to a record low two years later, and dipped again in 2023, when it was about 1 million square miles below the 1981–2010 average.
With less ice, less freshwater is released during the melt season, making the layer saltier.
Gayen said changes in the layer's density, thickness, and salinity could alter how the ocean stores and redistributes heat, and that impacts could arrive within five years, possibly including more frequent storms and ocean heat loss identified in a 2024 study.
Michael Meredith of the British Antarctic Survey called the Southern Ocean "the key juncture for all the world's oceans" and noted its major role in absorbing human-caused carbon dioxide. He cautioned that it is not yet clear whether the shift is permanent, since the observational record is short, and said global climate models used to guide policy still struggle to reproduce recent sea-ice losses. An earlier review this year concluded that the speed and persistence of the decline suggest the region may be moving into a new state. Meredith added that the upper ocean circulation can influence climate within years to decades rather than centuries, and that if low sea ice persists, the changes will keep spreading through the world's oceans.