Saltier water is appearing several hundred meters below the surface of the ocean surrounding Antarctica. The phenomenon is linked to the retreat of sea ice. But how can melting ice make the sea saltier?
It all comes down to the seasonal cycle. During the Antarctic winter, some seawater freezes at the surface and forms sea ice. In summer, this less salty ice partially melts and releases relatively fresh water into the seawater below.
Sea ice has retreated almost uniformly around Antarctica, except in the Amundsen Sea.
Image credit: NOAA Climate.gov, based on NSIDC data
Since 2016, Antarctic sea ice has experienced several years of sharp decline. Yet when there is less ice, there is also less to melt during the following summer. The ocean therefore receives less relatively fresh water from this seasonal melting.
This explains the paradox: even in a warming climate, the retreat of sea ice can reduce the input of relatively fresh water associated with its melting.
The researchers studied the consequences for a large body of water called Antarctic Intermediate Water. It forms in the Southern Ocean, then sinks to depths of about 700 to 1,200 m. It subsequently flows northward into the Atlantic, Indian and Pacific oceans.
For several decades, this water was gradually becoming less salty. But a study published on October 28, 2026, in Nature Climate Change shows that the trend reversed around 2016. Observations through 2024 now indicate saltier, denser water located at greater depths.
The decrease in seasonal melting is not the only explanation. The researchers also report an increased upwelling of deeper waters, which are warmer and saltier. These two phenomena alter the properties of the water feeding this layer.
To discover this change, the team notably used measurements from the Argo network's automated floats. These devices descend into the ocean and record its temperature and salinity. The scientists compared these data with satellite observations of sea ice.
This matters because the ocean consists of layers of water whose density varies according to temperature and salinity. Their organization influences the circulation of water and the transport of heat, oxygen and carbon.
The transformation observed near Antarctica could therefore have consequences much farther north. In particular, it could alter how heat penetrates the deep ocean. The scale and timing of these effects remain to be determined.