Southern Ocean Layer Turns Saltier After Sea Ice Collapse

A major ocean layer has switched from getting fresher to getting saltier.

Researchers published a paper Wednesday detailing this shift in the Southern Ocean. The change affects a vast band of water extending from 2,300 to 4,000 feet deep. This band, known as Antarctic Intermediate Water, encircles Antarctica. The new salinity levels reverse a trend that persisted for decades. The speed of the change matches shifts previously seen only over thousands of years.

Bishakhdatta Gayen, a professor at the University of Melbourne, co-authored the study. He explained that this water layer spreads northward into the Indian, Pacific, and Atlantic oceans. It carries heat, carbon, oxygen, and nutrients toward the tropics. The flow even crosses the equator. Gayen noted that the study combined six independent datasets to track these changes from 2004 to 2024. The data came from Argo floats. These devices drift through the ocean, descend to various depths, and measure water properties like density and salinity. They then resurface to transmit the readings. A separate 2025 study also detected rising surface salinity linked to the same ice decline.

The trigger for this ocean change was the collapse of Antarctic sea ice. Satellite records show sea ice reached its highest extent in 2014. Just two years later, it dropped to a record low. The ice has remained well below the 2014 levels since then. It hit another low point in 2023. At that time, the ice cover measured about 1 million square miles below the 1981 to 2010 average. That represents an area roughly four times the size of Texas disappearing from the region. As the ice dwindles, less freshwater enters the ocean during the melt season. This lack of fresh input makes the intermediate water saltier.

Changes in the density, thickness, and salinity of this layer influence how heat moves between the ocean surface and deeper waters. Gayen stated that these changes could affect how the ocean stores and redistributes heat. He emphasized that more monitoring is needed to track the ongoing shift. He warned that the impacts are potentially knocking on the door within five years.