Decline in Antarctic sea ice has increased salinity: Study

IISc said that freshwater from Antarctic sea ice melt helps maintain a relatively fresh layer of cold water near the surface. | Photo Credit: File Photo

A new study has revealed that the Antarctic Intermediate Water (AAIW), a major water mass that carries the influence of the Southern Ocean into subtropical oceans, has become saltier and denser since 2015.

The study, carried out by researchers from the Indian Institute of Science (IISc), the University of Melbourne, and the University of Southampton, noted that this increase in salinity could have implications for the ocean’s structure and circulation.

According to IISc, sea ice forms across the Southern Ocean every year during Antarctica’s austral winter between May and July.

Seasonal cycle

“When temperatures rise, much of this ice melts, releasing freshwater into the ocean. This seasonal cycle helps shape the properties of the water beneath the surface, particularly the AAIW,” IISc said.

The study has found that before 2015, the AAIW had been freshening for decades, however since 2015, a sharp decline in Antarctic sea ice has led to increasing salinity.

IISc said that saltier AAIW, which forms in the Southern Ocean and spreads northward through the Atlantic, Indian and Pacific Oceans, typically hundreds of metres below the surface, causes a disruption in how ocean layers are stratified and allows stored carbon to escape into the atmosphere more easily, contributing to increased warming.

“Changes to this water mass can carry the effects of changes near Antarctica far beyond the polar region,” said Kaushik Mishra, the study’s lead author and a PhD student at the Centre for Atmospheric and Oceanic Sciences (CAOS) at IISc.

The researchers found that reduced freshwater from melting sea ice is directly linked to the reversal in AAIW’s salinity, alongside an increased upward movement of warmer, saltier water from the ocean’s depths.

The institute added that freshwater from Antarctic sea ice melt helps maintain a relatively fresh layer of cold water near the surface.

“This water mixes with deeper water and sinks beneath the surface, eventually contributing to the AAIW as it moves northward. The layer of cold water formed during the Antarctic winter, known as winter water, plays an important role in this process,” it said.

When less meltwater enters the system, and more salty deep water reaches the upper ocean, the balance changes. The water feeding into the AAIW becomes saltier and denser.

“Initially, we were interested in how the intermediate water flows in the Indian Ocean sector, how it mixes with other water masses, and how it is changing over the long term,” said Bishakhdatta Gayen, corresponding author of the study and Associate Professor at CAOS.

Changes in AAIW

The team of researchers while examining the observations found changes in AAIW and traced a connection to the changing sea ice conditions.

The team analysed ocean temperature and salinity measurements, including data from Argo – an international network of robotic floats that measure conditions beneath the ocean surface – alongside satellite observations of sea ice and other ocean datasets. They found an increase in AAIW salinity of about 0.0048 grams per kilogram per decade around the Southern Ocean, with stronger changes in some regions.

The findings also highlight the importance of closely monitoring the ocean beneath Antarctic sea ice. “These water masses connect the polar ocean to the tropical ocean, far from Antarctica,” Mishra said.

Published - October 28, 2026 07:47 pm IST