Antarctic Sea Ice Decline Coincides with Saltier Intermediate Ocean Water, Indian-Led Study Finds

On 28 October, three researchers published a finding that begins with a very small number: Antarctic Intermediate Water, a major layer of the Southern Ocean, has grown saltier by 0.0048 grams per kilogram per decade. The change is tiny in absolute terms. What interested the team is that it runs against the direction the water had been moving for decades.

The study comes from researchers at the Indian Institute of Science, the University of Melbourne and the University of Southampton. The lead author is Mishra. His co-authors are Bishakhdatta Gayen, a professor at IISc and the University of Melbourne, and Alberto C Naveira Garabato, a professor at the University of Southampton.

Salinity was not what they set out to study. Mishra and his team wanted to understand how Antarctic Intermediate Water flows and mixes into the Indian Ocean, and how its properties shift over time. Going through recent measurements, they saw a sudden reversal around 2015-2016. For decades the water mass had been getting fresher. After Antarctic sea ice declined, that trend turned.

The mechanism, as the researchers describe it, runs through meltwater. Sea ice forms in the Antarctic winter and melts as temperatures rise, releasing freshwater into the ocean that feeds the intermediate water. That water sinks below the surface and slowly drifts northwards. Gayen said that until 2016, the seasonal summer melt was triple the Amazon River's entire annual flow of roughly 6,600 cubic km. From 2016, sea ice shrank to record lows, and meltwater delivery fell to roughly double the Amazon's annual flow. With less freshwater arriving, saltier water from the ocean's depths is moving upward, changing the balance between layers.

The team relied mainly on satellite observations of sea ice and on measurements from Argo floats, small robotic instruments that record ocean temperature and salinity at different depths. Mishra said the change may look small, but it affects seawater density and how ocean layers mix.

He framed the stakes by asking why anyone in India should care. His answer is that this intermediate water acts as a barrier between the upper and deeper oceans. If the barrier reorganises, he said, the ocean's heat content will reorganise too, and 90% or more of human-caused heat is stored in the global oceans.

The researchers are careful about what they have not shown. The water is part of the Atlantic Meridional Overturning Circulation, a system of currents that moves heat across the Atlantic, and changes here could influence that larger system, though this remains to be proven. Changes in ocean layering also affect how much carbon the sea can store, but the study did not quantify that. A saltier intermediate water might eventually affect India's summer monsoons, but Mishra said a single decade of observations is not enough for accurate predictions.

His practical conclusion concerns the models. Current climate models, he said, have not yet learnt to factor in these changes, and oceanographers should begin adjusting them rather than wait another 20 or 30 years. He added that many of the impacts would already have played out by the time humanity learns to foresee them.

The work also reflects something Mishra wanted to pursue. Climate coverage in India, he said, is mostly about the monsoon, and few people in the country research Antarctica or the Southern Ocean. He believes these changes will affect India, and soon. What to watch is whether more years of Argo data confirm the reversal, and whether modellers build it in.