Antarctic Intermediate Water Changes as Sea Ice Shrinks
Antarctic Intermediate Water has rapidly turned saltier and denser across the Atlantic, Indian, and Pacific ocean basins following a sharp drop in polar sea ice. Researchers analyzed data from the global Argo program—a network of roughly 4,000 robotic floats sampling the upper oceans—alongside satellite observations to track the shifts, Xinhua reported.
The vast layer of water spans much of the Southern Hemisphere and drives global ocean currents, stores carbon, and supplies oxygen. According to findings published in Nature Climate Change, summer sea-ice melt helped maintain the water layer from 1995 to 2015. After sea ice declined to record lows beginning in 2016, reduced freshwater input caused the intermediate layer to become saltier, denser, and deeper.
Polar Freshwater Engine Under Strain
Antarctic sea ice acts as a giant climate engine that regulates surface water properties. The Indian Institute of Science (IISc) noted that freshwater from melting sea ice helps maintain a relatively fresh layer of cold water near the surface, which mixes with deeper water and sinks to form the Antarctic Intermediate Water as it moves northward.
Between 1993 and 2015, Antarctic sea ice expanded slowly, releasing more freshwater that made the waters beneath it fresher and shallower. That pattern reversed entirely between 2016 and 2024 as seasonal meltwater inputs fell sharply by 20% to 30%. Stripped of its polar freshwater supply, the layer turned saltier by about 0.0048 grams per kilogram per decade around the Southern Ocean, with stronger changes occurring in specific regions.
Antarctic Water Changes Push Heat into Deep Oceans
Changes in the Antarctic Intermediate Water carry the effects of conditions near Antarctica far beyond the polar region. Kaushik Mishra, a PhD student at the Centre for Atmospheric and Oceanic Sciences at IISc and lead author of the study, explained that these water masses connect the polar ocean to the tropical ocean. When less meltwater enters the system and more salty deep water reaches the upper ocean, stratification is disrupted.
The altered mixing allows stored carbon to escape more easily into the atmosphere and pushes excess heat into deeper waters. Researchers warned that existing climate models may underestimate deep-ocean warming and miscalculate regional sea-level rise. The last time the water layer underwent a comparable shift was around 130,000 years ago at the end of the last interglacial period when Antarctica held very little ice, though that transition took thousands of years rather than two decades.
Frequently Asked Questions About Antarctic Intermediate Water
What is the Antarctic Intermediate Water?
It is a major subsurface water mass formed in the Southern Ocean that spreads northward into the subtropical Atlantic, Indian, and Pacific oceans hundreds of meters below the surface. It helps drive global ocean circulation, distribute heat and freshwater, and regulate carbon storage.
How do researchers track changes in this water layer?
Scientists use data from the Argo program—an international network of about 4,000 autonomous robotic floats that measure ocean temperature and salinity beneath the surface—combined with satellite observations of sea ice and atmospheric datasets.
Why did the water layer reverse its trend after 2015?
A sharp decline in Antarctic sea ice driven by ocean warming and atmospheric shifts choked off the seasonal freshwater surge from summer melt. Without that freshwater input, alongside an increased upward movement of warmer, saltier water from the depths, the layer became saltier, denser, and deeper, according to Xinhua and IISc.
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