Ocean measurements show a vast Southern Hemisphere water layer growing saltier and denser, a shift that could send more heat into deep waters than climate models predict.
Every Antarctic summer, melting sea ice sends a vast pulse of fresh water into the ocean. That seasonal supply helps sustain Antarctic Intermediate Water (AAIW), a layer of relatively fresh seawater that spreads across much of the Southern Hemisphere’s ocean. The AAIW helps store carbon, supply oxygen, and drive global currents, redistributing heat, fresh water, and carbon around the planet.
From 1995 to 2015, summer ice melt helped maintain this layer. But from 2016 to 2024, reduced freshwater input left it saltier, denser, and deeper, abruptly reversing its long-term trends in salinity, or salt content, and density.
The findings, published in Nature Climate Change, provide the first direct observational evidence of this change in the AAIW’s properties. The international research team analyzed data from the global Argo program, a network of around 4,000 robotic floats that sample the upper oceans, alongside satellite and atmospheric datasets.
Less sea ice, less fresh water
The study was led by Associate Professor Bishakhdatta Gayen, of the University of Melbourne and the Indian Institute of Science (IISc), and Kaushik Mishra, a PhD student at IISc. Gayen said:
“The Antarctic region is the freshwater engine of oceans around the globe. Melting Antarctic sea ice influences circulation thousands of kilometers away. Until 2016, the seasonal summer melt was triple the Amazon River’s entire annual flow of roughly 6,600 cubic km.
“From 2016, Antarctic sea ice shrank to record lows. This abruptly choked off that critical seasonal surge, and the meltwater delivery dropped to roughly double the Amazon’s annual flow.”
Co-author Professor Alberto Naveira Garabato, of the University of Southampton in the United Kingdom, described the changes as an urgent reminder that climate change is affecting almost every part of Earth’s surface.
“As this altered, saltier and denser water mass travels north, it moves across the Atlantic, Indian and Pacific oceans. Because this water layer forms part of the upper limb of the ocean’s global ‘overturning circulation’, a sustained shift could, over years to decades, feed into the Atlantic Meridional Overturning Circulation – the deep-ocean current system often linked to climate stability in the North Atlantic.
“It could also ultimately influence atmospheric circulation, weather, and storm tracks in the Southern Hemisphere. However, the impact can cascade globally.”
The globe shows sea ice around Antarctica month by month. The first time-series graph shows whether sea-ice area was above or below normal (the 1993-2024 mean), based on satellite observations. The bottom graph tracks the saltiness of Antarctic Intermediate Water, a layer that sinks from the surface into the ocean’s interior, using Argo float data from 2004. From 1993 to 2015, Antarctic sea ice slowly expanded (blue trend line), releasing more freshwater, making the waters beneath it fresher. After 2016, sea ice dropped sharply (red trend line), and the layer turned saltier. Less melting ice means less freshwater reaching the ocean interior. Credit: Kaushik Mishra
More heat could reach deeper waters
Changes in how water moves between the surface and deeper layers could also affect the oceans’ role as a “climate buffer,” as Gayen described it. Oceans absorb and redistribute most of the excess heat and a large share of human CO₂ emissions, storing them for decades to centuries. This slows and moderates atmospheric warming and its immediate effects.
Mishra said the abrupt reversal revealed a critical missing feedback loop.
“The ocean absorbs over 90 percent of all heat released into the environment by human activities, with most of that heat stored in the upper ocean. A weakened AAIW layer could push more excess heat deeper. That would mean existing models are underestimating deep-ocean warming and therefore miscalculating regional sea level rise and long-term ocean and land surface temperatures.”
“If the altered AAIW changes the way surface water is carried down into deeper layers, it could affect how much carbon the ocean locks away – although the magnitude and timing are uncertain,” Gayen said.
“This could be the tipping point for the Atlantic Ocean and the AAIW current, leading to larger, longer-lasting changes to the climate in the Southern Hemisphere.”
According to Gayen, the last time the AAIW switched was at the end of the last interglacial period, around 130,000 years ago, when there was very little ice in Antarctica.
“That transition took thousands of years, not two decades,” he said.
Reference: “Reversal of Antarctic Intermediate Water trends triggered by sea ice decline” by Kaushik Mishra, Bishakhdatta Gayen and Alberto C. Naveira Garabato, 28 October 2026, Nature Climate Change.
DOI: 10.1038/s41558-026-02758-y
K.M. acknowledges the Prime Minister’s Research Fellowship (grant no. PMRF-0201843). B.G. acknowledges the Ministry of Earth Sciences Grant (grant no. MOES/11/2024) and the Australian Research Council Discovery Grant (grant no. DP240102823). A.C.N.G. acknowledges U.K. Research and Innovation guarantee funding for a European Research Council Advanced Grant (grant no. EP/X025136/1).
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