The study, led by the University of Cambridge with collaborators at the University of California, Scripps Institution of Oceanography, and UCLA, drew on long-term ocean measurements collected by research ships and autonomous robotic floats. The data reveal that a warm water mass known as circumpolar deep water has expanded and shifted poleward toward the Antarctic continental shelf over the past 20 years.
"It's concerning, because this warm water can flow beneath Antarctic ice shelves, melting them from below and destabilizing them," said Joshua Lanham, lead author of the study at Cambridge Earth Sciences.
Ice shelves act as buttresses, restraining Antarctica's inland ice sheets and glaciers. Those frozen reserves collectively contain enough freshwater to raise global sea levels by approximately 58 metres if fully discharged.
Previous observations of the Southern Ocean were largely confined to transects conducted by ships on roughly decadal intervals. These hydrographic sections, gathered under a long-running international programme, captured detailed snapshots of temperature, salinity, and nutrients throughout the water column. Without continuous data, however, scientists faced significant uncertainty about long-term trends in heat distribution.
To close those observational gaps, the researchers supplemented ship records with publicly available data from the global Argo array - autonomous profiling floats that drift through the upper ocean and deliver continuous measurements. Because the Argo programme has not been operating as long as shipboard surveys, the team applied machine learning to blend the float data with long-term patterns from the hydrographic sections, constructing a new monthly record spanning roughly four decades.
That combined record allowed the researchers to detect the poleward migration of warm water for the first time in observational data rather than in model output alone. "It's something that had been predicted by climate models due to global warming, but we hadn't seen it in data," said Lanham.
Prof. Sarah Purkey of the Scripps Institution of Oceanography, a senior author of the study, described the shift using a vivid analogy. "In the past, the ice sheets were protected by a bath of cold water, preventing them from melting. Now it looks like the ocean's circulation has changed, and it's almost like someone turned on the hot tap and now the bath is getting warmer!"
The expansion of this warm water pool is consistent with the known heat budget of the global ocean. More than 90 percent of excess heat generated by global warming is absorbed by the oceans, with the Southern Ocean taking up the largest share of that anthropogenic warming.
The findings carry implications beyond ice melt and sea level rise. Prof. Ali Mashayek, another senior author from Cambridge Earth Sciences, noted that the Southern Ocean is central to regulating global heat and carbon storage, meaning that shifts in its heat distribution have consequences for the entire climate system.
The physical mechanism underlying the observed change is linked to the production of dense, cold bottom water near Antarctica. In the frigid polar seas, extremely cold and saline water sinks to the deep ocean, driving a global thermohaline circulation - the so-called conveyor belt of ocean currents - that includes the Atlantic Meridional Overturning Circulation. Climate models, including those assessed by the IPCC, project that warmer air temperatures and freshwater input from melting ice are reducing the formation of this dense bottom water, which in turn weakens the overturning circulation.
Analogous changes have been forecast for the Southern Ocean, with models suggesting that declining cold bottom water production near Antarctica will allow warmer circumpolar deep water to migrate into the space vacated by the retreating dense water. The new observational record now shows that process is already underway.
"We can now see this scenario is already emerging in the observations," said Lanham. "This isn't just a possible future scenario suggested by models; it's something that is happening now, bringing wider implications for how carbon, nutrients and heat are cycled through the global ocean."
Research Report:Poleward migration of warm Circumpolar Deep Water towards Antarctica
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