Researchers at UC San Diego’s Scripps Institution of Oceanography participated in the study, led by the University of Cambridge, that compiled long-term ocean measurements collected by ships and autonomous floats to show that a warm water mass called “circumpolar deep water” has expanded and shifted toward the Antarctic continental shelf over the past 20 years.
“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,” said Scripps physical oceanographer Sarah Purkey.
Ice shelves play an important role in holding back Antarctica’s inland ice sheets and glaciers, which collectively hold enough freshwater to raise sea level by about 58 meters (190 feet).
“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.
It’s the first time that scientists have observed the shift in deep-ocean heat throughout the Southern Ocean, said Lanham. “It’s something that had been predicted by climate models due to global warming, but we hadn’t seen in data.”
The study was published in the journal Nature Communications Earth & Environment.
Previous observations of the Southern Ocean, which encircles Antarctica, were limited to transects recorded by ships roughly once a decade. This information, collected as part of a long-running international program, provided detailed snapshots of temperature, salinity, and nutrients throughout the water column, but without continuous data, scientists were more uncertain about long-term changes in heat distribution.
To fill the gaps in the record, the researchers, including scientists from Scripps Oceanography and UCLA, supplemented the ship measurements with publicly available data collected by a global array of robotic floating instruments known as Argo. They merged the two datasets to gather monthly snapshots that allowed them to uncover the shift in warm waters.
It makes sense that this pool of warm water is expanding, Purkey said. More than 90% of excess heat from global warming is stored in the ocean, with the Southern Ocean absorbing most of the human-induced heat.
Oceanographer Sarah Purkey describes the workings of SOLO floats used in the Argo network. (Video credit: Argo Project)
The findings have implications beyond Antarctic ice melt and sea-level rise, said Ali Mashayek, one of the senior authors of the study from Cambridge Earth Sciences. “The Southern Ocean plays a key role in regulating global heat and carbon storage, so changes in heat distribution here may also have wider implications for the global climate system.”
In the frigid waters around the poles, extremely cold, dense water forms and sinks to the deep ocean. As the water sinks, it draws down heat, carbon, and nutrients, setting in motion a global ‘conveyor belt’ of currents, including the Atlantic Meridional Overturning Circulation (AMOC), which shuttles water around the Atlantic Ocean.
Climate models, including those used by the IPCC, indicate that warmer air temperatures and added freshwater from ice melt are reducing the formation of this dense water in the North Atlantic, potentially leading to a weakening of the AMOC.
Similar changes have recently been forecast for the Southern Ocean. Climate models suggest that the production of cold, dense water will decline in Antarctica, causing the warmer, circumpolar deep water to draw toward the continent to occupy the space left by the shrinking cold water.
“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.”
Additional study co-authors are Matthew Mazloff of Scripps Oceanography, Kaushik Srinivasan of UCLA, and Laura Cimoli of Cambridge.
