Southern Ocean Salinity Could Be Triggering Sea Ice Loss

The Southern Ocean exists in a state of precarious stability. The sea is layered, with chilly floor water sitting atop comparatively heat water. It’s an inherently unstable state of affairs—all else being equal, the nice and cozy water ought to rise to the highest. But it’s saltier and subsequently denser, so it lurks under. The chilly higher layer, in the meantime, is freshened by snowfall and sea ice, which varieties close to the coast, then drifts northward into the open ocean earlier than melting.

For the previous 10 years, sea ice cowl has been in decline as ocean temperatures have warmed. The speedy melting has delivered much more freshwater to the floor, which ought to strengthen the cold-water-layer’s insulative capability, permitting the ocean ice to ultimately increase once more.

But that suggestions loop seems to have been disrupted. New satellite tv for pc knowledge have revealed that the ocean round Antarctica, in opposition to all expectations, is getting saltier.

The study was printed in Proceedings of the National Academy of Sciences of the United States of America.

Measuring Where It’s Hard to Measure

Sea ice, tough seas, and 24-hour darkness make it practically inconceivable to observe Southern Ocean salinity from a ship within the winter. Only in recent times has it been attainable to measure salinity within the Southern Ocean from area. Satellites can observe the ocean floor’s brightness temperature—a measure of radiation given off on the sea floor. The brisker the water, the upper the brightness temperature.

The method works effectively in hotter waters, however in chilly waters, brightness temperature doesn’t shift as a lot as salinity modifications. Because these modifications are typically fairly delicate to start with, satellites haven’t been capable of precisely detect them in polar areas. In these areas, sea ice has additionally clouded the sign.

Recent advances in satellite tv for pc know-how, nevertheless, have tremendously improved the sensitivity of brightness readings, and new algorithms enable researchers to wash up noise from sea ice.

Oceanographer Alessandro Silvano of the University of Southampton and his colleagues analyzed the previous 12 years of salinity data from the European Space Agency’s Soil Moisture and Ocean Salinity (SMOS) satellite tv for pc. Team member Alex Haumann, a local weather scientist with Ludwig-Maximilians-Universität München, in Germany, stated having these broad knowledge, which cowl the whole Southern Ocean at 25-square-kilometer decision, is a sport changer. “Due to the big coverage and the time series you can get, it’s super valuable. It’s really a new tool to monitor this system.”

When the staff noticed that salinity had elevated over that interval, nevertheless, they couldn’t assist however query the know-how. To floor fact what they had been seeing, they turned to Argo floats—automated buoys that pattern water as much as 2,000 meters deep. A community of the floats dots the world’s seas, together with the Southern Ocean.

To Silvano’s shock and shock, the floats corroborated the satellite tv for pc knowledge. “They show the same signal,” he stated. “We thought, okay, this is a real thing. It’s not an error.”

Matching the salinity knowledge to traits in sea ice, the staff observed a disturbing sample. “There is a very high correlation between the surface salinity and the sea ice cover,” Haumann stated. “Whenever there’s high salinity, you have low sea ice. Whenever it is low salinity, there is more sea ice.”

“With warming, we expect more freshwater to be flowing into the ocean. So having this saltier water appearing at the surface is quite shocking,” stated Inga Smith, a sea ice physicist on the University of Otago in New Zealand who was not concerned within the analysis.

A Shifting Regime

The most believable rationalization for the enhance in salinity, Silvano stated, is that the fragile layers of Antarctic water have been upset, and the hotter, saltier water under is now bursting by to the floor, making the floor too heat for sea ice to type.

Though he pressured it’s too early to pinpoint a trigger for the upwelling, Silvano postulated that it might be pushed by stronger westerly winds round Antarctica—a results of the warming local weather. He stated he fears that Antarctica’s pure harm management mechanism, by which ice soften releases freshwater, which in flip traps the nice and cozy deep water and ultimately permits extra sea ice to type, is now irreversibly damaged.

The weakening of the ocean’s stratification as an alternative threatens to arrange a harmful new suggestions, whereby highly effective convection currents convey up much more heat, salty water from depth, resulting in runaway ice loss.

“We think this could be a regime shift—a shift in the ocean and ice system, where you have permanently less ice,” Silvano stated.

Wolfgang Rack, a glaciologist with the University of Canterbury in New Zealand who was not concerned within the analysis, stated the satellite tv for pc file will not be lengthy sufficient to indicate whether or not the rise in salinity is an anomaly, or a brand new state of regular, however “it is quite unlikely that it is a simple anomaly, because the signal is so significant.”

Zhaomin Wang, an oceanographer with Hohai University in Nanjing, China, who was not concerned within the analysis, stated the research was a “very firm result,” however cautioned that it’s nonetheless too early to conclusively pin the ocean ice retreat on upwelling. “It’s quite difficult to disentangle the cause and effect between Antarctic sea ice change and the surface salinity change,” he stated, “because it’s a coupled system, making it difficult to determine which process initiates the changes.”

For Haumann, the findings present how essential new know-how is for monitoring modifications within the Southern Ocean. “We have to find ways to monitor the system, because it’s changing very rapidly,” he stated. “This is one of the most distant regions on Earth, but one of the most critical for society. Most of the excess heat we have in the climate system goes into this region, and this has helped us keep the planet at a relatively moderate warming rate.”

“Now we don’t really know what will happen to that,” he stated.

—Bill Morris, Science Writer

Citation: Morris, B. (2025), Southern Ocean salinity could also be triggering sea ice loss, Eos, 106, https://doi.org/10.1029/2025EO250276. Published on 29 July 2025.

Text © 2025. The authors. CC BY-NC-ND 3.0
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