The Southern Ocean is experiencing an unexpected increase in its salinity that could trigger the irreversible loss of sea ice in Antarctica.
This is revealed by new satellite data collected in a study published in Proceedings of the National Academy of Sciences of the United States of America (PNAS).
This change is especially difficult to percibir, however, it is essential to understand how this occurs.
It’s because, “when salinity is high, sea ice is scarce. When salinity is low, there is more sea ice,” explains Alex Haumann, a climate scientist from Ludwig-Maximilians University in Munich, Germany, who participated in the analysis.
Thus, the salinity level of the Southern Ocean is a key point: its increase above normal levels disrupts the delicate oceanic balance and threatens to create a dangerous feedback loop.
The delicate balance of Antarctica and its ocean
Why does higher salinity in the Southern Ocean threaten the marine ecosystem? To understand this, it is necessary to comprehend how its delicate balance works.
This ocean has a system of stratified layers: cold and less salty water on the surface, and warm and salty water in the depths.
This marine balance is maintained thanks to snowfall and sea ice that provide freshwater to the upper layer.
Thus, the cold layer acts as a natural insulator that prevents the warm water from the depths from rising to the surface.
Generally, the sea ice forms near the Antarctic coast and moves north before melting in the open ocean, thus maintaining the stratification.
But now, the main problem is that the salty surface water is too warm for ice to form.
This weakens the stratification and creates convection currents that bring even more warm water from the depths, increasing its temperature and causing an uncontrolled loss of ice.
Why salinity is increasing in the Southern Ocean
Although it was initially believed that greater ice melting should release more freshwater to the surface, thus reinforcing the stratification, today the opposite is happening: salinity is increasing.
This is explained by Inga Smith, a physicist specializing in sea ice from the University of Otago: “With warming, we expect more freshwater to flow into the ocean. Therefore, it is quite surprising that this saltier water appears on the surface.”
Alessandro Silvano, an oceanographer from the University of Southampton and leader of the study, suggests that the warm and salty water from the depths is rising to the surface due to “a regime change, a change in the oceanic and glacial system.”
The most likely cause is the westerly winds around Antarctica, which have strengthened as a result of climate change.
These winds alter the currents and cause the deep water to rise.
The technology that revealed the problem
Measuring ocean salinity in Antarctica was practically impossible during the winter due to the sea ice and darkness.
But now, the satellites of the European Space Agency (SMOS) can observe the entire Southern Ocean with a resolution of 25 square kilometers.
Haumann highlighted the value of these data: “Due to the large coverage and the time series you can obtain, it is super valuable. It is truly a new tool to monitor this system.”
Additionally, the team verified the satellite data with Argo buoys, which take water samples up to 2000 meters deep.
“They show the same signal [as the satellites],” confirmed Silvano. “We thought, okay, this is real. It’s not an error.”
Thus, with the change in the marine balance, Haumann warned about the urgency of monitoring.
“This is one of the most remote regions on Earth, but one of the most critical for society. Most of the excess heat we have in the climate system ends up in this region,” he noted.
