In these polar waters, cold, fresh surface water overlays warmer, saltier waters from the deep. In the winter, as the surface cools and sea ice forms, the density difference between water layers weakens, allowing these layers to mix and heat to be transported upward, melting the sea ice from below and limiting its growth.
Since the early 1980s, the surface of the Southern Ocean had been freshening, and stratification – that density difference between the water layers – had been strengthening. This was trapping heat below and sustaining more sea ice coverage.
Now, new satellite technology, combined with information from floating robotic devices which travel up and down the water column, shows this trend has reversed; surface salinity is increasing, stratification is weakening, and sea ice has reached multiple record lows – with large openings of open ocean in the sea ice (polynyas) returning.
This is the first time scientists have been able to monitor these changes in the Southern Ocean in real time.
Aditya Narayanan, a postdoctoral research fellow at the University of Southampton and co-author on the paper, said: “While scientists expected that human-driven climate change would eventually lead to Antarctic Sea ice decline, the timing and nature of this shift remained uncertain.
“Previous projections emphasised enhanced surface freshening and stronger ocean stratification, which could have supported sustained sea ice cover. Instead, a rapid reduction in sea ice – an important reflector of solar radiation – has occurred, potentially accelerating global warming.”
What this all means is that – according to Professor Alberto Naveira Garabato, co-author on the study and Regius Professor of Ocean Sciences at the University of Southampton – our current understanding “may be insufficient” to accurately predict future changes.
“It makes the need for continuous satellite and in-situ monitoring all the more pressing, so we can better understand the drivers of recent and future shifts in the ice-ocean system.”
The paper – ‘Rising surface salinity and declining sea ice: a new Southern Ocean state revealed by satellites is published in Proceedings of the National Academy of Sciences and is available online.
This project has been supported by the European Space Agency.
Click here for more from the Oceanographic Newsroom.