In the mid-1970s, the first available satellite images of Antarctica during the polar winter revealed a huge ice-free region within the ice pack of the Weddell Sea. This ice-free region, or polynya, stayed open for three full winters before it closed.
Subsequent research showed that the opening was maintained as relatively warm waters churned upward from kilometres below the ocean's surface and released heat from the ocean's deepest reaches. But the polynya - which was the size of New Zealand - has not reappeared in the nearly 40 years since it closed, and scientists have since come to view it as a naturally rare event.
Now, however, a study led by researchers from McGill University suggests a new explanation: The 1970s polynya may have been the last gasp of what was previously a more common feature of the Southern Ocean, and which is now suppressed due to the effects of climate change
on ocean salinity.
The McGill researchers, working with colleagues from the University of Pennsylvania, analyzed tens of thousands of measurements made by ships and robotic floats in the ocean around Antarctica over a 60-year period. Their study, published in Nature Climate Change
, shows that the ocean's surface has been steadily getting less salty since the 1950s. This lid of fresh water on top of the ocean prevents mixing with the warm waters underneath. As a result, the deep ocean heat has been unable to get out and melt back the wintertime Antarctic ice pack.