The iciest waters around Antarctica are less icy

The sea ice that surrounds Antarctica, like that in the Arctic, is seasonal: it increases in extent during the winter and largely melts in the summer. The exception has been the Weddell Sea, east of the Antarctic Peninsula, which retains much of its ice from year to year because of cold winds from the south and a circular current, or gyre, that keeps the ice from drifting into warmer waters that would cause it to melt more.

That makes the Weddell an important area for species of penguins and seals and other creatures that prefer colder, icier conditions.


But from 2016 to 2017, the Weddell had a sharp drop in sea ice coverage during the Southern Hemisphere summer compared with the previous summer. The loss was nearly 350,000 square miles, or about one-third of the recent average summer coverage. What’s more, that loss of sea ice has persisted, with only slight increases in coverage each summer since then.

So what’s going on? And is this reduced sea ice coverage just temporary, or will it continue, with potentially harmful effects on marine ecosystems?


After analyzing satellite data and weather records, researchers from the British Antarctic Survey and other institutions have answered the first question. They say two elements led to the initial sea ice loss in 2016-17, and the loss has persisted because of a phenomenon also seen in the Arctic.

First, they report in a recent study in Geophysical Research Letters, unusually strong westerly winds in September 2016 — the start of spring in Antarctica — pushed a lot of sea ice out of the Weddell. And that December, the start of summer, record strong storms brought in warm air from the northeast.

“That caused a very early melting of sea ice,” said John Turner, a British Antarctic Survey meteorologist and lead author of the study.

The second element was equally unusual, Turner said in an interview. In November 2016 a large opening in the sea ice, called a polynya, appeared in the eastern part of the sea. Such a large opening had last appeared four decades before.

The 2016 polynya and the storms were no doubt linked, Turner said, as the rough conditions served to break up the sea ice.

“But that would just freeze over again,” he said. “You need an ocean element to keep it open.” That came in the form of warmer deep-ocean waters that rose to the surface, keeping the polynya from refreezing.

With less ice in the Weddell in 2016-17, a feedback mechanism came into play. Ice reflects most of the sun’s rays, while the darker ocean absorbs more of them. As a result the ocean warms, which in turn causes more sea ice to melt, leading to more open water and still more ocean warming and melting ice. It’s the same phenomenon that occurs in the Arctic, and has led to steep declines in sea ice extent there over the last four decades.

In the Weddell, the researchers said, the initial loss of sea ice that winter led to a rise in the sea temperature of about 0.5 degree Celsius, or 0.9 degree Fahrenheit. That led to the near-runaway melting of ice during the rest of the winter and is why summer ice coverage has only slightly increased in the three summers since, including this year.


But that leads back to the second question: Will this reduced summer coverage in the Weddell persist, or will the sea ice eventually recover?

“It might rebuild,” Turner said. “We just don’t know. "

The recent history of Antarctic sea ice is much different from that of the Arctic, Turner said. While Arctic sea ice has been steadily declining, Antarctic sea ice was growing until about 2014, largely because of the presence of a large ozone hole over the region that led to colder conditions.

Since then, sea ice around the continent has been declining overall, with the drop in the Weddell accounting for much of the change. And with the ozone hole slowly healing, most climate models are forecasting that Antarctic sea ice will continue to decline. What’s happened in the Weddell may be a sign of that emerging long-term decline.

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