In September 2017, satellites spotted a vast patch of open water in the middle of the frozen Weddell Sea, off Antarctica. By late October it covered roughly 80,000 square kilometres, about the size of Scotland. It was not a hole in the ground or in the continent itself, but a gap in the floating sea ice called a polynya. Scientists have since worked out much of why it formed: a combination of unusual ocean conditions over an underwater mountain called Maud Rise and powerful winter storms.
Here is what a polynya is, what happened in 2016 and 2017, what later research found, and why oceanographers care so much about a patch of open water few people will ever see.
The photograph above shows Antarctic icebergs and sea; the polynya itself was far out in the pack ice and visible mainly from space.
What a polynya is
A polynya is an area of open water surrounded by sea ice. The word comes from Russian and was picked up by polar explorers in the nineteenth century. Polynyas come in two broad types:
- Coastal (latent heat) polynyas form when strong, cold winds blowing off the continent push newly formed ice away from the shore. Ice keeps forming in the gap and is swept away, so these areas act like sea-ice factories.
- Open-ocean (sensible heat) polynyas form far from land when relatively warm water rises from below and keeps the surface from freezing. The Maud Rise polynya belongs to this rarer type.
Coastal polynyas are common and reappear every year. Open-ocean polynyas of this size are unusual, which is why the 2017 event made headlines around the world.
The 2017 opening and the 1970s giant
The hole sits over Maud Rise, a submerged mountain in the eastern Weddell Sea. NASA's Terra satellite photographed it on 25 September 2017. According to NASA's Earth Observatory, it grew from about 9,500 square kilometres in mid-September to about 80,000 square kilometres by late October. A smaller opening had appeared in the same area during the 2016 winter.
A satellite-based map of the eastern Weddell Sea, with latitude and longitude marked, showing the open water of the polynya (dark area) within the surrounding pack ice.
The location was remote enough that even scientists depended on satellites to know it existed. Atmospheric physicist Kent Moore of the University of Toronto told Motherboard at the time: "This is hundreds of kilometers from the ice edge. If we didn't have a satellite, we wouldn't know it was there."
Polynyas here are not new. Satellites first recorded one over Maud Rise in 1973, and in the winters of 1974, 1975 and 1976 a much larger Weddell Polynya opened, by NASA's description each time larger than the state of California. After that it largely disappeared for four decades, which is why some researchers had wondered if it had become a thing of the past.
What caused it: storms, salt and an underwater mountain
When the hole reappeared, researchers openly admitted they did not know why. Winter fieldwork in the Southern Ocean has to be planned years ahead, and few direct measurements of such events existed. Since then, at least three studies have filled in much of the picture.
Salty water and strong storms (Nature, 2019)
A team led by oceanographers at the University of Washington combined satellite images with data from robotic floats in the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project, and from instruments carried by elephant seals. They found that the polynyas formed when several factors lined up: unusually salty surface water and intense winter storms with winds close to hurricane strength. Under those conditions, mixing brings warmer deep water to the surface, where it cools, becomes denser and sinks again. That loop keeps delivering heat to the surface and stops ice from forming.
Polar cyclones as the trigger (JGR Atmospheres, 2019)
A study led by Diana Francis of New York University Abu Dhabi focused on the atmosphere. In winter 2017, air currents carried heat and moisture from lower latitudes toward Antarctica more strongly than in 2016, producing frequent, intense cyclones over the sea ice. Cyclonic winds push floating ice away in opposite directions around the storm's centre, opening water. Once open, the ocean takes over and keeps the gap from refreezing.
A stronger gyre and a "corkscrew" of salt (Science Advances, 2024)
A later study led by Aditya Narayanan at the University of Southampton found that in 2016 and 2017 a strengthened Weddell Gyre, the large circular current in the region, brought heat and salt closer to the surface. Winds then helped draw salty water upward around the flanks of Maud Rise. Saltier water freezes at a lower temperature, which helped the polynya open and stay open.
A satellite view of the open-water area of the Maud Rise polynya, the dark shape surrounded by white pack ice.
Why scientists care about a hole in the ice
Open water in an Antarctic winter acts like a window between a relatively warm ocean and an extremely cold atmosphere. Heat pours out of the sea, and the ocean surface cools and becomes denser. This is part of the process that forms Antarctic Bottom Water, the cold, dense water that spreads along the sea floor and helps drive the global overturning circulation of the oceans.
Polynyas also matter for carbon. The deep Southern Ocean holds a large store of carbon dioxide. When deep water is mixed up to the surface, some of that carbon can escape into the air. University of Washington oceanographer Ethan Campbell warned that a large release of this kind "could really whack the climate system if it happened multiple years in a row."
And polynyas leave a mark long after they close. Researchers on the 2024 study noted that their imprint on ocean temperature and salinity can last for several years.
For wildlife, open water in the ice is a lifeline. Polynyas let light reach the water, feeding plankton blooms, and give seals and penguins access to air and food. Studies of coastal polynyas have found that most Adélie penguin colonies in East Antarctica sit next to one.
Myths and quick answers
- Is it a hole in Antarctica's land ice? No. It is open seawater inside floating sea ice, hundreds of kilometres from the coast.
- Is it related to the ozone hole? No. The ozone hole is a thinning of ozone high in the atmosphere. The polynya is an ocean and sea-ice event.
- Did climate change cause it? At the time, Kent Moore called that conclusion premature, and the research since then points to storms, salinity and ocean currents as the direct causes. How climate change will affect how often such polynyas open is still an active research question.
- Were aliens involved? Online jokes compared it to science fiction films, but the physics of wind, heat and salt explains it well.
What to watch next
The key questions now are how often the conditions will line up again and whether a warming Southern Hemisphere with stronger winter winds will make large open-ocean polynyas more frequent. Robotic floats, seal-borne sensors and satellites now watch the region year-round, so the next time the Weddell Sea opens up, scientists will be measuring it from the inside rather than guessing from orbit.
Sources: NASA Earth Observatory: Deciphering the Maud Rise Polynya (2019); ScienceAlert: why there are holes in the Antarctic sea ice (11 June 2019); Live Science on the 2024 Science Advances study (May 2024); Wikipedia: Polynya. Kent Moore's comments were given to Motherboard in October 2017.
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