Mysterious holes in the Antarctic ice floe explained by years of robotic data

The winter ice on the surface of the Weddell Sea in Antarctica sometimes forms a huge hole. A hole appeared in 2016 and 2017 aroused a great curiosity from scientists and journalists. Although even larger gaps were formed decades ago, it was the first time that oceanographers had the opportunity to truly monitor the unexpected gaps in the Antarctic ice pack.

A new study by the University of Washington combines satellite images of sea ice cover, robotic dinghies and even seals equipped with sensors to better understand the phenomenon. The research explores why this hole appears in just a few years and what role it could play in the wider ocean circulation.

The study was published on June 10 in the journal Nature.

"We thought that this wide hole in the pack ice – called polynya – was something rare, maybe a process that had disappeared, but the events of 2016 and 2017 forced us to reevaluate it," said the main author. Ethan Campbell, PhD student in oceanography at UW. "The observations show that recent polynyas were caused by a combination of factors, one being unusual ocean conditions, and the other a series of very intense storms that swept the Weddell Sea with winds almost a hurricane force. "

A "polynya", a Russian word meaning "hole in the ice", can form near the shore when the wind pushes it. But it can also appear far from the coast and stay for weeks or even months, where it serves as an oasis where penguins, whales and seals can make their appearance and breathe.

This particular place, far from the Antarctic coast, often has small openings and has already seen large polynyas before. The largest known polynyas at this location were in 1974, 1975 and 1976, just after the launch of the first satellites, when a region the size of New Zealand remained ice-free for three consecutive winters in Antarctica, despite air temperatures well below freezing.

Campbell joined UW as a graduate student in 2016 to better understand this mysterious phenomenon. In a stroke of scientific luck, a big one appeared for the first time in decades. A satellite image of NASA in August 2016 drew public attention to an area of ​​33,000 km 2 (13,000 km 2) that appeared for three weeks. An even larger gap of 50,000 square kilometers (19,000 square miles) appeared in September and October 2017.

The Southern Ocean is believed to play a key role in global ocean currents and carbon cycles, but its behavior is poorly understood. It hosts some of the fiercest storms on the planet, with winds blowing continuously around the continent in the dark of 24 hours of polar winter. The new study used observations from the Southern Ocean Carbon and Climate Observing and Modeling Project, SOCCOM, which provides instruments that drift with currents to monitor Antarctic conditions.

The study also used data from the long-established Argo oceanographic observation program, elephant seals that transmit data to the shore, weather stations, and decades of satellite imagery.

"This study shows that this polynya is actually caused by a number of factors that must all be taken into account for this to happen," said co-author Stephen Riser, professor of oceanography at the University of Toronto. 39, University of Washington. "Every year, many of these things could happen, but unless you have them all, you do not get polynya."

The study shows that when the winds around Antarctica approach the shore, they favor a stronger upward mix in the east of the Weddell Sea. In this region, an underwater mountain called Maud Rise forces the dense seawater around her and leaves a swirl in rotation above. Two SOCCOM instruments were trapped in the whirlwind over Maud Rise and recorded years of observation there.

Analyzes show that when the surface of the ocean is particularly salty, as seen throughout 2016, heavy winter storms can trigger a traffic reversal. The warmer and saltier water of the depths rises to the surface, where the air cools it and makes it denser than the water below. When this water flows, a relatively warm deep water of about 1 degree Celsius replaces it, creating a feedback loop where the ice can not reform.

Under the effect of climate change, fresh water from melting glaciers and other sources will make the surface layer of the Southern Ocean less dense, which could mean less of polynyas in the future. But the new study challenges this hypothesis. Many models show that winds that revolve around the Antarctic will become stronger and move closer to the coast – the new paper suggests that this would encourage more polynyas to form, not less.

These are the first observations that prove that even a smaller polynya, like that of 2016, moves water from the surface to the deep ocean.

"It 's essentially a whole ocean reversal, rather than an injection of surface water when going from the surface to the surface. the depths, "said co-author Earle Wilson, who has just completed his Ph.D. in oceanography at the University of Washington.

One of the ways in which a surface polynya is of importance for the climate is for the deepest waters of the oceans, called Antarctic bottom waters. This cold and dense water is hidden under all other waters. Where and how it is created affects its characteristics and would affect other major ocean currents.

"Right now, people think that most bottom waters are forming on the Antarctic Plateau, but these large offshore polynyas may have been more common in the past," said Riser. "We need to improve our models to be able to study this process, which could have broader climatic consequences."

Large, long-lived polynyas can also affect the atmosphere because the deep waters contain carbon from life forms that have sunk over the centuries and have dissolved during their descent. Once this water has reached the surface, the carbon could be released.

"This deep carbon tank has been locked up for hundreds of years and, in a polynya, it could be broken down to the surface through this very violent mix," Campbell said. "A big carbon degassing event could really affect the climate system if it happened several years in a row."