Scientists solve mystery of hole in Antarctic sea ice

Scientists have discovered how an open body of water formed in the middle of thick ice during the middle of the Antarctic winter.

The polynya – the scientific name for an area of water where you would expect to find ice – developed and persisted for several weeks at a time in two consecutive winters, in 2016 and 2017, in Antarctica’s Lazarev Sea.

Scientists have been puzzled about why the phenomenon – which was first reported at the location in 1974 and had returned every decade or so – had reappeared at such a scale. It stretched to almost 50,000 square kilometres, almost three times the size of Kuwait.

A team from the University of Southampton, the University of Gothenburg and the University of California San Diego studied the Maud Rise polynya, as it is known, after the submerged mountain-like feature in the Weddell Sea, over which it grows.

They concluded that it formed due to complex interactions between the wind, ocean currents, and geography of the ocean floor, which transported heat and salt towards the surface.

The study found that during 2016 and 2017, the large circular ocean current around the Weddell Sea became stronger, which caused that the deep layer of warm, salty water to rise, making it easier for salt and heat to mix into the surface water.

The surface of the ocean of Antarctica freezes each year, covering an area twice the size of the US.

Polynyas are common off the coast, where strong coastal winds blow off the continent, pushing the ice away and exposing seawater below. But they are rare in areas of deep sea.

Aditya Narayanan, a postdoctoral research fellow at the University of Southampton who led the research, told The National that 2017 was the first time a large and long-lived polynya had developed in the area.

“It’s sporadically shown up once in about 10 years but it has never been as long lived as it was in 2017,” he said.

“The last time we saw such a long lived opening was in the 1970s. So for three years between 1974 and 1976 we had polynyas opening up in the same region and since then, 2017 is the first year when we have had such a long lived polynya over quite a large footprint of about 50,000 square kilometres.”

Dr Narayanan said the conditions involved in the shrinking of sea ice appear to also be present during the formation of a polynya.

“This is forming at a time when we are seeing dramatic reduction in sea ice and it may indicate that we have increased heat supply from below,” he said. “It might also indicate that we are seeing a change in how Antarctic waters are behaving.”

Since 2016 sea ice has entered a “negative trend” in the Southern Ocean, he said, with 2023 representing a record low.

Previous work by scientists at New York University Abu Dhabi into the 2017 polynya, which used a combination of satellite observations and data to check the weather at the time, showed how strong winds from severe and frequent cyclones had caused the sea pack ice to break up and move in opposite directions.

The area of unfrozen ocean, from underneath the pack ice, filled the space left behind.

Early explorers who ventured into a polynya often believed they had discovered a new ocean. But even with the advent of technology, they are still not completely understood by scientists.

Some remain open for years at a time, while others only for weeks or months.

According to US information centre the National Snow and Ice Data Centre, polynyas are important for wildlife.

In addition to providing access to between the ocean and the atmosphere for a variety of animals like seals and penguins, they bring nutrients including phytoplankton to the surface.