Researchers at the University of Southampton have uncovered the mystery behind a rare giant opening, almost twice the size of Wales, in the Antarctic sea ice during the winters of 2016 and 2017.

A study published on Friday in Science Advances offers new insights into how the opening was able to form and remain for several weeks.

The team, from the University of Southampton, the University of Gothenburg and the University of California San Diego, examined the Maud Rise polynya, named after a submarine mountain-like feature over which it expands in the Weddell Sea.

The scientists discovered that the polynya resulted from intricate interactions between the wind, ocean currents, and the unique geography of the ocean floor, delivering heat and salt to the surface.

It's much rarer for these polynyas to appear over the open ocean, hundreds of kilometres from the coast, where the seas are thousands of metres deep.

In the winters of 2016 and 2017, the substantial circular ocean current around the Weddell Sea intensified. As a result, the deep layer of warm, salty water rose, facilitating the vertical mixing of salt and heat into the surface water.

The researchers used various tools including sea ice maps, marine mammal tags, autonomous floats, and computational ocean modelling in their study.

A process known as 'Ekman transport' was crucial in shifting the salt to where the polynya first started.

Polynyas are crucial for the heat and carbon exchange between the ocean and the atmosphere, potentially impacting the heat and carbon budget of the region.

The researchers suggested that some of the processes responsible for the formation of the Maud Rise polynya, such as the upwelling of deep and salty water, are also driving a generalized reduction in Southern Ocean sea ice.

Aditya Narayanan, a postdoctoral research fellow at the University of Southampton, who led the research, said: "The Maud Rise polynya was discovered in the 1970s when remote sensing satellites that can see sea ice over the Southern Ocean were first launched. It persisted through consecutive winters from 1974 to 1976 and oceanographers back then assumed it would be an annual occurrence.

"2017 was the first time that we’ve had such a large and long-lived polynya in the Weddell Sea since the 1970s."

This research was funded by the European H2020 project SO-CHIC (Southern Ocean - Carbon and Heat Impact on Climate).