Amidst the vast, brilliant white landscape of Antarctica, the surface of the ocean freezes in winter and Sea ice covers an area roughly the size of all of South America. However, this surface is not without openings and one in particular has caused intrigue since its first appearance.
After 50 years without explanation, Researchers finally uncovered the disturbing mystery of the giant 40,000-square-kilometer hole in Antarctica which is equivalent to almost three times the Metropolitan Region.
In the vast Antarctic, openings in the sea ice occur every year in coastal areas, where melting is most likely to occur. This helps marine wildlife, such as seals or whales, catch their breath.
These openings are known as polynya, a term used to designate the maritime areas of the poles that do not freeze throughout the year. They usually form simply because heat prevents an ice sheet from creating, or because katabatic wind or ocean currents move the ice away from the coast, thinning the ice sheet and creating a hole. It is a natural process that scientists are accustomed to.
However, A rare event emerged 50 years ago over the Antarctic Weddell Sea, a polynya called Maud Rise, which does not always appear, It was first discovered in 1974 and reappeared in 2016 and 2017. Although polynyas are common, this particular one occurred in an unusual location and of giant size.
Until recently there was no explanation for how Maud Rise originated, but a study published in Science Advances revealed that the polynya was caused by complex interactions between wind, ocean currents and the unique geography of the ocean floor, transporting heat and salt to the surface.
During 2016 and 2017, the large circular ocean current around the Weddell Sea became stronger and as a result the deep layer of warm, salty water rose, making it easier for salt and heat to mix vertically with the surface water.
Fabien Roquet, professor of Physical Oceanography at the University of Gothenburg and co-author of the research, said in a statement: “This updraft helps explain how sea ice could melt. But as sea ice melts, surface water cools, which in turn should stop mixing. So, another process must be occurring for the polynya to persist. There must be an additional supply of salt from somewhere.”
The team of researchers from the University of Southampton, the University of Gothenburg and the University of California, San Diego discovered through remotely captured sea ice maps, observations of autonomous floaters and tagged marine mammals, together with a computational model of the state of the ocean, that, As the Weddell Sea current flowed around Maud Rise, turbulent eddies carried salt to the top of the seamount.
A process known as “Ekman transport” helped move salt toward the northern flank of Maud Rise, where the polynya first formed. Salt can significantly lower the freezing point of water, so if the water in the polynya is particularly saline, that could explain the persistence of the hole.
“Ekman transport was the essential missing ingredient and it was necessary to increase the salt balance and keep the mixture of salt and heat towards the surface water,” says co-author Professor Alberto Naveira Garabato, also from the University of Southampton.
Professor Sarah Gille from the University of California, San Diego, another co-author of the research, said: “The imprint of polynyas can remain in the water for several years after they form. They can change the way water moves and how currents carry heat toward the continent. The dense waters that form here can spread throughout the global ocean.”
This may have implications for climate change, since once the sea ice melts, a huge temperature contrast is generated between the ocean and the atmosphere. Denser, colder water sinks to the bottom of the ocean, while warmer water rises to the surface.
Professor Gille added: “For the first time since observations began in the 1970s, There is a negative trend in sea ice in the Southern Ocean, which started around 2016. Before that, it had been somewhat stable.”
Climatologists already predict that Antarctic winter winds will become stronger and more frequent, which could lead to huge and more frequent polynyas in the coming years.
