Researchers have found the missing piece of the puzzle behind a rare hole in the sea ice around Antarctica, almost twice the size of Wales, that formed in the winter of 2016 and 2017.
A published study Science Developments, It reveals an important process that has been overlooked by scientists as to how the hole called polynya can form and persist for weeks. The paper is titled “Ekman-assisted salt transport as a key mechanism for polynya formation in the Eastern Maud open ocean.”
A team of researchers from the University of Southampton, the University of Gothenburg and the University of California, San Diego studied the Maud Rise polynya, named for the submerged mountain on which it grows in the Weddell Sea.
They found that polynyas result from a complex interaction between wind, ocean currents, and the unique geography of the ocean floor, transporting heat and salt to the surface. In Antarctica, the surface of the ocean freezes in winter, and sea ice covers an area roughly twice the size of the continental United States.
Expansion of sea ice occurs in coastal areas every year. Here, strong coastal winds blow the continent away and push the ice, exposing the seawater below. Much less frequently, these polynyas form on sea ice over the open ocean, hundreds of kilometers from shore, where seas are thousands of meters deep.
Research scientist Aditya Narayanan from the University of Southampton, who led the study, said: “The Maud Rise polynya was discovered in the 1970s, when remote sensing satellites capable of viewing sea ice over the Southern Ocean were first launched. This continued throughout successive winters from 1974 to 1976.” and oceanographers at the time assumed it would be an annual event, but it has occurred only occasionally and in short bursts since the 1970s.
“2017 was the first time such a large and long-lived polynya occurred in the Weddell Sea since the 1970s.”
The Great Circulation Ocean Current around the Weddell Sea strengthened in 2016 and 2017. One consequence of this is that the deep layer of hot brine rises, facilitating the vertical mixing of salt and heat into the surface water.
Fabien Roque, professor of physical oceanography at the University of Gothenburg and co-author of the study, said: “This upwelling helps explain how sea ice can melt. But when sea ice melts it leads to a freshening of surface waters, which should stop mixing. So the extra salt has to come from somewhere.” “For that to happen, a different process needs to continue.”
The researchers used remotely sensed sea ice maps, observations from autonomous marine vessels, and tagged the marine mammals along with a computational model of the state of the ocean. They found that when the Weddell Sea Current flows around Maud High School, turbulent eddies carry salt to the top of the seamount.
From here, a process called “Ekman transport” helped transport salt to the northern edge of the Maud High, where the polynya first formed. Ekman transport involves water moving at a 90-degree angle to the direction of the wind blowing over it, influencing ocean currents.
“The main missing ingredient required to increase salt balance and promote the infusion of salt and heat into surface waters was Ekman transport,” said co-author Professor Alberto Naveira Garabato from the University of Southampton.
Polynyas are areas where there is a large exchange of heat and carbon between the ocean and the atmosphere. So much so that they can affect the heat and carbon budget of the region.
Another co-author of the study, Professor Sarah Gill from the University of California, San Diego, said: “The imprint of polynyas can remain in water for years, even after they have formed. They can change the way water moves and the way currents transport heat. The thick waters formed here could spread across the world’s oceans.” “
Some processes, such as the upwelling of deep, salty water involved in the formation of the Maud Upwelling polynya, are also responsible for the overall reduction in sea ice in the Southern Ocean.
Professor Hille 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, the situation was somewhat stable.”
Source: Port Altele
