Researchers have discovered the missing piece of the puzzle behind a rare hole in the sea ice around Antarctica, which was nearly twice the size of Wales and occurred during the winters of 2016 and 2017.

Aerial view of Maud Rise Polynia.  IAerial view of Maud Rise Polynia.  I

Aerial view of Maud Rise Polynia. Image credit: NASA Earth Observatory photo by Lauren Dauphin, using MODIS data from NASA EOSDIS/LANCE and GIBS/Worldview

A study published in Advances in science shows an important process that eluded scientists in how the blooms, known as polynyas, were able to form and persist for weeks.

A team of researchers from the University of Southampton, the University of Gothenburg and the University of California San Diego studied the mood rise polynya – named after the mountain-like feature submerged in the Weddell Sea, on which it grows.

They found that polynyas were brought about by complex interactions between wind, ocean currents, and the unique geography of the ocean floor, driving heat and salt toward the surface.

In Antarctica, the sea surface freezes in winter, with sea ice covering an area nearly twice that of the continental United States.

Blooms annually in coastal areas, sea ice. Here, strong coastal winds blow off the continent and push the ice away, exposing the ocean water below. These polynyas are rarely formed in sea ice over the open ocean hundreds of kilometers offshore where the oceans are thousands of meters deep.

Aditya Narayanana post-doctoral 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 could see sea ice over the Southern Ocean , were first launched. It persisted through the winters of 1974 and 1976, and oceanographers at the time assumed it would be an annual event. But since the 1970s, it has only been intermittent and short-lived It has been taken.

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

During 2016 and 2017, the large circular tidal current around the Weddell Sea strengthened. One result is that a deeper layer of warm, salty water rises, making it easier for salt and heat to mix vertically into the surface water.

Fabian Roquet, professor of physical oceanography at the University of Gothenburg and co-author of the study, said: “This increase helps explain how sea ice can melt. Freshening of the water occurs, which must prevent mixing.Therefore, there must be an additional input of salt from somewhere to maintain the polynya.

The researchers used computational models of ocean state as well as remotely sensed sea ice maps, observations of autonomous floats and tagged marine mammals. They found that as the Weddell Sea Current flows around the Moody Rise, turbulent eddies move salt over the top of the seamount.

From here, a process called ‘Ekman transport’ helped move the salt to the northern part of the Moody Rise, where polynyas first formed. Ekman transport involves moving water at a 90-degree angle to the upwind direction, which affects ocean currents.

“Ekman transport was an essential missing component required to enhance salt equilibria and maintain mixing of salt and heat toward the surface water,” says the co-author. Professor Alberto Naveira Garabatoalso from Southampton University.

Polynyas are areas where you have a lot of heat and carbon transfer between the ocean and the atmosphere. So much so that they can affect the region’s heat and carbon budget.

Professor Sarah Gale of the University of California San Diego, another co-author of the study, said: “Polyany can stay in the water for years after they form. They can change how the water circulates and how the current heats up. The dense waters formed here can spread out into the global ocean.

Some of the same processes involved in the formation of the Moody Rise polynya, such as the rise of deep and salty water, are also causing a general decrease in sea ice in the Southern Ocean.

Professor Gale 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 was fairly stable.”

Paper Ekman-driven salt transport as a key mechanism for the formation of open-ocean polynyas at the Mode Rise. Published in Advances in science.

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

Source: Southampton University