Researchers have found the answer to the mystery of a rare hole in the sea ice around Antarctica, which was almost double the size of Wales and appeared during the winters of 2016 and 2017. The study unveils a crucial process that had puzzled scientists in understanding how the hole, known as a polynya, was able to form and last for several weeks.The formation and persistence of a polynya, an opening in the ice, has been a mystery to scientists for a long time. 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 in the Weddell Sea to understand how it forms and lasts for weeks. They discovered that complex interactions between wind, ocean currents, and the unique geography of the ocean floor play a key role in creating the polynya and bringing heat and salt to the surface. In Antarctica, the ocean surface freezes over in the winter.er, with sea ice covering an area about twice the size of the continental United States.
Every year, openings in the sea ice occur in coastal areas. Strong coastal winds blow off the continent and push the ice away, exposing the seawater below. It is much less common for these polynyas to form in sea ice over the open ocean, which is hundreds of kilometres away from the coast and where the seas are thousands of meters deep.
Aditya Narayanan, a Postdoctoral Research Fellow at the University of Southampton, led the research and stated that “The Maud Rise polynya was discovered in the 1970s through remote sensing satellites that can see sea ice over th rnrnThe initial observations of the polynya in the Weddell Sea in the Southern Ocean took place in the 1970s, where it persisted for several winters from 1974 to 1976. At the time, scientists believed it would occur annually. However, since the 1970s, it has only appeared sporadically and for short periods of time. In 2017, researchers noted that it was the first time such a large and long-lasting polynya had been observed in the Weddell Sea since the 1970s. This phenomenon coincided with a period of increased strength in the large circular ocean current around the Weddell Sea from 2016 to 2017. As a result, the deep layer of warm and salty water in the area rose, facilitating the vertical mixing of salt and heat.ocean’s surface water, the upwelling process brings warmer, saltier water to the surface, which contributes to the melting of sea ice. However, the melting sea ice freshens the surface water, which should inhibit the mixing process. The researchers concluded that there must be another source of salt input to sustain the polynya. Using data from sea ice maps, autonomous floats, tagged marine mammals, and a computational ocean model, the researchers identified the additional salt input from the Weddel ocean’s upwelling process.The sea current flowed around Maud Rise, causing turbulent eddies to move salt to the top of the sea mount. The process of ‘Ekman transport’ then helped to transfer the salt to the northern flank of Maud Rise, where the polynya first appeared. Ekman transport involves water moving perpendicular to the direction of the wind above, which affects ocean currents.
“Ekman transport was the crucial missing element needed to rebalance the salt and maintain the mixing of salt and heat towards the surface water,” said co-author Professor Alberto Naveira Garabato, also from the University of South.Hampton.
Polynyas are areas where there is a significant exchange of heat and carbon between the ocean and the atmosphere. This exchange can have a major impact on the heat and carbon balance of the region.
Professor Sarah Gille from University of California San Diego, who is also a co-author of the research, stated: “The effects of polynyas can persist in the water for several years after their formation. They can alter the movement of water and the way currents transport heat towards the continent. The dense waters that are created here can spread throughout the global ocean.”
Many of the same processes that were involved in the formation of these polynyas are also at play in other oceanic phenomena.The Maud Rise polynya is being influenced by various factors, including the upwelling of deep and salty water, which is also contributing to a general decrease in sea ice in the Southern Ocean.
According to Professor Gille, there has been a negative trend in sea ice in the Southern Ocean since around 2016, which is a change from the previous stability observed since the 1970s.
Journal Reference:
- Aditya Narayanan, Fabien Roquet, Sarah T. Gille, Birte Gülk, Matthew R. Mazloff, Alessandro Silvano, Alberto C. Naveira Garabato. Ekman-driven salt transport as a key mechan rnrnThe process of open-ocean polynya formation at Maud Rise is explained in a recent article in Science Advances (2024; 10(18)). The DOI for the article is 10.1126/sciadv.adj0777.