Researchers have made a significant discovery regarding the conditions for the formation of a huge ice hole around Antarctica - it is the Great Weddell Polynya in the Weddell Sea, which is almost twice the size of Wales, and its last appearance was recorded in 2016-2017.
This is reported by the publication Science Advances.
The published study made it possible to understand how the opening, called polynya, could form and persist for several weeks. A research team from the universities of Southampton, Gothenburg, and the University of California, San Diego studied polynya, which is named after a seamount in the Weddell Sea. The scientists found the polynya was brought on by complex interactions between the wind, ocean currents, and the unique geography of the ocean floor, transporting heat and salt towards the surface.
In coastal areas, openings in the sea ice occur every year. Strong winds blowing from the land push the ice away, exposing the seawater below. In the open ocean, far from the coast and over deep waters, where the depth reaches several thousand meters, such phenomena occur much less frequently.
Aditya Narayanan, a Postdoctoral Research Fellow at the University of Southampton, points out: "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. But since the 1970s, it has occurred only sporadically and for brief intervals."
During 2016 and 2017, the large circular ocean current around the Weddell Sea became stronger. This has led to warm, salty water rising up from the depths, making it easier for salt and heat to mix vertically into the surface water.
Fabien Roquet, a Professor in Physical Oceanography at the University of Gothenburg, said:
"This upwelling helps to explain how the sea ice might melt. But as sea ice melts this leads to a freshening of the surface water, which should in turn put a stop to the mixing. So, another process must be happening for the polynya to persist. There must be an additional input of salt from somewhere."
The process of Ekman transport helped to move the salt onto the northern flank of the of Maud Rise, where the polynya first formed. It involves water moving at a 90-degree angle to the direction of the wind blowing above, influencing ocean currents.
Another important factor affecting the conditions in the Weddell Sea was the topography of the ocean floor, in particular the underwater plateau with gentle rises. Winds also play an important role, controlling the movement of water masses and causing the Ekman transport. Temperature differences between local waters and those moved by the wind, as well as variations in salinity and other parameters, are also important.
Ultimately, when the Ekman transport coincides with the right wind direction, which varies from year to year, wind-driven water flows collide with underwater mountain plateaus as they move under the ice. This leads to the formation of an underwater vortex, which actively erodes the ice cover above it, forming the mysterious Great Weddell Polynya.
Professor Sarah Gill from the University of California, San Diego adds: "For the first time since observations began in the 1970s, there’s a negative trend in sea ice in the Southern Ocean, which began around 2016. Before then it had remained somewhat stable."
Earlier, we wrote that scientists discovered huge black hole near Earth.