In Antarcticain the Weddell Sea, for decades a huge “hole” in the sea icemore or less the size of Switzerland, opened several times and then closed again without apparent regularity, in the presence of temperatures well below zero. Its origin, after remaining unknown for a long time, has finally been clarified by a study by researchers at the University of Gothenburg in Sweden, Southampton in England and California published in 2024 in the magazine Science Advances. The hole, called “Maud Rise polynya”, was analyzed in 2017when its size reached i 50,000 km2. To do this, satellite observations and data from field monitoring of ocean conditions were combined. To cause it would be complex interactions between the wind, ocean currents and seabed morphology.
The big hole in Antarctica in the ice of the Weddell Sea
A huge hole has been spotted for the first time in the ice shelf that extends into the Weddell Sea starting from the Antarctic ice sheet during thewinter of 1974. Subsequently, the hole closed and then reappeared several times at irregular time intervals, in the presence of air temperatures well below zero. The hole usually appears near the coast and can remain visible for years after it has closed. The phenomenon was called “Maud Rise polynya“: Maud Rise is the seamount above which it forms, while “polynya” is a term for a hole in the sea ice. The first time oceanographers had the opportunity to adequately monitor the sinkhole was in 2016-2017when after a long time a large, extensive one opened again 50,000 km2. The study of the phenomenon took place using satellite images of sea ice, data collected in the field from drifting robots and tools with currents to monitor Antarctic conditions and even those obtained from sensors applied to seals.
Why sinkholes like the polynye in Antarctica form: the study on the causes
The researchers’ study showed how the formation of a polynya depends on the concomitance of several factors. Among these is the strengthening of the ocean current which moves in a circular fashion around the Weddell Sea (called Weddell Gyre), which determines the phenomenon called upwelling: the warmer, saltier water that flows deeper rises to the surface, mixing with the surface water. The latter becomes saltier and warms: the salt lowers the freezing temperature of the water, so the sea ice above can melt, creating a hole. By melting, however, the ice should lower the temperature of the surface waters and therefore interrupt the phenomenon. The researchers discovered that if this does not happen immediately it is thanks to a process called “Ekman transportThis process consists of transport of water due to the interaction between the wind and the ocean surface. This transport also brings accumulated salts from the ocean current eddies up to the top of the Maud Rise seamount. The result is a greater intake of saltsbut also of heat, in surface waters.
Studying the phenomenon of polynyes is important because these holes can influence the trend of ocean currents and how they transport heat to the continents. The process of rising deep and salty water which is at the origin of the formation of the holes is the same one which contributes to a general reduction of sea ice in the Southern Ocean.
