In the 1970s we began monitoring the seasonal trends of Antarctic sea ice through images collected by satellites. This allowed us to learn many things and, above all, to study the temporal evolution of its cycles of contraction and expansion. But these images have also highlighted a phenomenon whose origin has so far remained shrouded in mystery: we are talking about a huge one hole, which reached almost twice the size of Wales during the Antarctic winters of 2016 and 2017. Today, thanks to a study just published on Science Advanceswe might finally have one explanation for its formation, to which a series of concomitant factors would have contributed, including a process known as “Ekman transport”. Let’s see what it’s about.
The history of Maud Rise polynya
The hole in question has been baptized by experts with the name “Maud Rise polynya”. Polynya is the word used in jargon to indicate this type of phenomenon, which regularly occurs near the it costswhile it is much rarer for it to occur at the offshore, as happened in the case examined by the researchers. The Maud Rise polynya was in fact formed in the pack ice that covers the Weddel Seaand in particular in correspondence with the submerged mountain “Maud Rise”, which gave its name to the hole.
The Maud Rise polynya was first observed in 70’s and it recurred every winter from 1974 to 1976.”Oceanographers at the time thought it was an annual event”, explains Aditya Narayanan, who led the study and is a researcher at the University of Southampton (UK). But, contrary to predictions, in the following years the hole appeared only sporadically and for short periods. “2017 was the first time we had such a large and long-lived polynya in the Weddell Sea since the 1970s”, continues Narayanan.
Salt and currents
Among the causes, explain the authors of the study, there is certainly the fact that during 2016 and 2017 the great circular ocean current around the Weddell Sea it has strengthened. One of the consequences of this current is that the deep layer of water that is warmer and richer in salt than the surface water mixes with the latter, increasing its temperature and salinity. This phenomenon of upwelling, that is, the rise of deep water towards the surface, helps to explain the reason for the melting of sea ice, says Fabien Roquet, professor at the University of Gothenburg (Sweden) and co-author of the research. In fact, the increase in salt concentration lowers the freezing temperature of water. “But melting sea ice leads to a drop in surface water temperatures, which in turn should stop mixing“, continues Roquet: “Thus, another process must occur for the polynya to persist. There must be an additional supply of salt somewhere”.
Ekman transport
And it is at this point that the process known as “Ekman transport” comes into play, which concerns the movement of water depending on the winds that blow on its surface. This is a phenomenon that can influence marine currents and which in this specific context would contribute to bring the salt to the surface accumulated on the submerged mountain Maud Rise due to the circular current we were talking about before. “Ekman transport was the essential missing ingredient needed to increase salt balance and support the mixing of salt and heat to surface waters”, adds Alberto Naveira Garabato, professor at the University of Southampton and co-author of the research.
Studying this type of formation is important, underlines Sarah Gille, co-author of the study and professor at the University of California San Diego, both because polynyas can have strong repercussions on circulation of ocean currentsand therefore on the distribution of heat between ocean and continent, both because the phenomena that lead to their onset are the same ones that are causing a reduction of ice present in the Southern Ocean: “For the first time since observations began in the 1970s – concludes the researcher –there is a negative trend in sea ice in the Southern Ocean, which began around 2016. Before then it had remained rather stable”.
