Maud Rise polynya finally explained
The gigantic Maud Rise polynya in the Weddell Sea has long intrigued researchers. We now have a better understanding of all the factors behind its formation and persistence. And it’s a story of winds, currents, topography and a lot of salt.
First identified in the 1970s thanks to remote sensing satellites, the Maud Rise polynya in the Weddell Sea, north of Antarctica, reappeared sporadically and briefly in the decades that followed. So far, nothing very exceptional, polynyas being a normal phenomenon. But during the winter of 2016-2017, Maud Rise polynya took on gigantic proportions. Covering an area of over 40,000 km2, the size of Switzerland, it persisted for weeks, much to the astonishment of scientists who tried to understand why.
A team of researchers from the University of Southampton, the University of Gothenburg and the University of California, San Diego, have been looking into the matter. Their findings were published on May 1 in the journal Science Advances. And it seems that it was a combination of factors that led to the formation of Maud Rise polynya.
Taking its name from the Russian “polynia”, which means hole in the ice, a polynya is a stretch of open water in the sea ice. This phenomenon is completely natural and occurs in both the Arctic and Antarctic. Polynyas usually appear during the summer months and in the same place.
Polynyas can form thanks to vertical ocean currents that bring warmer water up from the depths to the surface, preventing ice formation. These bodies of water can also be formed by winds and currents that break up the pack ice, creating openings in the sea ice.
However, to create Maud Rise polynya, it took a combination of wind, ocean currents and the unique geography of the ocean floor to transport heat and salt to the surface. These conditions prevailed during the winter of 2016-2017, when the Weddell Gyre, the great circular ocean current, became stronger, helping the layer of warm, salty deep water to rise towards the surface.
But that’s still not enough: “This upwelling helps to explain how the sea ice might melt.”, says Fabien Roquet, Professor in Physical Oceanography at the University of Gothenburg and co-author of the research, in a press release issued by the University of Southampton on May 1. “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.”
Using remotely sensed sea ice maps and tagged marine mammals, the researchers discovered that the Weddell Sea’s very active marine currents played a role in the particular topography of the Maud Rise, a continental shelf that rises to the surface like a huge underwater mound, displacing salt at its summit.
A process that goes by the name of Ekman transport and explains why the Maud Rise polynya is so gigantic: “Ekman transport was the essential missing ingredient that was necessary to increase the balance of salt and sustain the mixing of salt and heat towards the surface water.”, notes Alberto Naveira Garabato, Professor in Physical Oceanography at the University of Southampton and co-author of the study.
Mirjana Binggeli, Polar Journal AG
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