ABU DHABI 11 May 2019: A study led by NYU Abu Dhabi Institute (NYUAD) has helped unravel a four decade long mystery surrounding the occurrence of a mid-sea Polynya – a body of unfrozen ocean that appeared within the sea-ice pack during Antarctica’s winter, almost two years ago.
The Maud-Rise Polynya was spotted in mid-September 2017 in the center of a sea-ice pack in Antarctica’s Lazarev Sea, causing researchers to question how this phenomenon occurred during Antarctica’s coldest, winter months when sea ice is at its thickest.
Due to its difficult access location, NYUAD scientists used a combination of satellite observations and reanalysis data to discover that cyclones (as intense as category 11 in the Beaufort Scale) and the strong winds that they carry over the sea-ice pack cause the ice to shift in opposite directions, which leads to the appearance of the Polynya.
At the time of the discovery, the Maud-Rise Polynya was approximately 9,500 square kilometers large (equivalent to three times the landmass of the Emirate of Dubai), and grew by over 700 percent to 800,000 square kilometers within a month. Eventually, the Polynya merged with the open ocean once the sea ice started to retreat at the beginning of the austral summer. Prior to 2017, the largest previous event was seen in the 1970s when satellite observations began, with smaller Polynya events occasionally occurring since then. The phenomenon has intrigued scientists ever since it first appeared.
The research was carried out by Research Scientist Diana Francis and co-authored by Clare Eayrs and David Holland, all from the Center for Global Sea Level Change (CSLC), and Juan Cuesta of the University of Paris-East.
Holland, who has worked on the oceanographic aspects of the Polynya, remarked that “Previously we have shown that the large Maud Rise Seamount, which sits on the sea floor beneath this Polynya, preconditions the manner by which warm, deep water can rise up to the surface of the ocean, thus keeping the ocean surface warm and preventing the formation of sea ice. It is an unstable oceanographic situation and a large atmospheric cyclone can trigger the opening of a Polynya over the seamount as we have seen here. The Polynya can then be sustained for months, long after the cyclone has passed, by the ocean instability that continues to cause the rise up of warm, deep water.”
“Once opened, the Polynya works like a window through the sea ice, transferring huge amounts of energy during winter between the ocean and the atmosphere,” said Francis. “Because of its large size, a mid-sea Polynya is capable of impacting the climate regionally and globally as it modifies the oceanic circulation. It is important for us to identify the triggers for their occurrence to improve their representation in models and their effects on climate.”
“Given the link between Polynya and cyclones we demonstrated in this study, it is speculated that Polynya events may become more frequent under warmer climate because these areas will be more exposed to more intense cyclones. Previous studies have shown that under a warmer climate, polar cyclone activity will intensify, and extratropical cyclone tracks will move toward Antarctica which could decrease the sea-ice extent and make Polynya preconditioned areas closer to the cyclones formation zone,” Francis added.
- Polynya is a geographical term that is used to describe an area of unfrozen water surrounded by a large area of sea ice.
- Researchers at the NYU Abu Dhabi Institute have discovered an atmospheric trigger for the occurrence of the Maud-Rise Polynya that appeared in September 2017 in the Lazarev Sea. A much larger Polynya was spotted in the same location in 1974 and may also have had an atmospheric trigger.
- Researchers have found that when intense cyclones occur over the ice pack, a mid-sea Polynya in a preconditioned oceanographic area can be initiated as the cyclonic winds push the ice in opposite directions away from the cyclone center.
- The Maud Rise Seamount preconditions the near-surface ocean waters such that warm, deep water is brought close to the surface above the seamount. A cyclone passing over the seamount, in addition to creating ice divergence, can further enhance the rise of warm, deep water, bringing it to the surface, and thus allowing a Polynya to be sustained.