By the end of austral winter, the seas around Antarctica are blanketed with as much as 18 million square kilometers of ice—an area about twice the size of the continental United States. But that vast span of ice is not always continuous. Cracks can open up and expose the seawater below. Sometimes state-sized areas of ice go missing from the middle of the ice pack. This phenomenon is known as a polynya, and scientists have been investigating these features for decades.
The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite acquired these images of the Maud Rise polynya in the eastern Weddell Sea on September 25, 2017. The first image is natural color and the second is false color to better differentiate between areas of ice (blue) and clouds (white).
Coastal polynyas form when strong offshore winds move sea ice away from the continent. In contrast, open-ocean polynyas like the one near Maud Rise grow amid the ice far from shore. This type of polynya is somewhat more complex, with circulation in the atmosphere and ocean both playing a role in creating and sustaining them. These openings can be large and long-lived.
The Maud Rise polynya—named for the submerged mountain-like feature over which it grows—routinely occurs in early spring and occasionally in winter. According to Joey Comiso, an emeritus scientist at NASA’s Goddard Space Flight Center, the shape of the seafloor “causes the ocean current driven by the Weddell Gyre to bring warm water up to the upper layer of the ocean and causes the sea ice to melt.”
In winter 2017, the Maud Rise polynya was especially large, growing from 9,500 square kilometers in mid-September to about 80,000 square kilometers by late October (nearly the same size as South Carolina). The polynya was among largest in this area since the famous Weddell polynyas that formed in 1974, 1975, and 1976, which reached areas larger than the size of California.
How polynyas can form, grow, and persist during Antarctica’s cold winter has long been a topic of investigation. Recent research points toward strong cyclonic winds—some as strong as hurricanes—as the trigger for open-ocean polynyas. Diana Francis, a scientist at New York University Abu Dhabi and leader of that study, explained that cyclonic winds “drag the floating sea ice in opposite directions around the cyclone center, creating the opening.”
In some winters, atmospheric circulation moves a significant amount of heat and moisture from mid-latitudes to Antarctica, allowing large cyclones to develop over the sea ice. For example, a cyclone passed over the sea ice in winter 2016 before a small, short-lived polynya formed.
Compared to 2016, the movement of heat was stronger and more consistent in 2017, according to Francis. “The result was more frequent and intense cyclones, making the 2017 event bigger and keeping the polynya open for a longer period.”
Comiso agrees that the atmosphere plays an important role, especially in initiating polynyas. “But the oceanographic contribution is just as important in sustaining the polynya,” he said.
Indeed, conditions in the ocean and atmosphere likely work together. “I think the atmospheric conditions play the role of the trigger for the initial opening,” Francis said. “Once the area is free of ice, ocean dynamics bring warmer water near the surface and prevent the formation of new ice and sustain the polynya over longer period of time. Satellite images are a powerful tool to help us understand such a complex system where interactions between atmosphere-ice-ocean take on full meaning.”
NASA Earth Observatory images by Lauren Dauphin, using MODIS data from NASA EOSDIS/LANCE and GIBS/Worldview. Story by Kathryn Hansen.