When sea ice breaks apart or is pushed away from the coast by wind, what’s left behind are pockets of open water surrounded by ice, known as polynyas. The polynyas that form around Antarctica can be identified and measured using satellite data going back to 1979.
A team analyzing those data unexpectedly found that in the Ross Sea region, the area of polynyas appears to fluctuate on a 16-year cycle.
“It’s quite remarkable, actually. I haven’t seen such periodicity in polynya area before.”
“It’s quite remarkable, actually,” said Kent Moore, a distinguished professor of geophysics at the University of Toronto Mississauga in Canada who was not involved in the study. “I haven’t seen such periodicity in polynya area before.”
The discovery comes at a time when Antarctic sea ice is declining overall. But the changes in polynya area do not consistently mirror sea ice changes, said Ceridwen Fraser, a biologist at the University of Otago in New Zealand. She and colleagues published their findings in Proceedings of the National Academy of Sciences of the United States of America.
For the past several years, Fraser said, “we’ve seen really dramatic and terrifying lows in Antarctic sea ice.” She expects this year will be another record low. “But what people haven’t really looked at quite as much is where the holes in the sea ice occur.”
Climate-Driven Colonization
With climate change, various marine species, such as kelp typically found much farther north, are moving poleward in search of more hospitable habitats, something Fraser is exploring in her research. Polynyas could represent a new path forward for them.
“If there are big polynyas butting up against the coast, then there is ice-free coastline that could potentially be colonized by these coastal species that we know we’re getting in from further north.”
“If there are big polynyas butting up against the coast, then there is ice-free coastline that could potentially be colonized by these coastal species that we know we’re getting in from further north,” Fraser said.
One of Fraser’s Otago colleagues, ecologist Grant Duffy, used daily satellite observations to identify polynyas and calculate changes in their size.
Under current conditions, most of the species that survive the journey won’t find suitable habitats. But warming temperatures could allow new species to gain a foothold in Antarctica, Fraser said.
“That is bad news for Antarctic ecosystems because these things will probably outcompete a lot of slow growing Antarctic species,” she said. “But I guess you can also look at it as good news for those species that are able to reach somewhere where they can grow in the future.”
Although polynya area and sea ice concentration are related, polynya area hasn’t always changed in sync with sea ice concentration. This study suggests that the 16-year cycle in the Ross Sea shows a further decoupling of the two.
Temperature Records Offer a Clue
The polynya modeling was initially a bit of a sideline to Fraser’s research into the potential for new plants and animals to stake claims in Antarctica. Identifying the periodicity surprised the researchers and led to questions about what creates these recurring holes. Coauthor Ariaan Purich, a climate scientist at Monash University in Australia, said there is another data set with a similar cycle length.
“Examining the longer 68-year record for meteorological stations, a 16-year periodicity has been observed in winter air temperature on the Antarctic Peninsula,” she said.
Purich also thinks the Amundsen Sea Low, a low-pressure system off the coast of Antarctica in the Pacific sector, could affect both temperature and polynya area.
“We can physically understand how it might influence both the temperature at stations on the peninsula, as well as the polynya area in the Ross Sea,” she said. “That, I guess, is a bit of a hint that it could be the atmosphere forcing this rather than the ocean.”
She is also looking at ways the Southern Annular Mode, a circumpolar climate feature, interacts with the Amundsen Sea Low.
But, of course, the atmosphere and the ocean constantly interact, and Purich said the ability to gather relevant ocean information at the resolution needed to study a 16-year cycle remains elusive.
“They’re on the right track by looking for an atmospheric component,” Moore said. “The challenge is that 16 years is an odd cycle in the atmosphere.” The El Niño–Southern Oscillation, for example, has a periodicity of 3–5 years.
Both Moore and Purich said further study of the deep-ocean formation and water mass circulation could enhance understanding of the findings. Moore said he would start by investigating the peaks and valleys in the polynya area.
With a limited data set and a 16-year cycle, finding answers could be slow going.
“We’re not 100% sure that it’s a long, long-term pattern,” Fraser said. “And, so, looking at that into the future will be really interesting. But that’s going to be something that happens over decades, not years.”
—Amy Mayer (@amyhmayer), Science Writer