A satellite image shows blue sea ice in the Nares Strait and open water toward the bottom of the image.
Scientists have speculated that a weakening ice arch in the Arctic, pictured here, could disrupt the North Water polynya to the south. Credit: European Union, Copernicus Sentinel-2 imagery

When the springtime Sun hits a patch of open water in the Arctic known as the North Water polynya, or the Pikialasorsuaq in Greenlandic, it kick-starts a massive phytoplankton bloom. As the microscopic organisms multiply, they lure an abundance of critters such as fish, whales, seals, seabirds, and polar bears. The North Water is one of the most biologically productive areas in the Arctic and has supported Inuit communities for millennia. New research suggests unique weather in the region might keep the productivity of the North Water safe. But not all experts are convinced.

Directly north of the polynya, in the Nares Strait between Canada and Greenland, are ice bridges—also called arches—that act as a plug at the strait’s northern and southern ends, preventing sea ice from entering the North Water. Scientists have speculated that as the arches weaken because of warming in the Arctic, more ice will enter the North Water. The extra ice, which can be thick, multiyear ice, could block out sunlight, preventing phytoplankton blooms in the future.

The presence of the Nares Strait arches is already tenuous: The mainstay arch at the southern end of the strait has failed to form in 6 of the past 20 years. Kent Moore, an atmospheric physicist at the University of Toronto Mississauga in Canada, and his colleagues analyzed the North Water to see whether ice and phytoplankton conditions looked any different in the years that the arch didn’t form.

Satellite images over the Nares Strait and the North Water between February and June from 2003 to 2022 showed that the amount of ice over the polynya was the same with or without the usual southern ice arch.

Strong Winds Made Stronger

Moore and colleagues wondered why the North Water remained mostly ice-free even without an ice bridge. They turned their attention to another distinctive characteristic of the Arctic: the wind.

Because the strait is narrow, northerly winds entering it from the north speed up, like air moving into a tunnel. “The winds that blow down Nares Strait are quite strong.” Moore said. “They can be hurricane-force winds.”

“The polynya may be more stable than we think.”

But sea ice is a rough surface that causes friction, which slows the wind. The satellite data showed less sea ice filled the Nares Strait in the years without an arch. “The winds are actually stronger in the absence of sea ice,” Moore said. Using modeled hindcast wind speeds from the Copernicus Arctic Regional Reanalysis, the researchers found that in the years without an ice arch, winds through the strait and over the North Water were faster than in years with an ice arch. The researchers suspect that strong winds sweep out the ice that enters the North Water before it can block out the Sun and prevent a phytoplankton bloom. Moore and his colleagues published their findings in Scientific Reports.

Even though the North Water remained largely ice-free without the arch, the researchers still wanted to see whether passing ice made any difference in phytoplankton production. Phytoplankton produce chlorophyll, a green pigment that can be measured from space. The researchers found that the green hue of the North Water remained fairly constant, indicating that phytoplankton were abundant regardless of the ice bridge.

“The polynya may be more stable than we think,” Moore said. “Even if the arches don’t form, the winds will always be strong enough to keep the ice moving southwards and keep the North Water present.”

Seeing the polynya form even in years without an ice arch is consistent with previous findings, said Andreas Preußer, a sea ice researcher at the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research who was not involved in the study. “This observation is something that we saw in earlier papers, but that is now within recent years, and with more frequent ice arch disappearance, more proven,” he said.

“This bold conclusion of the study that primary productivity is independent of the ice arch is not supported by field studies.”

But Sofia Ribeiro, a climate scientist at the Geological Survey of Greenland and Denmark who was not involved in the study, warned that there may not be enough data from years without an ice arch for the results to be conclusive. In some of the years in which the southern ice arch failed, an arch did form at the northern end of the strait. Because satellite imagery showed that ice cover was very similar to that in the years without an ice arch, the authors considered these situations equivalent, but Ribeiro said that making this assumption could skew results further. “It’s a rather uneven comparison,” she said.

Measuring blooms by satellite doesn’t account for shifts in species, Ribeiro added. Evidence suggests that the species responsible for the primary productivity, or the conversion of carbon dioxide into organic material, in the region is changing, which has implications for the food chain, she said. “This bold conclusion of the study that primary productivity is independent of the ice arch is not supported by field studies.”

Moore acknowledged that measuring productivity using satellite data has many challenges such as clouds obscuring the satellite’s view. But he said he is confident in their conclusions because the trends of the phytoplankton bloom across several months following winters with and without an ice arch are so similar and are also consistent with what other research groups reported previously.

Many other factors, such as fresh water entering the polynya, not just the weakening ice arch, could threaten the polynya’s productivity. “We should act cautiously rather than assuming it’s going to be fine,” Ribeiro said.

Moore also pointed to other effects of climate change that will affect the polynya: “We still have to be mindful that there probably will be changes. Even the ocean temperatures are changing, so that will also impact the ecosystem,” he said. The fate of the polynya rests on “more than just the arch.”

—Andrew Chapman (@andrew7chapman), Science Writer

Citation: Chapman, A. (2023), Winds clear sea ice through fertile Arctic waters, Eos, 104, https://doi.org/10.1029/2023EO230340. Published on September 11 2023.
Text © 2023. The authors. CC BY-NC-ND 3.0
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