An enormous hole in the the wintertime sea ice surrounding Antarctica is attracting considerable scientific attention. Researchers think the so-called Weddell polynya is part of a natural cycle, but its present size—the biggest it’s been since it was first spotted in the 1970s—could help us understand the processes controlling Antarctic circulation, and how the Southern Ocean is changing due to human-caused climate change.
Polynyas are regions of open water that occur in the Arctic and Southern Oceans where you’d expect to see ice, typically around coastlines that experience fierce wintertime winds. The Weddell polynya is unusual in that it occurs far offshore, in a shallow water region known as the Maud Rise. It was first spotted in the winter of 1974, when a hole roughly the size of Oregon emerged in the Antarctic sea ice in the dead of winter. The polynya cropped up again for the next two winters, before going dormant for decades—although low sea ice concentrations persisted in the region.
In the winter of 2016, a sea ice gap similar to what scientists had spotted in the ‘70s, although considerably smaller, re-appeared, drawing the attention of Antarctic scientists. Now, the polynya has opened yet again. At more than 16,000 square miles, it’s larger than the Netherlands, and quite a bit bigger than it was last year. Though it’s still about five times smaller than it was in the 1970s, according to Torge Martin, a meteorologist and climate modeler at the Helmholtz Centre for Ocean Research in Kiel, Germany, who has been tracking the feature since mid-September.
Other Antarctic researchers have spent the last few days getting excited over the weird ice gap on Twitter:
The Weddell polynya is thought to be driven by the upwelling of warm water, which releases heat to the air, before becoming cooler and denser, and sinking.
“The polynya is like a big window,” Martin told Earther. “Through the hole in the ice heat escapes from the ocean, warming the atmosphere above but more so cooling the ocean underneath.”
According to Céline Heuzé, a physical oceanographer at the University of Gothenburg, Sweden, sinking water within the polynya contributes to the formation of very cold, very dense Antarctic Bottom Water, which feeds into the global ocean conveyor belt.
“There’s a bit of a mystery going on in Antarctica at the moment,” Heuzé told Earther. “From global circulation, we know how much deep water should be formed, but the areas we know are forming water now just aren’t forming enough. We’ve got a source of deep water that’s missing, somewhere. Maybe [the Weddell polynya] contributes to that.”
The polynya’s cycle—of heat release, followed by sinking water—is expected to continue until it’s halted by warm springtime air, or the addition of fresh water from melting sea ice, which stratifies the ocean. While the polynya is a naturally-occurring feature, some researchers thought it would disappear due to global climate change, “which is expected enhanced surface freshening putting ‘a lid’ on top of the ocean and thus shutting down the polynya,” Martin explained.
Martin said that his group’s climate models supported the view that the Weddell polynya could make a comeback, as it seems to be doing now.
“While many climate models tend to produce such a large open ocean polynya, the feature was viewed more as a disruptive model glitch than a true phenomenon in the past,” Martin said. “Its recurrence supports our hypothesis... that the Weddell Polynya was not a one-time event but possibly occurred regularly in the past.”
Importantly, Martin thinks the polynya’s resurgence suggests that global warming “is not yet strong enough to suppress this internal variability” in the Southern Ocean. The Southern Ocean is indeed a bit of an anomaly when it comes to climate change: It’s one of the slowest-warming places on Earth, and, with the noted exception of last year, its sea ice has been growing, rather than shrinking, over the satellite record.
A study published last year attributed the Southern Ocean’s ability to keep its chill to powerful currents that siphon warm water northwards, only to be replenished by very old, cold water from the deepest parts of the ocean. Changes in the winds surrounding Antartica have also been implicated. The truth is, we aren’t totally sure why the Southern Ocean isn’t heating up faster, and the Weddell polynya is another piece of a very complex puzzle.
“We really don’t know what’s going on,” Heuzé said. “We don’t have enough observations of the Southern Ocean yet.” She hopes that the polynya’s reemergence will prompt scientific expeditions to the region—there’s only so much we can learn from satellites.
“At the moment any expedition into the Southern Ocean needs to be planned years in advance,” Heuzé said. “But I do hope that now that [the polynya] has formed people will go observe it. You can monitor it by satellites, but for really what’s happening in the ocean we need to go down there.”