In February 2018, a vast expanse of open water appeared in the sea ice above Greenland, a region that normally has sea ice well into the spring. The big pool of open water in the middle of the ice, known as a polynya , was a scientific puzzle.
New analysis by researchers at the University of Washington and the University of Toronto Mississauga shows that odd winds are to blame, not simple global warming. The study was published Dec. 6 in Geophysical Research Letters.
Although last winter did see unusually warm temperature spikes in the Arctic, the authors identify the cause to be strong surface winds triggered by a dramatic warming in Earth’s upper atmosphere, known as a " sudden stratospheric warming.”
"During these events, temperatures in the stratosphere - about 30 kilometers above ground level - can warm by 10 or 15 degrees Celsius in just a few days,” said lead author Kent Moore , at the University of Toronto Mississauga.
The sudden warming of 18 to 27 F at some 18 miles elevation shifts air pressure and thus circulation patterns. In February 2018, it caused winds from Siberia to blow cold air into Northern Europe, creating a weather system that became known as the " Beast from the East.” That same weather pattern drew warmer air north up the east coast of Greenland, and generated persistent strong winds.
"This [wind pattern] lasted a week, and these were the warmest temperatures and strongest winds observed in north Greenland since observations began in the 1960s,” Moore said. "Winds were close to hurricane force and temperatures were above freezing. Once we got that piece of the puzzle, we realized it could be wind rather than warmth that caused the polynya.”
The study relied on a UW tool, the Pan-Arctic Ice Ocean Modeling and Assimilation System , or PIOMAS, to reconstruct sea ice conditions in the Arctic Ocean.
The authors used PIOMAS to run a simulation with the atmospheric conditions of 2018 but with thicker sea ice that was present in the Arctic in 1979, to see if thinner sea ice due to climate change caused the open water to appear. The patch of open water in that area was unprecedented in observations and lasted about three weeks, from mid-February through the first week of March.
"We used to ask the question hypothetically: What would have happened if the ice had been as thick as in 1979?” said co-author Axel Schweiger , a polar scientist at the UW’s Applied Physics Laboratory. "Now, we simulate it. The answer was that the thinning of the ice didn’t matter much, but strong winds were responsible.”
A longtime sea ice researcher, Schweiger was surprised. He thought thinning ice would be the decisive factor.
"But when we looked closer, it wasn’t. Letting your intuition guide your hypothesis, then letting yourself be convinced otherwise - that’s science,” he said.
The other co-authors on the study are Jinlun Zhang and Mike Steele , both in the UW Applied Physics Laboratory.
The UW tool is also commonly used to gauge the total volume of Arctic sea ice in a given month. Overall the UW tool shows the minimum volume of Arctic sea ice, reached in September, has recovered slightly from its all-time low in 2012, but is still following a long-term decline over the past four decades.
"We’ve lost about half of the extent, we’ve lost half of the thickness, and if you multiply these two things, we’ve lost 75 percent of the September sea ice,” Schweiger recently told the Washington Post.