Study Finds Arctic Cyclone Had Insignificant Impact on 2012 Ice Retreat
Last summer, after an extraordinary storm swirled across the Arctic Ocean, churning sea ice like some great Waring blender, much was speculated about the cyclone’s role in the enormous retreat of sea ice that followed.
A new modeling study by the Applied Physics Laboratory at the University of Washington, replaying last summer’s Arctic Ocean ice conditions with and without the storm, shows that the short-term influence of all that ice churning probably played almost no role in the final ice retreat in September. The study also revealed that the ice melting that was caused by the storm was the result of heat coming from the stirring of seawater layers
below the ice. Here’s an excerpt from the university news release:
“The effect is huge in the immediate aftermath of the cyclone, but after about two weeks the effect gets smaller,” said lead author Jinlun Zhang, an oceanographer in the UW’s Applied Physics Laboratory. “By September, most of the ice that melted would have melted with or without the cyclone….”
Results showed the storm caused the sea ice to pass the previous record 10 days earlier in August than it would have otherwise, but only reduced the final September ice extent by 150,000 square kilometers (almost 60,000 square miles), less than a 5 percent difference. By comparison, the actual minimum ice extent was 18 percent less than the previous record set in 2007.
The study also revealed a surprising mechanism for the cyclone-related melting. Earlier discussions about the cyclone’s effect had focused on winds breaking up the ice or driving ice floes into areas of warmer water. The results suggest that neither process led to much increase in melting.
Relatively recent research shows that in the summertime, thin ice and areas of open water allow sunlight to filter down to the water below. As a result, while a layer of ice-cold fresh water sits just beneath the sea ice, about 20 meters (65 feet) down there is a layer of denser, saltier water that has been gradually warmed by the sun’s rays.
Blowing on polar water is like blowing on a layered cocktail. When the cyclone swept over the drifting ice floes, underside ridges churned up the water to bring sun-warmed seawater to the ice’s bottom edge. The model suggests that during the cyclone there was a quadrupling of melting from below, and that this was the biggest cause for doubling ice loss during the three-day storm. [Read the rest.]
This is one modeling study, so the question is hardly resolved. In September, NASA scientist Claire Parkinson, in a NASA release, had described the storm’s influence this way:
“The storm definitely seems to have played a role in this year’s unusually large retreat of the ice”, Parkinson said. “But that exact same storm, had it occurred decades ago when the ice was thicker and more extensive, likely wouldn’t have had as prominent an impact, because the ice wasn’t as vulnerable then as it is now.”
I’ll keep track as work continues on the complex mix of oceanic and atmospheric factors that shape the Arctic Ocean’s ice.