News•February 1, 2013

Major Storm Accelerated Arctic Sea Ice Loss, Study Finds

The “Great Arctic Cyclone of 2012,” which struck the Arctic at the height of the sea ice melt season in early August, was not responsible for causing sea ice extent to plunge to a record low just a few weeks later. That is one of the conclusions of a new study published in the journal Geophysical Research Letters. It is the first study to quantify the impacts that the storm had on the fragile Arctic sea ice cover, which has been rapidly shrinking and thinning in response to rapid Arctic warming.

An unusually strong storm formed off the coast of Alaska on August 5 and tracked into the center of the Arctic Ocean, where it slowly dissipated.
Click to enlarge the image. Credit: NASA

The study found that while the extraordinarily powerful storm did, in fact, accelerate the melting of Arctic sea ice, the sea ice extent record would have occurred regardless. The sea ice cover has been so depleted by warming air and water temperatures during the past few decades that it was “preconditioned” to reach a new record low, according to the study. Other studies have shown that manmade global warming is responsible for much of the sea ice loss by causing Arctic air and water temperatures to increase.

The research, by a team of University of Washington polar scientists, relied on a combination of computer models and meteorological data to simulate the effects that the storm had on the sea ice. They ran computer simulations of the sea ice cover interacting with last summer’s weather and compared it against simulations that did not include the storm. The simulations did not include every possible storm-related impact, as the model used left out wave-related effects, for example.

The study found that the storm-related ice loss accelerated due to the way the storm caused warmer waters to rise from deeper ocean layers, melting sea ice from below.

In the summertime, thin sea ice cover and areas of open water allow sunlight to filter to the water below, creating a layer of denser, saltier water about 65 feet below the surface. That water can be warmed over time by the sun’s rays, and when stirred to the surface by a storm or other factors, it can help melt sea ice.

Monthly September ice extent for 1979 to 2012 shows a decline of 13 percent per decade.
Click to enlarge the image. Credit: NSIDC.

During the storm, which peaked during a three-day period from August 6-8, sea ice volume decreased at about twice the normal rate, due to a quadrupling in the melting of sea ice from below as the storm roiled the Arctic waters, transporting ocean heat upward. In fact, the three-day, storm-related sea ice volume loss was the largest three-day volume loss during the 1979-2011 period, the study said.

The study also found that the storm reduced the sea ice minimum at the end of the 2012 melt season by about 4.4 percent, or about 60,000 square miles, and forced the 2012 ice extent to drop below the 2007 minimum 10 days earlier than if the storm had not occurred.

However, sea ice extent at the end of the 2012 melt season was 18 percent below the 2007 record minimum, which indicates the storm's contribution was not the driving factor behind the record.

“By September, most of the ice that melted would have melted with or without the cyclone,” said lead author Jinlun Zhang, an oceanographer, in a press release.

Ted Scambos, a scientist at the National Snow and Ice Data Center in Boulder, Colo., who was not involved in the study, said the results demonstrate that the long-term decline in sea ice cover is more consequential than the storm was. “I think the storm was remarkable, and for a brief time had the effect of pushing the ice extent a week or two ahead of where it would have been. But in the end, nothing extra was needed for 2012 to set a major new record low ice extent,” he said in an email conversation.