Is stored heat causing Arctic sea ice to freeze later each year?

Graph showing Arctic sea ice minimum dates from 1979 through 2012, derived from satelite records This graph shows the yearly trend toward later Arctic sea ice minimum dates, but also illustrates the wide variability from year to year. Credit: NSIDC

A reader recently asked if the date of the annual Arctic sea ice minimum is shifting later each year. And if so, is that shift a sign of heat being stored in the Arctic region? According to the satellite record, Arctic sea ice generally melts to its minimum annual extent between the first and third week of September, after which ice begins freezing again. In recent decades, the Arctic has been gaining heat: Air, land, and ocean temperatures in the region have been slowly rising, and scientists have noted dramatic reductions in summer sea ice extent, as this heat causes more ice to melt away. But is this heat causing sea ice to form later each fall?

Walt Meier, a research scientist at NSIDC, said, “Overall, one would expect to get later minimums because with more open water during summer, more heat is absorbed by the ocean, and that heat has to be dissipated before freezing can begin.” Oceans dissipate heat more slowly than land surfaces and air do, and this trapped heat may delay sea ice formation even after the brief Arctic summer has waned. Meier said that the record does reveal a slight shift. He said, “There is a general shift toward a later minimum of about two days per decade.” But, he noted, there’s a catch. “The record is pretty noisy—a lot of ups and downs from year to year—so I'm not sure how significant it is,” Meier said.

Mind the weather

The Arctic environment is complex, and heat is only one of the factors affecting annual freeze up. The catch is often the weather. "The minimum’s high variability is influenced by the weather," Meier said. "If a cold front or a warm front moves through a key location near the minimum date, strong winds may push the ice edge in or out.”

Satellite image showing a large storm that churned over the Arctic in August 2012 This satellite image mosaic shows a massive storm that churned over the central Arctic for several days in early August 2012. Although sea ice extent was already low, this storm may have left the remaining ice more vulnerable to melting. The storm is located in the center of the Arctic ocean; ice-covered Greenland is visible near the lower left corner. Credit: NASA/Goddard/MODIS Rapid Response Team

Timing is crucial. At the end of summer when ocean waters have cooled, winds that push the ice edge outward will halt declining sea ice extent, resulting in a higher minimum extent than would have occurred otherwise. Winds that push the ice edge inward can cause the ice extent to continue dropping, leading to a lower seasonal minimum. In early or mid summer, however, the same wind patterns can have different effects: winds that blow sea ice floes outward and further south may scatter the ice into warmer ocean waters where it melts.

This year, for instance, a massive storm formed near the coast of Alaska in August, and then spun over the central Arctic for several days. Strong storms can make the ocean choppy, break up existing ice packs, and churn warmer waters up to the surface. This turbulence, especially arriving when sea ice is already extremely low, does not bode well for a strong ice pack going into the fall freeze up. In fact, this storm dislodged a large expanse of sea ice, making the remaining ice pack more vulnerable to the storm’s fury and to subsequent melting at the end of summer. Storms like this illustrate how factors like day-to-day weather still play a large role in determining when sea ice freezes up each year. “You can see the big swings from year to year, but also the general trend towards later dates,” Meier said. The annual freeze-up is creeping later in the season, but heat stored in the Arctic may only be part of this particular equation.