Arctic sea ice hit its fourth-lowest annual minimum in the satellite era, as melting appeared to come to a halt Friday, the National Snow and Ice Data Center said in a preliminary report issued Tuesday.
With sea-ice extent reaching a minimum of 1.7 million square miles (4.41 million square kilometers), 2015 ranked closely behind 2011, when ice extent retreated to 1.67 million square miles (4.34 million square kilometers), the third-lowest annual minimum on record.
The center's report analyzes satellite data collected since 1979. It defines extent as areas with at least 15 percent ice coverage.
This year's low ice conditions fit the long-term trend of decline, said Mark Serreze, director of the Boulder-based center. He noted that the nine lowest extents in the center's records have all occurred in the last nine years.
"That's pretty telling," he said. "This is sort of the new normal."
Within the long-term decline, there are year-to-year variations driven by weather. Weather this year was "reasonably favorable" for faster melt, as it was in 2011, a year with a similar melt pattern, Serreze said.
Specifically, this year's melt season was dominated by high atmospheric pressure and anticyclone conditions, or counter-clockwise wind patterns, the center said in its report. Cyclonic weather patterns in the Northern Hemisphere tend to bring low temperatures and help ice converge, the center said. Anticyclonic patterns in the Arctic, by contrast, tend to bring sunny and warm conditions, the center said.
This year's anti-cyclone conditions were not nearly as strong as in 2007, when the second-lowest annual ice extent was hit, Serreze said. "That was a year when we had sort of a perfect storm," he said.
Arctic sea ice hit a record-low extent in 2012, when it fell to 1.31 million square miles. In that year, the ice was particularly thin, Serreze said.
Mid-September is when the annual melt usually stops as seasons change and temperatures drop. But the 2015 numbers released Tuesday are only preliminary, Serreze said. A shift in conditions could mean some slight additional melting, he said.
If 2015 is similar to 2011, will next year be similar to the record-low ice year of 2012?
"We know it will be low," Serreze said. He declined to make a more specific prediction, but noted that multiyear ice melted extensively this year, as it did in the prelude to the 2012 record low."The ice is not in good shape," he said.
Along with the atmospheric conditions in the air above, movement below the water's surface is contributing to sea-ice melt, said a scientist sailing about 90 miles north of Barrow Tuesday on the research vessel Sikuliaq.
Jennifer MacKinnon of the Scripps Institution of Oceanography and chief scientist for a mission aboard the University of Alaska Fairbanks-operated research ship, said her team is measuring underwater turbulence that has been sending warmth up to the underside of the ice layer.
In the Arctic Ocean, unlike other ocean waters, the freshwater surface tends to be colder, with the warmer and saltier water lurking below, MacKinnon said in an email sent from the ship. Open-water conditions allow the winds to create more underwater turbulence, bringing up the subsurface heat and likely playing a "substantial role" in ice melt, she said.
Measurements being taken this year show the turbulence is dramatic, she said. "Even though we thought something like this might be happening, we have been genuinely thunderstruck by how incredibly strong the turbulence is below the surface. Our instruments are seeing billows of turbulence 30 or more feet high, events that look just like a wave breaking on the beach, but much larger!" she said.
Marine shippers might be benefiting from this year's low ice. The Northern Sea Route across Arctic Russia has been free of ice and open to traffic this year, and the Northwest Passage over the top of Canada is partially open.
A southern route around the islands of Arctic Canada, pioneered a century ago by Norwegian explorer Roald Amundsen, has been navigable this year, as it has in many recent years, Serreze said. That was almost the case this year for a more northern route, through deeper waters, where full ice melt is rare, he said. "We got pretty close this year to opening up that northern channel of the Northwest Passage," he said. That northern route of the Northwest Passage did open in 2007.