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Arctic sea ice is being melted from below because of increased ocean mixing.

Renato Granieri/Alamy Stock Photo

Warm Atlantic waters wage a new assault on Arctic ice from below

A new enemy is undermining ice floating on the Arctic Ocean: heat from below.

Sensors that have plumbed the depths of Arctic seas since 2002 have found warm currents creeping up from the Atlantic Ocean and helping drive the dramatic retreat of sea ice there over the last decade. A new study shows this “Atlantification” of the Arctic Ocean as a new, powerful driver of melting, alongside losses due to rising air temperatures.

The paper shows “a massive shift” in the behavior of the Arctic Ocean over a short time, says Finlo Cottier, a physical oceanographer with the Scottish Association for Marine Science in Oban who was not part of the study team. “Here we’re seeing an ocean basin changing on a generational timescale—or less,” he adds.

Deep below the Arctic sits a ridge that splits the ocean roughly in half: The Amerasian basin sits on the North American side, whereas the Eurasian basin lies north of Europe and most of Asia. Both basins are losing ice fast. Across the entirety of the Arctic Ocean, it’s disappearing at an eye-popping rate of 13% per decade since satellite data was available. It has also thinned by 1.7 meters since the 1970s.

Warm Atlantic waters, delivered by an offshoot of the Gulf Stream, have long been known to prevent ice formation north of Scandinavia, on the western side of the Eurasian basin. Satellite data show that, in general, sea ice is much more prevalent in the eastern side of the basin, north of Siberia. But over the last decade, ice has also begun to disappear here, too. It used to persist through the sunny summers, allowing several years of ice growth to accumulate. Now, the ice melts in summer, causing the total time without floating ice in the region to jump from less than 1 month per year to more than three.

Sensors on tethered moorings showed warm Atlantic currents creeping into part of the Arctic Ocean.

©Ilona Goszczko

To understand this new trend, scientists in 2002 began installing sensors on lines tethered to the floor of the Eurasian basin, called moorings. The team relied on a total of nine moorings, augmented with satellite data and sensors bolted below drifting sea ice and along ice frozen to the shore. When they retrieved data from the moorings in 2015, they found that the ocean had experienced a dramatic change over the previous decade, especially during the winter.

Between Norway and Greenland in the western Eurasian basin, Atlantic currents flow into the Arctic at a depth of 200 to 250 meters, about 4°C warmer than the surface water. In winter, cold air cools surface waters until they fall and mix with the warm waters below. That creates an overall warmer, well-mixed ocean over the top 250 meters, and one with little sea ice.

On the eastern side of the basin, however, the warm Atlantic waters were kept at bay—until recently. The currents lurked at a depth of about 150 meters, but they didn’t mix much with surface layers, because of a barrier called the cold halocline layer (CHL)—a boundary between salty deep waters and fresher water on top. Summer ice, as it forms, rejects salt, leading to the creation of dense, salty waters just below the ice. Those waters are heavier, and as they fall they create a highly stratified ocean. “Previously this monster, Atlantic warm water, was well covered from the surface” by the CHL, says Igor Polyakov, a physical oceanographer at the University of Alaska in Fairbanks, who led the study. “The new data show this layer has disappeared in winter.”

The result, he says, is an increased “Atlantification” of the Arctic, where the eastern side of the Eurasian basin is becoming more like the western side, the team reports today in Science. The top of the Atlantic water, according to one mooring, had risen from a depth of 140 meters in the winter of 2003–04 to a depth of 85 meters just a decade later. Without summer sea ice forming to establish the CHL, he says, the ocean mixes more—and less ice forms.

On the eastern side of the Eurasian basin, say Polyakov and his colleagues, air temperatures were the main culprit for ice melting in the 2000s. Now, however, they believe air temperatures and warm waters share the blame about equally. Polyakov says a positive feedback loop is underway, in which less summer sea ice will lead to warmer winter waters and even less summer ice in subsequent years. One unknown is how the addition of massive flows of freshwater from Siberian rivers, bolstered by thawing permafrost, could affect the system, says study co-author Eddy Carmack, an oceanographer with Fisheries and Oceans Canada in Sidney. That new freshwater could encourage more sea ice to form on the basin, unless winds wash the new water away.