The melting of Greenland's largest floating ice tongue, an extension of a glacier that projects from the coastline, may be influenced by changes in ocean currents several hundred kilometers away — and the same may be true for glaciers elsewhere.

Scientists measured the speed of water flowing into and out of the cavity beneath the Nioghalvfjerdsfjorden Glacier, also known as the 79 North Glacier or 79NG, over the course of a year. They found that the velocity varied significantly over periods of days to months and was linked to water-circulation patterns along the neighboring continental shelf. These phenomena should be considered in studies exploring the interactions between oceans and glaciers, the researchers reported April 29 in JGR: Oceans.

Melting from the Greenland Ice Sheet is responsible for 15% of the current rate of sea-level rise, the researchers noted in the study. This is partly due to melting on the surface of the ice. However, a significant amount of melting also happens from below, where glaciers meet the sea.

"You can think of Greenland as one big blob of ice, and from this there are icy rivers that flow down to the coast," said Luisa von Albedyll, a Ph.D. student at the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, in Germany, and first author of the study. "When they are then in contact with the ocean, the ocean can also warm and melt the glaciers."

The tongue of the 79 North Glacier in northeastern Greenland juts about 80 kilometers (50 miles) into the ocean, she said. It has a large east-facing front and a smaller front facing to the north. Parts of the eastern front are anchored by little islands, which have helped keep the tongue "remarkably" stable in recent years, von Albedyll said, while the neighboring Zachariæ Isstrøm Glacier's tongue completely disintegrated. Nonetheless, scientists have estimated that 79NG's floating ice tongue thinned by 30% between 1999 and 2014, von Albedyll and her colleagues wrote.

"It's important to know how much heat the ocean transports to the glacier," von Albedyll said. This depends on both the temperature and the velocity of the water, she added, "because if the ocean is flowing faster, it can provide more heat." 

The heat is provided by waters traveling from the Atlantic Ocean through the Fram Strait, which lies between Greenland and Svalbard, and onto the northeast Greenland continental shelf. This warm water can enter or exit the cavity beneath the glacier tongue at three sites along the eastern front and one at the northern front.

Von Albedyll and her colleagues planted sensors at these four gateways to measure the ocean speed from August 2016 to September 2017. They found that warm water was directed into the cavity beneath the tongue through the deepest gateway on the eastern front. More than half of the water was eventually transported out through the gateway at the smaller northern front. 

The researchers also compared their measurements of water flowing into the cavity to those from another sensor 170 kilometers (106 miles) to the south of the glacier tongue, on the continental shelf. There they found similar variability, suggesting that "changes in the ocean currents several hundred kilometers away from the 79NG can have an impact on its melting," according to the paper.

"We didn't expect such a good correlation between the shelf circulation and the glacier-front circulation," von Albedyll said. "We knew that the water masses were passing through that pathway, but it was still shocking to see how well those records fit together."

It's not clear why the variations in ocean speed at these two sites matched so well, von Albedyll said, but they may be driven by large-scale wind patterns that send waves deep beneath the sea surface from the edge of the continental shelf toward the glacier. 

The researchers were able to pinpoint brief variations in ocean speed that likely came from tidal currents as well as patterns that played out over several months, with water flow speeding up as winter arrived.

Recent research using satellite imagery indicates that glacier melting has accelerated worldwide over the past two decades. Many computer models that simulate glacier melting take into account only yearly variability in ocean velocity, von Albedyll said.

"What our study now showed is that there is considerable variability also on the sub-yearly time scales and that this should be represented realistically in models that want to assess the ocean-ice interactions," she said.

In the future, she and her team plan to measure how ocean speeds near the glacier vary over longer periods. Another important step will be to investigate whether the kind of connection the researchers observed between 79NG and the continental shelf also affects other glaciers in Greenland.

"It's important to check for this and to take into account remote drivers for glacier melting," von Albedyll said.  "If we're trying to understand which parameters drive the increased heat flux to the glaciers and the increased melting of the glaciers, then we also need to take into account phenomena that are changing the circulation farther away."

The study, "Ocean variability at Greenland's largest glacier tongue linked to continental shelf circulation," published April 29 in JGR: Oceans, was authored by Luisa von Albedyll and Janin Schaffer, Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research; and Torsten Kanzow, Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research and University of Bremen.