Atmospheric blocking slows ocean-driven melting of Greenland’s largest ice tongue

Northeast Greenland is home to the 79°N glacier, the country’s largest floating glacier tongue, but also one of the most threatened by global warming. Warm Atlantic water is melting it from below. However, experts from the Alfred Wegener Institute have now determined that the temperature of the water flowing into the glacier cavern decreased from 2018 to 2021, even though the ocean has been steadily warming in the region over the past few decades. This could be due to temporary changes in atmospheric circulation patterns.

In a study just published in the journal ScienceResearchers discuss how this affects the ocean and what it could mean for the future of Greenland’s glaciers.

In recent decades, the Greenland ice sheet has been losing more and more mass, which has also weakened its stability. This is mainly due to the warming of the atmosphere and oceans, which accelerates the melting of ice, thus contributing to a rise in average sea level. The Northeast Greenland Ice Stream alone, which feeds the huge Nioghalvfjerdsfjorden glacier, also known as the 79°N glacier, could cause a rise in sea level of one meter if it were to melt completely.

Beneath the glacier tongue is a cavern into which ocean water flows. Data collected by the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) now indicate that the temperature of the water flowing into the cavern decreased between 2018 and 2021.

“We were surprised to find this abrupt cooling, which contrasts sharply with the long-term regional warming of the ocean that we observed in the inflow to the glacier,” said Dr. Rebecca McPherson, an AWI researcher and first author of the study. “Since the ocean water in the glacier cavern became colder, this means that less ocean heat was transported under the ice during this period, and therefore the glacier melted more slowly.”

But where does this cold water come from under the glacier if the surrounding ocean temperatures continue to rise? To find out, AWI researchers collected data from 2016 to 2021, using an oceanographic mooring.

The monitoring platform continuously recorded parameters such as seawater temperature and flow rate at the calving front of the glacier 79°N, where water flows into the cavern. While the Atlantic water temperature had initially increased, reaching 2.1 degrees Celsius in December 2017, it dropped again by 0.65 degrees from the beginning of 2018.

“We were able to trace the source of this temporary cooling from 2018 to 2021, upstream, to the Fram Strait and the vast Norwegian Sea,” McPherson says. “In other words, circulation changes in these distant waters can directly affect the melting of the 79°N glacier.”

So the lower water temperatures in Fram Strait are the result of atmospheric blocking. When this blocking occurs, stationary high-pressure systems in the atmosphere force the normally dominant air currents to deflect. This is also what happened over Fram Strait: several atmospheric blockings over Europe allowed more cold Arctic air to flow through Fram Strait into the Norwegian Sea. This slowed down the Atlantic water flowing toward the Arctic, so that it cooled more than usual along the way.

The cooled water then flowed through Fram Strait to the Greenland shelf and the glacier at 79°N. The entire process, from the appearance of the atmospheric blocks to the influx of colder Atlantic water into the glacier cave, took two to three years.

“We assume that atmospheric blocks will remain an important factor in multi-year cooling phases in the Norwegian Sea,” McPherson says. “They provide the atmospheric and oceanic conditions that influence the variability of water temperature in the Atlantic Ocean and, therefore, in the glaciers of northeast Greenland.”

Why? Because the mass of water flowing northward not only continues further into the Arctic, where it affects the extent and thickness of sea ice; in Fram Strait, about half of the water flows westward, where it determines the oceanic melting of Greenland’s glaciers.

“In the summer of 2025, we will return to the 79°N glacier on board the research icebreaker Polarstern. We already know that the water temperature in the Fram Strait is rising slightly again and we are eager to see whether the glaciers will melt further as a result.”

To more accurately predict the fate of the 79°N glacier, it is important to understand the causes of the changes taking place within it, as McPherson points out: “Our study provides new insights into how glaciers in northeast Greenland behave in a changing climate. This will help refine predictions of sea level rise.”

As their colleague Professor Torsten Kanzow from AWI adds: “Generally speaking, we consider the warm water influx into the cavern beneath the 79°N glacier to be part of the Atlantic Meridional Overturning Circulation (AMOC). Predictions indicate that this heat transfer chain could weaken in the future. A major challenge will be to set up long-term observation systems that can capture the effects of the large-scale ocean circulation extending to the Greenland fjords.”