Atmospheric blocking slows ocean-driven melting of Greenland's largest glacier tongue

Over the past few decades, the Greenland Ice Sheet has lost more and more mass, which has also lessened its stability. This is chiefly due to the warming of the atmosphere and oceans, which accelerates the melting of ice, contributing in turn to an increase in mean sea level. The Northeast Greenland Ice Stream alone, which feeds into the massive Nioghalvfjerdsfjorden Glacier – also known as the 79° N Glacier – could produce a metre of sea-level rise if it melted completely. Beneath the glacier tongue lies a cavern that ocean water flows into. Data gathered by the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) now indicates that the temperature of the water flowing into the cavern declined between 2018 and 2021. “We were surprised to discover this abrupt cooling, which is a marked contrast to the long-term regional ocean warming we’ve observed in the influx to the glacier,” says Dr Rebecca McPherson, a researcher at the AWI and the study’s first author. “Since the ocean water in the glacier cavern grew colder, it means less oceanic warmth was transported under the ice in this period – and in turn, the glacier melted more slowly.”

But where did this cold water below the glacier come from if temperatures in the surrounding ocean continued to climb? To find out, the AWI researchers collected data from 2016 to 2021, using an oceanographic mooring to do so. The monitoring platform continually took readings on parameters like the temperature and flow speed of the seawater at the calving front of the 79° N Glacier, which is where water flows into the cavern. Whereas the temperature of the Atlantic water initially rose, topping out at 2.1 degrees Celsius in December 2017, it dropped by 0.65 degree again from early 2018.

“We were able to track down the source of this temporary cooling from 2018 to 2021 upstream, to Fram Strait and the vast Norwegian Sea,” Rebecca McPherson explains. “In other words, circulation changes in these remote waters can directly affect the melting of the 79° N Glacier.” As such, the lower water temperatures in Fram Strait were the result of atmospheric blocking. When this blocking occurs, stationary high-pressure systems in the atmosphere force the normally dominant air currents to deviate. That’s also what happened over Fram Strait: several atmospheric blocks over Europe allowed more cold air from the Arctic to flow through Fram Strait into the Norwegian Sea. This slowed water from the Atlantic that was flowing toward the Arctic, so that it cooled more than usual along the way. The cooled water then flowed through Fram Strait to Greenland’s continental shelf and the 79° N Glacier. The whole process – from the appearance of the atmospheric blocks to the inflow of the cooler Atlantic water in the glacier cavern – took two to three years.

“We assume that atmospheric blocks will remain an important factor for multiyear cooling phases in the Norwegian Sea,” says Rebecca McPherson. “They provide the atmospheric and oceanic conditions that influence temperature variability in Atlantic Ocean water, and in turn the glaciers of Northeast Greenland.” Why? Because the northward-flowing water mass not only continues farther into the Arctic, where it affects the extent and thickness of sea ice; in Fram Strait, roughly half of the water veers to the west, where it determines the oceanic melting of Greenland’s glaciers. “In the summer of 2025, we’ll be returning to the 79° N Glacier on board the research icebreaker Polarstern. We already know that water temperatures in Fram Strait are now rising again slightly and we’re anxious to see if the glacier melting increases as a result.”

To more accurately predict the fate of the 79° N Glacier, it’s important to understand what is driving changes within it, as Rebecca McPherson stresses: “Our study offers new insights into the behaviour of Northeast Greenland’s glaciers in a changing climate. This will allow forecasts for rising sea levels to be refined.” As their colleague Prof Torsten Kanzow from the AWI adds: “Generally speaking, we consider the warm-water inflow into the cavern below the 79° N Glacier to be part of the Atlantic Meridional Overturning Circulation (AMOC). Forecasts indicate that this thermal conveyor belt could weaken in the future. One key challenge will be to establish long-term observation systems capable of capturing the effects of macro-scale ocean circulation extending as far as the fjords of Greenland.”