How AWI researchers investigate the currents at the 79° North Glacier
The ice shield of Greenland is shrinking - at an ever faster rate. The glaciers at the edge of the inland ice are particularly affected by this. Scientists do not yet understand why exactly they shrink at such high speed and what the underlying mechanisms are. Is it mainly the warming sea that gnaws on the ice tongues from below? This is the question AWI oceanographer Janin Schaffer examines using the Greenlandic 79° North Glacier as an example. In order to obtain the necessary measurements, in summer 2016 she had to take the Polarstern where no other research vessel had ever been - directly to the glacier front. An expedition report!
The first time I take the helicopter on board the Polarstern to the 79°North Glacier I realise how close we were to our destination. We want to go where no ship has ever been - to the eastern glacier front of the 79° North Glacier in the northeast of Greenland. Virgin soil in sight!
In the early 1990s, a major measurement campaign was carried out on the glacier tongue itself, which our former AWI colleague Christoph Mayer was involved in. His measurements have told us that there is a cavern of up to 900 metres depth underneath the 80 kilometre long floating glacier tongue. Below around 400 metres depth it is filled with one-degree Celsius Atlantic water, which causes the ice on the bottom of the glacier to melt at a high rate.
As part of my PhD thesis, I have been examining the question of how this warm Atlantic water is transported from the Fram Strait to the 79°North Glacier. But no data have so far been available for the area crucial for me, directly in front of the glacier, because a stable solid ice mass has made ship-based measurements impossible. We didn't even know how deep the seabed is, let alone how the Atlantic water flows to the 79° North Glacier. However, satellite images from the last few years showed that the solid ice mass in front of the glacier breaks up more and more often in late summer. And that's precisely what we want to take advantage of.
Can warm Atlantic water become a risk for the glacier?
If you look at the development of the glaciers overall, the 79°North Glacier is something of an exception. Unlike the glaciers on the western and southern coast of Greenland, which have significantly retreated and thinned in recent years, the 79° North Glacier has hardly changed at all. Embedded in a fjord, it pushes eastward against a chain of small islands, which protrude from the water at the glacier front and stabilise the glacier tongue.
It is likely that the warming Atlantic water, which circulates around the coast of Greenland, is partly responsible for the retreat of its neighbours further south. The few existing measurements by international researchers from the years 1997, 1998, 2009, 2011 and 2014 suggest that the ocean water in the cavern underneath the 79° North Glacier has warmed by 0.5 degrees Celsius. The question we are therefore asking ourselves is how soon and exactly how a temperature change in the North Atlantic can pave its way to this very glacier cavern? Is there a deep passage at the glacier front between the islands that allows a direct exchange of the water in the cavern with the environment? Or is the Atlantic water episodically lifted across a threshold by dynamic ocean processes? To answer these questions, we had to take the ship directly to the glacier front.
No research vessel has yet made it to this point
During the first five weeks of our Polarstern trip when we work in the Fram Strait we are spellbound as we see the satellite image and observe how every day big cracks develop in the huge closed sea ice layer. As we approach the 79° North Glacier, the situation on the Polarstern still seems tricky. Poor visibility and a lot of sea ice and icebergs protruding 10 to 20 metres from the water block our way. Do we need a plan B? But the wind turns, pushes the ice away from the front of the 79° North Glacier and, in the days that follow, we have bright sunshine.
Virgin soil in front of us! But virgin soil also means that this region has not yet been mapped. Only a few isolated measurements suggest that there have to be water depths of 450 metres here, as well as some shallow water. It's mostly the captain who is interested in those. We oceanographers are also very interested in the topography of the ground, because it plays a key role in determining the extent to which a direct exchange of the Atlantic water between the glacier cavern and the environment is possible.
We slowly feel our way along the glacier front and we do find a deep passage between the islands. Our measurements show that warm Atlantic water quickly flows underneath the glacier tongue directly on the sea floor. What's more, on the surface we observe meltwater bubbling from underneath the glacier tongue in a huge plume: like a gigantic water jet with a diameter of up to 20 metres.
A new sensor provides surprising insights
We work day and night and are excited when we first see the data. As well as our conventional measurements of temperature, salt and velocity profiles, we deploy mooringsg for long-term measurements in the water column and use a turbulence sensor to measure small-scale mixing processes. We are also using a mini CTD for the first time. It consists of a small 20-centimetre long sensor, which we have attached to a fishing rod. This allows us to take the helicopter to places that we can't reach with the Polarstern, allowing us to obtain a temperature profile from an ice floe, for example, within a short time and without much effort. And we are angling deeper than we thought: instead of the expected 50 metres, the water here is up to 650 metres deep.
On the glacier itself, we are building two ice radar measurements stations for our AWI glaciologists. These will be collected in a year's time and will then provide insight into the extent of the melt at the bottom of the glacier. As it is propelled by Atlantic water, the data will perfectly complement our oceanographic measurements at the glacier front.
We have anchored instruments in the water column which we will use to record currents, temperature and salt content for a year in order to gain more detailed insight into the inflow of the warm Atlantic water under the glacier tongue and the outflow of the meltwater.
But it's not just our measurement results that are exciting, the landscape in Greenland is also impressive. I can't get enough of the glaciers, mountains and islands in the sunlight – even though right now we are spending our days travelling up and down the glacier front in order to take as many measurements as possible.
But the beauty of these ice masses seems more transient than ever. We will try to do our part in creating a better understanding of the processes that can trigger a possible collapse of the glacier tongue. How quickly will the ice tongue melt from below? How long will the 79° North Glacier survive?
Our new measurement data will answer some of the questions. We are looking forward to analysing our treasured set of data, and we are currently planning our return to the glacier next year. That's when we will retrieve our moorings, and we hope that the wind and the ice will allow us to make our way to the 79° North Glacier once again.