Uncharted territory! – The secrets of the ocean sea floor and its role in melting Greenland’s largest floating ice tongue
The 79 North Glacier in Northeast Greenland has the largest floating ice tongue around the coast of Greenland. Our recent work shows how the bathymetry constrains the ocean heat flux below the glacier tongue - and thereby melting at the underside of the glacier.
Summer 2016. Beautiful sunshine along the coast of Northeast Greenland. The captain of R/V Polarstern navigates us through sea ice and icebergs. We are heading to the 79 North Glacier - where no ship has ever been. When we take the helicopter to check for a route through the sea ice, I realise for the first time how close we got. I feel touched by the beauty of this landscape and related to this glacier I’ve been studying from my home desk since more than 2 years.
The 79 North Glacier and its neighbouring glacier Zachariæ Isstrøm together drain the Northeast Greenland Ice Stream – and thereby 1/5 of the entire mass of the Greenland Ice Sheet. Any instabilities at the glacier margins may trigger an increased mass loss from the ice sheet contributing to sea level rise. Recent measurements show, that both glaciers have been started to thin and retreat in the past years. The main suspect to drive the glacier’s mass loss has been the warming ocean – however, only few station-based measurements were taken in the vicinity of the glaciers yet.
Embedded in a fjord, the 80 km long floating tongue of the 79 North Glacier pushes eastward against a chain of small islands. Underneath the floating glacier there is a cavern of up to 900 m depth that is filled with 1°C-warm Atlantic water below around 400 m depth. These waters cause the ice on the bottom of the glacier to melt at a high rate. But which mechanisms are driving the oceanic heat to flow from the continental shelf below the glacier? - 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.
R/V Polarstern in front of the 79 North Glacier in summer 2016. Photo credit: Nat Wilson.
In summer 2016, virgin soil is in front of us! The solid ice mass in front of the glacier broke up giving way to this unmapped region. 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, we find a critical bathymetric sill behind which the warm Atlantic water flushes down a slope. The bathymetric control means that already a little thickening of the Atlantic water layer (far away from the glacier!) will lead to an enhanced ocean heat flux below the glacier tongue, driving increased melt – and as a consequence thinning of the ice tongue.
To gain more detailed insight into the time-variability of the ocean heat flux, we anchor instruments in the water column recording currents, temperature and salt content for a year. In September 2017, we recover our instruments. The one-year long records confirm that the bathymetry constrains the heat flux and thereby melting at the glacier tongue. However, it shows surprisingly large changes in the ocean heat flux over the year leading to new open questions: Is the wind driving the ocean variability? And what governs the variability of outflowing meltwater plumes? There are a lot more secrets to reveal from the ocean near Greenland’s largest glacier tongue.
Mooring recovery through sea ice in September 2017. Photo credit: Richard S. Jones.