Ice sheet-ocean interaction - The future of the 79° North Glacier
The Nioghalvfjerdsbræ or 79° North Glacier (79NG) is the largest of the marine terminating glaciers which are fed by the Northeast Greenland Ice Stream (NEGIS), and one of the few with a floating ice tongue. Considering the decrease in thickness of the 79NG and also of the neighboring Zachariae Isstrøm (ZI), we are investigating the fate of the coupled ice sheet-ice cavity-ocean system in the near future. Our goal is to understand the processes that potentially lead to the decay of these and other floating glacier tongues, such as Jakobshavn Isbræ. We plan to create projections for the future evolution of this region, with the ultimate goal to examine the contribution of the NEGIS to global sea level rise. In this subproject we will conduct simulations with a coupled ocean-sea ice-ice shelf-ice sheet model, based on the models FESOM and ISSM.
What was the main question you aimed to answer during the first project phase?
Within the first project phase (2017-2020) we have been working on various scientific questions targeting the 79° North Glacier (79NG) within the two subprojects TP2 (Basal melt and ocean circulation) and TP3 (Grounding line dynamics). Our main research questions can be summarized as follows:
- In subproject 2, one of our main objectives was to analyze the relation between the oceanic circulation at the entrance of the cavern of the 79NG and the basal melt rates at the bottom of the glacier. This is particularly interesting as the basal melt rates can be considered as an important factor for the destabilization of the glacier and the feeding ice stream.
- The main objective of subproject 3 was to investigate the response of the glacier dynamics to tidal forcing. This includes the variability of ice fluxes on short time scales and the vulnerability of the glacier to changes in the grounding line.
Which methods did you use to answer your research questions?
At the heart of TP2 were field measurements of time series and spatial distributions of oceanographic and glaciological variables. For instance, we deployed moorings close to the calving front of the 79NG to obtain long-term time series, and installed radar stations on top of the ice shelf to determine basal melt rates.
In TP3, we set up and analyzed viscoelastic model simulations that describe the movements of the glacier and the ice shelf. In addition, GPS observations served to validate the simulations as well as the model concept.
What were your main results?
- The analysis of ocean velocity and temperature time series revealed the spatial structure of the circulation at the calving front of the 79NG, the strength of the warm water inflow and meltwater outflow, and the temporal variability of the oceanic heat transport underneath the glacier tongue. Based on these time series we could both detect the process of topographic control on the oceanic heat transport and show that this process has an impact throughout the year on basal melt rates underneath the cavity (Schaffer et al., 2020).
- Based on circulation and stratification measurements, we described the processes controlling the time variability of the oceanic heat transport into the cavity of the 79NG (von Albedyll et al., submitted to JGR: Oceans).
- Glaciological campaigns allowed us to analyze the spatial variation and temporal variability of basal melt rates. In particular, radar measurements revealed large spatial differences in the basal melt rates with maxima of 145 m/year at the transition from the grounded to the floating part of the glacier tongue. In addition, a strong seasonal variability was detected (Zeising et al., submitted to Nat. Comm.).
- Viscoelastic simulations and GPS observations enabled an analysis of the different components of glacier dynamics, such as sliding, elastic, and viscous deformation. By comparing the simulations with GPS observations, we found that tidal forcing reaches by far less upstream than for Antarctic ice streams (Christmann et al., 2019; Christmann et al., submitted to Nat. Comm.).
What are your goals for GROCE-2?
In GROCE-2, we will investigate the reasons for high melt rates at the base of the 79NG by considering oceanic and glaciological processes. We will contrast the present state of the glacier (including mean state, seasonal and interannual variability) with future changes revealed by climate projection scenarios. In order to achieve these aims, we focus on the development and operation of a fully coupled ice sheet/ice shelf/ocean model (based on FESOM and ISSM) for the region around the 79NG.