Interaction between marine terminating glaciers and the ocean circulation in Northeast Greenland

The Greenland ice sheet has been subject to strong mass loss in the last decades. Greenland meltwater fluxes have a strong impact on global sea level and can affect the strength of the Atlantic Meridional Circulation. Anomalously high transport of Atlantic Water through the Nordic Seas was observed in the last decades and is thought to increase melting of marine-terminating glaciers around Greenland. Future projections of climate warming around Greenland indicate particularly large melting rates between Fram Strait and Denmark Strait at the east coast of Greenland. In Northeast Greenland, the Zachariæ Isstrøm lost its entire floating ice tongue between 2012 and 2014, while Mayer et al. (2018) report on a substantial thinning of the 79-North Glacier (79NG) since the late 1990s. Both Zachariæ Isstrøm and the 79NG drain the Northeast Greenland Ice Stream (NEGIS), so that changes in these two ice tongues are likely to have substantial implications for global sea level. A warming in Atlantic Water temperatures has been suggested to be the main driver for the collapse of the Zachariæ ice tongue. Warm Atlantic Water has to pass through the trough system of the Northeast Greenland continental shelf to reach the glacier fronts. Based on observations from 1979–2016, Schaffer et al. (2017) showed that warm Atlantic Water mainly reaches the 79NG via a southern pathway through Norske Trough. However, the observational data are largely restricted to the summer season. An indispensible requirement for modelling the processes on the Greenland continental shelf is a realistic representation of the Atlantic Water recirculation in Fram Strait. In earlier phases of this project, we have set up a global FESOM configuration with 4.5 km mesh resolution in the Nordic Seas and Arctic Ocean, and an additional refinement of 1 km applied in the larger Fram Strait area and on the Northeast Greenland continental shelf. A comparison with the Rossby radius of deformation shows that this mesh resolution is sufficient to resolve eddies. Comparing the model results to a coarser, only eddy-permitting set-up (4.5 km resolution everywhere) shows that mean circulation, hydrography and eddy kinetic energy in the fully eddy-resolving 1-km simulation yield a much better agreement to data of the long-term mooring array in Fram Strait. The mesh resolution of 1 km in Fram Strait is clearly needed to realistically represent Atlantic Water recirculation pathways (Wekerle et al. 2017). Ocean heat transport into the cavity derived from oceanographic mooring data suggests an annual mean, area-averaged basal melt rate for the 79NG floating ice tongue of 14 ± 6 m/yr (Schaffer et al., in prep.). Radar measurements have revealed that basal melt rates exceed 75 m/yr close to the grounding line (Zeising et al., in prep.). Radar data also indicate the existence of englacial channels transporting meltwater toward the strongly sloping ice base downstream from the grounding line. Each of these processes is subject to a substantial variability on time scales of several months, but none of them shows a clear seasonal cycle. With regard to the interplay between ice and ocean, two hypotheses thus seem plausible: