Process-based quantification of subglacial melt rates

Predicting sub-glacial melting of ice shelves is a significant area of interest within the field of ice-ocean. The floating ice tongue of the 79°N Glacier (79NG) has been found to thin over the past two decades. The rapid, significant variation of the basal melt rate is suggested to be the primary reason for thickness variability, with the contributions from ice flux and surface ablation being negligible. The main challenge faced by many researchers in estimating the basal melting is an inadequate representation of physical processes in ice-ocean interaction models due to substantial uncertainties and limited observations of glacial fjords. This subproject aims at understanding the hydrodynamic processes which are responsible for basal melt rates and the heat flow in the area of the glacial tongue of the 79°N glacier. This is approached by configuring, using and analyzing idealized but realistic model simulations with a good numerical resolution of the various processes concerning the ice-ocean boundary. In this context, special attention is paid to the understanding of the thermohaline circulation in the approximately 100 km long, 20 km wide and 800 m deep cavern below the ice shelf of the 79°N glacier. 

Contacts: Markus Reinert and Prof. Dr. Hans Burchard

Figure 1: Ice base topography of an experiment with a synthetic network of basal channels (left). and map of basal melting using the ice base topography (right). (from Mahammadi-Aragh et al., in prep)

Figure 2: Melting rate between the channels (top), and inside the narrow channels (bottom). (from Mahammadi-Aragh et al., in prep.)

What was the main question you aimed to answer during the first project phase?

The central aim of the first phase of our study was to estimate the magnitude and spatial distribution of the basal melt rate of the floating ice tongue of the 79NG. However, our knowledge about the effect of basal channels as a contributing factor on basal melting at the 79NG is still very limited. Previous studies have suggested that the kilometer-wide, hundreds-of-meter high shape dimensions of basal channels are results of feedbacks relating ocean melting and ice-shelf basal slopes. Such basal channels mostly exist in the regions with high basal melting. It has previously been observed that basal channels are common in Greenland. 

Which methods do you use to answer the research question?

We therefore design a suite of idealized numerical experiments based on a realistic ice base topography of the ice tongue and general spatial characteristics of basal channels, including a synthetic network of basal channels, to estimate the basal melting and its spatial variability (figure 1, left). Using a horizontal two-dimensional numerical plume model, we investigate the spatial distribution of submarine melting and assess the importance of ice base morphology in controlling basal melting. 

What were your main results?

The results show that the maximum magnitude of basal melting is reached near the grounding zone and that melt rates decay rapidly towards the calving front. Figure 1 (right) shows that the spatial heterogeneity of basal melting is inferred from the magnitude of basal melting. We suggest that the sizeable lateral variability of melting is due to the presence of basal channels. Although the existence of the channels boost the melting, figure 2 shows that the magnitude of melting inside and outside the channels is in the same order. 

What are your goals for GROCE-2?

In the second phase of GROCE we will estimate the basal melt rate by means of a three-dimensional model, in order to consider the effect of vertical processes inside the melt water plume.