A document by Erika Henell. Click on the document to view its contents.

For better projections of sea level rise, two things are needed: an improved understanding of the contributing processes and their accurate representation in climate models. A major process is basal melting of ice shelves and glacier tongues by the ocean, which reduces ice sheet stability and increases ice discharge into the ocean. We study marine melting of Greenland’s largest floating ice tongue, the 79° North Glacier, using a high-resolution, 2D-vertical ocean model. While our fjord model is idealized, the results agree with observations of melt rate and overturning strength. Our setup is the first application of adaptive vertical coordinates to an ice cavity. Their stratification-zooming allows a vertical resolution finer than 1 m in the entrainment layer of the meltwater plume, which is important for the plume development. We find that the plume development is dominated by entrainment only initially. In the stratified upper part of the cavity, the subglacial plume shows continuous detrainment. It reaches neutral buoyancy near 100 m depth, detaches from the ice, and transports meltwater out of the fjord. Melting almost stops there. In a sensitivity study, we show that the detachment depth depends primarily on stratification. Our results contribute to the understanding of ice–ocean interactions in glacier cavities. Furthermore, we suggest that our modeling approach with stratification-zooming coordinates will improve the representation of these interactions in global ocean models. Finally, our idealized model topography and forcing are close to a real fjord and completely defined analytically, making the setup an interesting reference case for future model developments.

Realistically approximating the basal melting of ice shelves is critical for reliable climate model projections and the process representations in ice-ocean interaction. In this regard, extensive research attributes the massive thinning of vulnerable ice shelves to basal melting enhancement driven by ocean water warming, focusing mainly on oceanic warm water intrusion into the sub-shelf basins. However, climate models mainly underestimated the impacts of probable small-scale processes at the ice-ocean interface on basal melting by using smooth ice base topographies. This paper provides new insights into how small-scale features on the ice-ocean interface contribute to basal melting enhancement and spatial distribution. We developed a time-dependent, two-dimensional ice-shelf plume model as an optimal tool that allows a high-resolution representation of basal topography and with the unique ability to provide valuable information from the mixed boundary layer between ocean and ice shelves. In an exemplary case study for the floating ice tongue of the 79◦ North Glacier, systematic sensitive analyses were performed with the developed model. Our results show that the sub-km-scale basal channels with realistic dimensions increase the mean basal melt rate and generate extreme and sizeable lateral variability of melting at the grounding line. This mechanism is not reproducible with the tuning of drag coefficient. Besides, it reveals that the subglacial discharge in the channels has contradicting effects of reducing the melt rate by refreshing the sea water and increasing the freezing point while increasing the melt rate due to high water speed. However, the latter was dominant in our experiments.