Effects of subgrid-scale ice topography on the ice shelf basal melting simulated in NEMO-4.2.0

Abstract. At the interface between the ocean and the ice shelf base, in the framework of the shear-controlled melt parameterisation, the ice melts due to combined actions of temperature, salinity and friction velocity. In the NEMO ocean model, the friction velocity is usually computed based on a constant drag coefficient and an ocean velocity averaged vertically within a distance from the ice, which is often referred to as the Losch layer. Instead, in this study, we use a logarithmic approach, where a constant hydrographic roughness length detetermines the drag coefficient through the law of the wall and the horizontal current speed is sampled in the 1st wet cell. The aim is to reduce the vertical resolution dependency, to homogeneise the sampling of horizontal current speed between the thermodynamic and the dynamic drag equation and to enable the use of a variable drag coefficient based on the subgrid-scale (or unresolved) ice shelf basal topography. The motivation behind a variable drag based on the topography comes from observations showing that regions with rough topographic features such as crevasses or basal melt channels experience more melts than flat ones. We compare different experiments in a configuration of Amundsen Sea at 1/12°. We find that our approach is less sensitive (6 % melt rates difference) to a coarser vertical resolution, such as the one used in global Earth System Models, than the Losch layer approach (22 % melt rates difference). We also find that it succeeds in reproducing higher melt rates in rougher regions while keeping total ice shelf melt rate within the observation range. Finally, to assess the effect of increasing ice shelf damage, we tested the sensitivity of a higher hydrographic roughness length. If the roughness of all the ice shelf grid points were to increase to the highest value currently observed, the overall ice shelf melting would increase by 16 %. This suggests the possibility of a positive feedback in which more melting leads to more ice damage and increased roughness, in turn increasing melt rates.