Insights into ocean mixing processes driving ice shelf melting from highly resolved simulations

The melting of ice shelves into the ocean plays a major role, and is a key source of uncertainty, on sea level rise projections. Under ice shelves, meltwater moves upslope, setting up a stratified shear flow with the warmer, but critically saltier, ocean beneath, regulating the transfer of heat between ice and the ambient ocean. These stratified flows are incredibly difficult to access in situ, and so research has focussed on the use of numerical modelling, laboratory experiments and analytical models to understand these processes, each with their own assumptions, advantages and limitations.

Here we present novel direct numerical simulations that represent this basal melt process with highly resolved (sub-millimeter resolution) simulations that reveal this shear flow evolves as a unique mixed mode shear instability. Unusually, the combined input of buoyancy, and a solid boundary leads to paired Kelvin Helmholtz and Holmboe instabilities, which prove highly effective at mixing the water column. Due to the restoring effect of the boundary forcing, a cycle of growth, instability, turbulent mixing, and re-stabilisation is observed. Results for a non-rotating framework with equal diffusivities for heat and salt are contrasted to simulations with added complexity, including rotation and double-diffusive processes. These findings suggest that current ice-ocean parametrisations are fundamentally built on assumptions for stratified flow instabilities that may differ from these simulations, with potential implications for the turbulent transfer of heat and salt and ultimately basal melt rates.