Sub-Ice Platelet Layer Physics: Insights From a Mushy-Layer Sea Ice Model

The sub-ice platelet layer (SIPL) is a highly porous, isothermal, friable layer of ice crystals and saltwater, that can develop to several meters in thickness under consolidated sea ice near Antarctic ice shelves. While the SIPL has been comprehensively described, details of its physics are rather poorly understood. In this contribution we describe the halo-thermodynamic mechanisms driving the development and stability of the SIPL in mushy-layer sea ice model simulations, forced by thermal atmospheric and oceanic conditions in McMurdo Sound, Ross Sea, Antarctica. The novelty of these simulations is that they predict a realistic model analogue for the SIPL. Two aspects of the model are essential: (a) a large initial brine fraction is imposed on newly forming ice, and (b) brine rejection via advective desalination. The SIPL appears once conductive heat fluxes become insufficient to remove latent heat required to freeze the highly porous new ice. Favorable conditions for SIPL formation include cold air, supercooled waters, and consolidated ice and snow that are thick enough to provide sufficient thermal insulation. Thermohaline properties resulting from large liquid fractions stabilize the SIPL, in particular a low thermal diffusivity. Intense convection within the isothermal SIPL generates the SIPL-consolidated ice contrast without transporting heat. Using standard physical constants and free parameters, the model successfully predicts the SIPL and consolidated ice thicknesses at six locations. While most simulations were performed with 50 layers, an SIPL emerged with moderate accuracy in thickness for three layers proving a low-cost representation of the SIPL in large-scale climate models.