Dependence of Pine Island Glacier Ice Shelf Basal Melt Rates on Subgrid-Scale Parameterizations of Mixing

Pine Island Glacier Ice Shelf (PIGIS) is melting rapidly from beneath due to the circulation of relatively warm water under the ice shelf, driven primarily by buoyancy of the meltwater plume. Basal melt rates predicted by ocean models with thermodynamically active ice shelves depend on the representation of environmental characteristics including geometry (grounding line location, ice draft and seabed bathymetry) and ocean hydrographic conditions, and subgrid-scale parameterizations. We developed a relatively high resolution (lateral grid spacing of 0.5 km, 24 terrain following levels) model for the PIGIS vicinity based on the Regional Ocean Modeling System (ROMS). Initial stratification was specified with idealized profiles based on observed hydrographic data seaward of the ice front. Predicted basal melt rate distributions were compared with satellite-derived estimates and stratification beneath PIGIS was compared with Autosub profiles. As in previous studies, we found that the melt rate was strongly dependent on the (specified) depth of the thermocline separating cold surface waters from deep, relatively warm waters, and on the presence of a submarine ridge under the ice shelf that impedes circulation of warm deep water into the back portion of the cavity. Melt rates were sensitive to the model's subgrid-scale parameterizations. The quadratic drag coefficient, which parameterizes roughness of the ice shelf base, had a substantial effect on the melt rate through its role in the three-equation formulation for ice-ocean buoyancy exchange. Turbulent tracer diffusion, which was parameterized by a constant value or various mixed layer models, played an important role in determining stratification in the cavity. Numerical diffusion became significant in some cases. We conclude that flow of warm water into the inner portion of the PIGIS cavity near the deep grounding line is sensitive to poorly constrained mixing parameterizations, both at the ice base and as a mechanism for allowing inflowing ocean heat to cross the sub-ice-shelf sill. Improved understanding of mixing processes is required as the community moves towards fully coupled ocean/ice-sheet models with evolving ice thickness and grounding lines.