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