Ice Shelf Oceanography Program
Glacial Cavity Circulation and Ocean-Ice Interaction
Basal melting of floating ice shelves involves a complex interplay between ice and seawater, where the transfer of heat at the ice/water interface depends on both the departure of sea water from its freezing temperature (liquidus) and the fluid stress. The former depends on melt salinity and pressure, while the latter represents a combination of tides, local buoyancy forcing, and background general circulation. Direct measurements of ocean properties from beneath floating ice shelves are rare, and in terms of the exchanges within the ice/ocean boundary layer (IOBL) beneath thick glacial ice, data are virtually nonexistent. This project is a unique experiment designed to understand the turbulent exchange of heat and salt through the glacial IOBL that will be directly applied to modeling the dynamic evolution of the PIG.
The picture of sub-ice shelf circulation that emerges from measurements of ocean properties near ice-shelf fronts combined with numerical modeling is that relatively warm seawater flows into the cavity at depth, eventually reaching the grounding line where it encounters glacial ice. Melting occurs, and because the melt is relatively fresh, buoyant forcing accelerates flow up the basal slope toward the ice shelf front. The combination of stress and buoyancy flux (controlled mainly by salt at low temperatures) will determine the depth of the developing IOBL, which cools significantly from basal melting as it flows up under the glacier toward the front, because the density increase at the base of the fresh, buoayant layer severely inhibits heat conduction from the underlying warm water.
In contrast to ocean/glacier exchange, there is ample guidance on ice/ocean exchange in a sea-ice environment. An important result from those studies is that, particularly during rapid melt, the process is inherently double-diffusive. Because the molecular diffusivity of salt in seawater is roughly 200 times smaller than thermal diffusivity, it is salt that controls melting and, as the melt rate increases, this control strengthens. Although current ice-shelf/ocean models use parameterizations based on sea ice studies, we recognize that the sub-shelf ice/water interface differs from sea ice in important ways. First, the glacial ice itself is fresh and has been subjected to high pressures, thus its structure may differ substantially from relatively porous and brackish sea ice. Second, freezing point depression with increasing pressure means that for a given ocean temperature the driving thermal forcing increases substantially at basal depths.
The sub-ice-shelf cavity observational program will make unprecedented multi-year measurements of basal melt rate, turbulence in the IOBL, and ocean temperature/salinity and current structure across the full sub-ice cavity. Our strategy is evolutionary: in the first year we will develop the miniaturized Small-hole Ocean Flux Profiler (SOFP, described below), and test it in a polar sea-ice environment. In the 2nd year, we will deploy the first profiler at the PIG site to be determined from the first-year reconnaissance. ŇOne-dimensionalÓ data from this system will reveal fundamental structure of the sub-shelf environment, and will be combined with our models to design the optimal configuration of a triangular array to be centered on our initial profiler. The final array will thus include four profilers, arranged to provide vertical flux (momentum, heat, and salt) and basal elevation data at each site, as well as horizontal gradients of advective heat and salt flux at the central location. Given these measurements, we will be able to accurately parameterize exchange at the ice-ocean interface and entrainment fluxes at the base of the IOBL. In addition, the sampling strategy will provide a direct 'ground-truth's target against which sophisticated, 3-D models of the sub-shelf circulation and shelf-ocean interaction can be gauged.