Tidal currents and vertical mixing processes beneath Filchner-Ronne ice shelf

Abstract

Oceanographic measurements have been undertaken at sites on Filchner-Ronne Ice Shelf using hot-water drilled holes allowing conductivity-temperature-depth profiling and the deployment of instrument moorings. The data show that Western Shelf Water enters the sub-ice shelf cavity and occupies the lower portion of the water column. This is the water that provides the external heat necessary for melting within the sub-ice shelf cavity. A depth-averaged tidal model of the region has been used to show that in areas with shallow water and large topographic gradients, tidal oscillations with peak velocities up to 1 m s-1 play a significant role in the vertical mixing and transport of water masses. The estimated energy dissipation beneath Filchner-Ronne Ice Shelf resulting from surface friction is 25 GW, approximately 1% of the world’s total tidal dissipation. The model indicates that Lagrangian tidal residual currents have fluxes of up to 250,000 m3 s-1 and speeds of over 5 cm s-1 along the ice front, with over 350,000 m3 s-1 being exchanged between the sub-ice shelf cavity and adjacent continental shelf. These currents are particularly efficient in ventilating the sub-ice shelf cavity within 150 km of Ronne Ice Front. Furthermore, a one-dimensional turbulence closure ocean model has been applied to this sub-ice shelf environment which significantly lies near the critical latitude for the semi-diurnal tide. Here, the Coriolis frequency equals the tidal frequency, resulting in a strong depth dependent tidal current and thick boundary layers. Both the model and observations show that stratification significantly affects how the shape of the tidal current ellipse varies with depth. The model also shows that vertical mixing and basal melting are sensitive to tidal ellipse polarization with anticlockwise rotating tidal currents maintaining the highest melt rates. This sensitivity is due, in large part, to the proximity of the critical latitude.