Modeling tidal current profiles and vertical mixing beneath Filchner-Ronne Ice Shelf, Antarctica
One of the warmest water masses beneath Filchner-Ronne Ice Shelf (FRIS) is dense, high salinity shelf water (HSSW) that flows into the sub-ice-shelf cavity from the ice front and occupies the lower portion of the water column. A one-dimensional turbulence closure ocean model has been applied to this high latitude sub-ice-shelf environment to demonstrate that tidal currents mix HSSW vertically through the water column and cause melting at the bottom of the ice shelf. Significantly FRIS lies near the critical latitude for the semidiurnal tide, where the Coriolis frequency equals the tidal frequency, resulting in a strongly depth-dependent tidal current and thick boundary layers. Using the model, the effect of the critical latitude, stratification, and the polarization of the tidal current ellipse on boundary layer structure and subsequent vertical mixing are examined. The model shows that stratification significantly affects how the shape of the tidal current ellipse varies with depth and that both the depth to which the pycnocline initially develops and the longer term melt rates are highly dependent on tidal current ellipse polarization. The sensitivity to both the stratification and the polarization are due, in large part, to the proximity of the critical latitude. Positive polarizations (anticlockwise rotating current vectors) quickly develop deeper pycnoclines and maintain higher melt rates than negative polarizations (clockwise rotating current vectors). For many areas beneath FRIS the polarization ranges from -0.3 to +0.3; here the modeled pycnocline development is sensitive to polarization, though the effect on the time-averaged melt rate is suppressed for positive polarizations. However, in key areas where the polarization exceeds +/-0.3 and the ellipses are more open and circular, the effects of polarization are significant. Levels of tidal mixing and associated melting vary by more than an order of magnitude over the whole tidal ellipse polarization range, showing that very different mixing and melting regimes are present beneath FRIS.