Baroclinic Wave Drag and Barotropic to Baroclinic Energy Transfer at Sills as Evidenced by Tidal Retardation, Seiche Damping and Diapycnal Mixing in Fjords

Fluctuating barotropic flow over sills in stratified water is subjected to baroclinic wave drag and accompanying barotropic to baroclinic energy transfer. In fjords, this process has notable consequences, for instance, tidal retardation across fjord sills, damping of barotropic seiches and enhanced diapycnal mixing in the basin water. These may be computed from simple models only including local conditions in the sill area. This has been done for numerous fjords with quite satisfactory results as reviewed in this paper. It is therefore concluded that baroclinic wave drag and barotropic to baroclinic energy transfer only depend upon local conditions in the sill area. Energy transferred from the barotropic tide to baroclinic motions at sills is eventually dissipated by turbulence and diapycnal mixing. If radiated away from the sill by progressive internal waves, the energy may be transferred to turbulence in places remote from the sill. Radiation of baroclinic energy from fjord sills, however, is often less than the energy gain by baroclinic wave drag, so there also seems to be a possibility for a short spatial pathway to dissipation and mixing close to the sill. It is suggested in this paper that the magnitude of the radiated fraction is regulated by the capacity of the fjord basin to dissipate internal waves. In a fjord with small capacity to dissipate internal waves, much of the baroclinic energy gained at the sill has to dissipate close to the sill. The power for diapycnal mixing in most fjord basins is mainly derived from barotropic tides by baroclinic wave drag at sills. Diapycnal mixing in the abyssal ocean is probably driven in a similar way by baroclinic wave drag at bumps, sills, and ridges on the sea bottom. By analogy with the case in fjords, the energy transfer should be computed using local models. Furthermore, some of the transferred energy probably dissipates close to the topography and the rest is radiated away by internal waves and dissipated in remote places. The total mixing should thus be greater than the mixing related to the field of internal waves sustained by radiation from the topography. Estimates of the total diapycnal mixing in the abyssal ocean should therefore be based on observations covering the whole water column down to the seabed as also suggested by recently published dissipation profiles from the ocean.