Resistance to Barotropic Tidal Flow in Straits by Baroclinic Wave Drag

Abstract

Energy transfer from barotropic tides to baroclinic motions may take place at the ends of straits connecting stratified basins, implying generation of internal waves propagating into the basins. Different aspects of this have been described in the literature, including quantification of the energy transfer, studies of the resulting internal tides, and the relationship to diapycnal mixing in the basins. However, the accompanying resistance to oscillatory barotropic strait flows, by so-called baroclinic wave drag, has not been implemented earlier in models for strait flow. In the present paper a formulation for the instantaneous baroclinic wave drag force is suggested. This leads to a linear relationship between the strait flow and the synoptic sea level difference between the adjacent basins. The model is used to study tidal response in land-locked basins. The response is computed for three fjords where the resistance to the barotropic flow comes essentially from baroclinic wave drag. It is concluded that the strait-flow model handles baroclinic wave drag in a satisfactory way since the observed responses are well predicted by the model. Baroclinic wave drag should provide the major resistance to surface tides in the deep ocean, which indicates its importance in a wider context.