Quantifying the bias in estimates of the baroclinic energy flux in shelf seas

Internal tides in shelf seas exhibit temporal and spatial variability that resist prediction. In the Celtic Sea, estimates of baroclinic energy flux across the shelf vary by more than an order of magnitude. A simple model of an interaction between a barotropic and baroclinic tide suggests an explanation. Advection of the baroclinic tide by the barotropic tide introduces spatial variability into observed mean water column properties and observed internal wave properties and shifts energy from fundamental tidal frequencies into higher harmonics. Two parameters govern the strength of the interaction: the ratio of the tidal excursion length scale to the baroclinic wavelength, and the phase offset between the barotropic and baroclinic tides. In shelf seas, where the effect is greatest, standard approaches to tidal analysis are likely to underestimate the magnitude of the internal tide. This may also be a source of error for energy dissipation values inferred from the divergence of baroclinic energy fluxes.