Internal tides, nonlinear internal wave trains, and mixing in the Faroe-Shetland Channel

A semidiurnal internal tide and trains of near-bed nonlinear internal waves were observed on the southeastern bank of the Faroe-Shetland Channel. The depth-integrated M2 internal tide energy flux was 140 W m-1 up-slope and 154 W m-1 along-slope to the southwest. The majority of the energy flux was contained within the main pycnocline, where the slope was supercritical. A numerical model of the M2 tide successfully reproduces the observed maxima in the pycnocline but overestimates depth-integrated baroclinic energy fluxes by 15%–45%. The model results suggest that the internal tide in the channel is generated at multiple sites, including the northwestern bank and the Wyville Thomson Ridge. On the northern flank of the ridge, modeled energy fluxes are over an order of magnitude larger than in the channel, >5 kW m-1. The turbulent kinetic energy dissipation rate inferred from the observed internal tide energy flux, by assuming that all the energy in the pycnocline was dissipated on the slope, was 1.3 × 10-7 W kg-1, a factor of 4 larger than that inferred from Thorpe scale analysis (3 × 10-8 W kg-1). This suggests that the high level of mixing on the slope can be accounted for by the internal tide, even if the majority of the energy was reflected. The nonlinear internal wave energy flux was up-slope and intermittent; peak energy fluxes reached 200 W m-1 but were typically of order 10 W m-1. The wave trains were likely tidally forced and may have been a nonlinear manifestation of the internal tide