Impact of nonlinear waves on the dissipation of internal tidal energy at ashelf break

The vertical and temporal structure of the dissipation of turbulent kinetic energy within the internal tide at a location 5 km shoreward of the shelf break on the Malin Shelf has been determined using a combination of the fre e-falling light yo-yo profiler and acoustic doppler current profilers. Two distinct internal wave regimes were encountered: period I in which large-am plitude high-frequency nonlinear internal waves (NIWs) occurred (around nea p tides) and period II in which the internal wave spectral continuum was no t dominated by any particular frequency band (around spring tides). Empiric al orthogonal function analysis shows that for the low-frequency waves, 76% of the variance was described by mode 1, rising to 95% for the high-freque ncy waves. During period I the dissipation and vertical mixing were charact erized by the NIWs, and 70% of the dissipation occurred in the bottom bound ary layer. During period II the depth-integrated dissipation was more evenl y distributed throughout the tidal cycle, whereas vertical mixing was great ly enhanced during a single hour long episode of elevated thermocline dissi pation coincident with weakened stratification. During both periods I and I I similar to 30% of the total measured dissipation occurred within the ther mocline when averaged over 12.4 hours; the remainder occurred within the bo ttom boundary layer(BBL). Tidal average values for depth-integrated dissipa tion and vertical eddy diffusivity for period I (II) were 1.1 x 10(-2) W m( -2) (4.0 x 10(-2) W m(-2)) and 5 cm(2) s(-1) (12 cm(2) s(-1)), respectively . Decay rates and internal damping are discussed, and vertical heat fluxes are estimated. Observed dissipation rates are compared with a simple model for BBL dissipation.