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

Abstract

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 free‐falling light yo‐yo profiler and acoustic doppler current profilers. Two distinct internal wave regimes were encountered: period I in which large‐amplitude high‐frequency nonlinear internal waves (NIWs) occurred (around neap tides) and period II in which the internal wave spectral continuum was not dominated by any particular frequency band (around spring tides). Empirical 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‐frequency waves. During period I the dissipation and vertical mixing were characterized by the NIWs, and 70% of the dissipation occurred in the bottom boundary layer. During period II the depth‐integrated dissipation was more evenly distributed throughout the tidal cycle, whereas vertical mixing was greatly enhanced during a single hour long episode of elevated thermocline dissipation coincident with weakened stratification. During both periods I and II ∼30% of the total measured dissipation occurred within the thermocline when averaged over 12.4 hours; the remainder occurred within the bottom boundary layer(BBL). Tidal average values for depth‐integrated dissipation and vertical eddy diffusivity for period I (II) were 1.1×10−2 W m−2 (4.0×10−2 W m−2) and 5 cm2 s−1 (12 cm2 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.