Tidal dynamics and mixing over steep topography

Abstract

Coastal currents and water column stratification frequently are determined by local phenomena that interact to produce complex dynamics. A process study was designed to assess the role of tides over a coastal shelf break as a local source of internal waves spatially-inhomogeneous, baroclinic residual currents, and internal mixing. The temporal evolution of three dimensional stratification and currents were characterized within a 4km x 4km box close to the mouth of Monterey Canyon, California. Strong, baroclinic, unidirectional, spatially inhomogeneous jet-like currents were observed with 24 hour periods, together with the occurrence of semidiurnal thermocline oscillations and vertically mixed layers. The linear theory of Baines (1982) predicts the existence of significant tidally forced internal waves in the area, with an interfacial mode at the thermocline decoupled from the internal waves propagating in the weaker stratification below. A ray tracing method is developed to study the nonlinear propagation of these tidally forced internal waves by considering wave advection and straining by the topographically controlled barotropic tidal flow while neglecting internal wave-wave interactions. It is found that nonlinear interaction of tidally forced internal waves with the topographically controlled barotropic tidal flow is mechanism capable of forcing residual currents and internal wave shear instabilities that are capable of creating internal mixed layers similar to those observed at the mouth of Monterey Canyon.