Physics Colloquium - Monday, Feb. 2nd, 2009, 4:00 P.M.

Hepeng Zhang
Center for Nonlinear Dynamics
University of Texas at Austin

Away from shallow, well-mixed surface regions, the density of sea water increases with depth due to variation in salinity and temperature. This continuous density stratification supports internal gravity waves, which are the counterpart within the fluid interior of surface gravity waves. Internal gravity waves are important for many oceanic processes, such as sediment transportation and ocean mixing.

We study internal wave generation in a laboratory model of oscillating tidal flow on a continental margin. Waves are found to be generated only in a near-critical region where the slope of the bottom topography matches that of internal waves. Fluid motion with a velocity an order of magnitude larger than that of the forcing occurs within a thin boundary layer above the bottom surface. The resonant wave is unstable because of strong shear; Kelvin-Helmholtz billows precede wave breaking. We construct a model to extrapolate our results to oceanic conditions. Our work provides a new explanation for the intense boundary flows frequently observed on continental slopes, which may play a role in shaping global continental shelf.