INTERNAL WAVES 



L. H. Larsen, M. Rattray, Jr., W. Barbee, J. G. Dworski 

 Department of Oceanography, University of Washington 



ABSTRACT 



The interaction of tides and the continental shelf break is such 

 that energy can be put into internal gravity waves. The amount of 

 energy depends on the change in depth across the shelf break; larger 

 amplitude and more confined internal waves occur when the depth change 

 is large. A further dependence on the width of the shelf region is 

 noted. The stability of the ocean is such that waves at the semidiurnal 

 period and longer periods may be generated at most of the world's 

 continental shelf breaks. Because, at the tidal frequencies, frictional 

 damping is small, most of the internal -wave energy reaches the sea floor 

 where it may be reflected upward. A beam in which the energy of the 

 semidiurnal wave motion generated over the shelf break is concentrated 

 may be reasonably expected to reach the sea floor some 25 to 75 km sea- 

 ward of the shelf break. 



Studies conducted at the University of Washington have provided an 

 analytic model of the phenomenon and a laboratory demonstration, and 

 have obtained data from the North Pacific. In this presentation a 

 short film will be shown illustrating the generation of internal tides 

 in a laboratory model, and the results of measurements taken in the 

 North Pacific in September 1968 will be discussed. 



Analytic studies that have been carried out by the authors will 

 be set forth insofar as they provide insight into the properties of 

 internal gravity waves. One feature of particular importance is the 

 occurrence of a very complex modal structure over the shelf regions of 

 the ocean. 



INTRODUCTION 



In a recent publication Rattray et at. (1969) discuss the generation 

 of internal tides at the edges of the continental shelves by means of 

 bathymetric coupling with the surface tides. In the model the ocean 

 bathymetry is approximated by a shelf of uniform depth and given width 

 and a deep ocean of uniform depth. At the break of the shelf the surface 

 tide generates two internal wave systems: a standing internal wave 

 system over the shelf and a propagating wave system seaward of the shelf 

 break. 



Internal waves have orthogonal group and phase velocities aligned 

 so that the sum of the group and phase velocity vectors lies in a plane 

 perpendicular to the gravitational field. The angle of phase velocity 

 to the plane perpendicular to the gravitational field is 



390 



