The vertical displacement at the interface of a two-layer density system, den- 

 sities p and p' is given by 



Tj =: a cos (kx — a-t) 



The horizontal velocity of flow, u' in the upper layer is 



u' = — (a/h') c cos {kx — at) 

 where 



h' = average thickness of the upper layer 

 a = amplitude of wave at interface 

 c = wave velocity at interface 



The circulation of a simple progressive wave is shown in figure 19. The fine 

 arrows represent the trajectories of particles. 



Figure 19. Simple progressive internal wave between water of two densities p and p'. The 

 large arrow at the top shows the direction of the wave. The water motion trajectories are 

 shown by the small arrows and the normal location of the slick by the heavy bar. 



The significant motion is a surface convergence over the trailing slope of the 

 internal wave. Although the maximum expansion of the surface layer is over the 

 trough, the slicks are normally found at the active convergence zone. 



Wind Effect 



Since the wind in most cases was westerly, it is possible that the slick film was 

 displaced in its direction, that is, away from the trough towards the crest. On one 

 day, 26 June, the wind was from 205 to 250 degrees or southwesterly, and the 

 slicks did not bear a consistent relation to the waves. However, wind speeds of 

 2 to 5 miles per hour were required to make the slicks stand out in contrast to 

 the rougher adjacent water. 



In the southern California area, the winds during the day are predominantly 

 from the sea towards shore, that is, in the same direction as the internal wave; 

 therefore, some consistency can be expected whether or not the slick is set shore- 

 ward by the wind. 



26 



