ocean. Although observations of internal waves are gen- 

 erally lacking, studies of coherence by analysis of observed 

 data are perhaps the most important from the standpoint of 

 naval operations. 



Destruction 



The literature contains only one report that deals 

 directly with the destruction of internal waves. Instability 

 in a two-layer system depends on densities and velocities 

 of the layers and the wave number. In shallow water where 

 the thickness of both layers is small, instability depends 

 also on their thickness. The criterion for instability is 

 satisfied for values that are observed in the ocean and 

 internal breakers can happen according to this theory. 



Time-series observations of temperature at three 

 stations in shallow water off Mission Beach, California, 

 have been recorded for several years. In all these observa- 

 tions only one case has been observed that can be attributed 

 to instability of a progressive internal wave (fig. 3). The 

 instability in this case is more analogous to whitecaps of 

 wind waves than to plunging breakers at the shoreline. 

 Short-period changes in surf temperature can be detected 

 by an attentive swimmer. They might be caused by higher- 

 frequency internal waves such as that shown in figure 3. 



Viscosity attenuates motion, and it can be accounted 

 for with slight modification of the equations, but the effect 

 of viscosity seems to be small in the ocean. 



15 



