Figure 2. An example of the mode structure of 

 internal waves. Subscripts are the mode numbers, 



known to exist. No theory accounts for the interaction be- 

 tween various motions and internal waves, or indeed be- 

 tween internal waves of different frequencies and modes. 

 Linear superposition of different kinds of motion, including 

 internal waves, is probably unrealistic. 



Theoretical maximum and minimum frequencies for 

 free internal waves in the ocean have been established 

 within the last two decades. The theoretical maximum 

 frequency is known by various names, the most common 

 being the Vaisala frequency. The theoretical low limits 

 are defined by the Coriolis parameter. The Vaisala fre- 

 quency is expressed in terms of density, density gradient, 



13 



