gravity, and sound velocity in the medium. The maximum 

 frequency is consequently a function of depth. Its maximum 

 occurs in the thermocline, and its lowest value at the depth 

 where density gradient is lowest. Internal perturbations 

 will not progress as a gravity wave at frequencies above 

 the Vaisala frequency. Any internal oscillations at fre- 

 quencies below the theoretical lowest frequency cannot 

 exist according to the theory as free internal waves. 



The phase and group velocity of internal waves is 

 higher for lower-frequency waves. For any given frequency, 

 first-mode internal waves have the highest phase velocity. 

 The phase velocity of the second mode is approximately 

 one-half the phase velocity of the first. The third-mode 

 internal wave progresses at still lower phase velocity, and 

 the phase velocity continues to decrease with higher mode. 

 The mode theory of internal waves indicates that they are 

 highly dispersive waves. Ocean currents modify phase 

 velocity, but have little effect on the modal structure. 



Only three unaliased spectra on internal temperature 

 oscillations have been published. A continuous, monoton- 

 ically decreasing spectrum seems to exist between the 

 minimum and the maximum frequencies, but the relative 

 proportions of variance per bandwidth contributed by 

 internal waves and less regular motion remain unknown. 

 This is an important point that should be emphasized for 

 the benefit of naval operations, because wave motion is 

 easily predictable compared to nonperiodic or transient 

 phenomena. 



The effect of rotation of the earth is to increase the 

 amplitude of waves in a direction to the right of the direction 

 of progress, in the northern hemisphere. This effect is 

 apparently unimportant with reference to free internal 

 waves. Rotation also increases the phase velocity com- 

 pared to the nonrotating case. 



Present-day theory on existence of internal plane 

 waves, as opposed to origin or destruction, implies perfect 

 coherence of plane waves over space. According to this 

 theory, an internal wave originating on the coast of Japan 

 travels all the way across the ocean undistorted. If this 

 is true, changes in temperature structure caused by internal 

 waves should be easily predictable for any point in the 



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