waves constantly tends toward a velocity minimum. Sound 

 energy concentration in the sound channel is therefore en- 

 gendered by a retaining process. 



Another channel for horizontal energy transmission 

 exists in the ocean. The identity of this channel involves 

 transverse waves and their frequency maxima. These are 

 internal waves the amplitude of which is a function of 

 depth and the maximum frequency of which is the Vaisala 

 frequency (also a function of depth). 



The significance of the Vaisala, or stability, fre- 

 quency (usually designated by N) can be illustrated by con- 

 sidering a small particle of fluid. A bit of fluid adiabati- 

 cally displaced from its zero-order position and allowed to 

 move freely, will oscillate at the Vaisala frequency pro- 

 vided entropy remains constant. The bit responds to a 

 balance between buoyant and inertial forces. Pressure 

 inside the bit becomes that of the fluid outside. When the 

 magnitudes of density-change inside and outside the bit are 

 equal, it remains in its displaced position. N is zero in 

 this case and stability is neutral. When density changes 

 inside are larger in magnitude than those outside, the bit 

 continues its displacement without oscillating, and the fluid 

 is unstable. The fluid is stably stratified when density 

 change outside is greater than that inside, and the particle 

 will oscillate about its equilibrium position after initial 

 displacement. Application of the stability frequency to 

 internal wave motion is immediately apparent. 



Internal perturbations do not progress as free waves 

 at frequencies above the stability frequency. This phe- 

 nomenon creates energy channels or frequency channels 

 for internal waves that are as well defined as the sound 

 channels in the ocean. 



Higher-frequency internal waves are limited to 

 regions near the maximum of N, but lower frequencies are 

 not excluded. Frequencies lower than the minimum of J7* 



* Minimum of N is used for convenience with the understanding 

 that a channel can be defined as the region between z and 

 z where N (z) =» N(zJ. 



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