nonhomogeneous media, but adequate observations to 

 support or discredit any theory are virtually nonexistent. 



Existence 



The simplest theory of existence of internal waves 

 is the two-layer theory. This theory assumes a medium 

 composed of two layers of different density. It has become 

 clear that two-layer theory is not adequate to explain ob- 

 servations in the ocean except in a few special places. 

 The special cases are largely confined to northern latitudes 

 where fresh water overlies salt water. The Baltic Sea is a 

 good example, but even in this instance success of the 

 theory is limited. 



A more complicated theory, which deals with 

 existence of internal waves in a medium with continuous 

 density distribution, is more realistic. This idea shows 

 that it is mathematically possible for an infinite number of 

 modes of internal waves to exist at any one frequency. 



Figure 2 shows the modal distribution computed 

 from data taken at a point off the San Diego coast. The 

 abscissa can be regarded as amplitude in arbitrary units; 

 M , M , etc., are the various modes. The subscripts 

 indicate the number of the mode. The first-mode wave has 

 a single amplitude maximum, the second mode has two 

 amplitude maxima, and so on. It would be expected that 

 the lower modes in the ocean would be dominant and that 

 the higher modes would be less common because more 

 shear is involved. This theory shows that an infinite num- 

 ber of modes are possible, but does not indicate the am- 

 plitudes of the modes. Amplitudes can be established only 

 by computing from observation. Dominant modes are im- 

 portant and should be established, because they probably 

 affect the coherence of waves over space. 



Both two-layer theory and continuous -density theory 

 are usually simplified by the assumptions that mean motion 

 is constant from surface to bottom or nonexistent, and that 

 the bottom is a plane; both assumptions are unrealistic. 

 Turbulence, various kinds of surface waves, currents with 

 vertical and horizontal gradients, and other motions are 



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