surface, and the internal waves are moving in unknown directions, 

 it is difficult to establish definite structures of the thermocline or 

 its movements. However, some qualitative assessment of the 

 dominant direction of propagation can be made. 



It was indicated earlier that the slopes of the isothermal 

 surfaces might assist in determining the direction of internal- 

 wave motion. In addition, the autocorrelation and power spectrum 

 indicate a shoreward direction. Furthermore, a doppler effect 

 may be obtained by towing in different directions. 



If the ship with the chain is anchored, the wave trains will 

 move past and be recorded in accordance with their true frequen- 

 cies. If the chain is towed in a direction parallel to the "crests" 

 of long-crested waves, that is 90 or 270° to their direction of 

 propagation, the true frequency should again be recorded. How- 

 ever, if the chain is towed in any other direction relative to the 

 waves, their true frequencies will not be recorded. In other 

 words, the recorded frequency of waves will depend on the rela- 

 tive directions of ship and waves and their relative speeds. 



If some arbitrary speed is assumed for the waves, such as 

 1.5 knots, and the ship's speed is 6 knots (fig. 22), a frequency 

 factor can be computed for all relative directions of ship and 

 waves (fig. 23). Under these conditions the frequency factor 

 should be three times normal when the chain is towed in a direc- 

 tion with waves and five times when being towed counter to the 

 waves. When a towing is conducted in a relative direction of 

 75°31' and 284°29', no waves should be encountered if the fronts 

 are long and parallel, and if the waves are of constant period. 

 Unfortunately the waves are not so uniform, but it is believed that 

 there may be dominant waves that would be revealed by the doppler 

 effect. Thus, to test the directionality of internal waves, an ex- 

 periment was arranged (at position E, fig. 2, near 30°19'N, 

 120° 03 'W) whereby the direction of tow was changed 6 degrees 

 every minute. In this way the ship made a complete 2-mile- 

 diameter circle in 1 hour. Four such circles were made; the re- 

 corded isotherms are presented in figure 24. In addition, two 

 larger circles of 6-mile diameter were traversed in a similar 

 fashion; the resulting data are presented in figure 25. 



36 



