To relate the temperature variability to the associated salinity-depth 

 structure, the T-S curves of hydrographic stations 13 and 14 are shown in 

 Figure 10, enclosed by the T-S envelope of the data for the 23 stations 

 taken during the expedition (See statistical summary. Table 9). 



The sound velocity-depth curves based on station 13, 0800 September 17, 

 and station 14, 1522 September 28, are given in Figure 11. The sound veloc- 

 ity inversion which can be seen on the station 13 curve results from the 

 temperature inversion in the halocline shown in Figure 10 . 



The sound velocity inversion is sufficient to produce a secondary sound 

 channel at this level (Fig 12) . The sound channel is approximately 40 m in 

 thickness , 



Computations based on the sound velocity structure when no inversion 

 existed (Station 14) were as follows: With a source depth of 100 m, the 

 maximum range of the limiting ray at 100 m was 1 nm. The axis of the per- 

 manent sound channel was at 600 m and the convergence zone was at 25.6 nm. 



The intermittent existence of the secondary sound channel in this bound- 

 ary region appears to be associated with changes in the vertical structure 

 due to internal waves . Use can be made of such a secondary sound channel 

 because its existence can be inferred from synoptic BT observations . 



SUMMARY 



The analysis of the oceanographic data collected by FLIP and the R .V . 

 HORIZON during September 1963 indicates: (l) that the variability in the 

 temperature-depth structure beneath the isothermal layer is primarily 

 dependent upon internal waves; (2) the variation in depth of the top of the 

 thermocline was as great during a 12-hour period at the time of the full 

 moon as over the period of 26 days, and was almost as large in any given 12- 

 hour period; (3) the temperature in the isothermal layer decreased from 21 

 to 19.8° during the observational period. Although the range of the depth 

 oscillations in the thermocline was almost the same on the first day as the 

 last, the mean depth of the thermocline increased from 65 to 110 feet (20- 

 34 m) during the same period. The decrease of temperature in the isothermal 

 layer and the increase in mean depth of the thermocline are in agreement 

 with the expected seasonal changes; and (4) the basic sound velocity-depth 

 structure remained essentially the same throughout the period, excepting, of 

 course, the intermittent inversion near 300 ft. (91 m) . 



CONCLUSION 



These results have important implications for Navy operations. The 

 variability in the temperature-depth structure associated with internal 

 waves will always be the limiting factor in prediction. It is the environ- 

 mental condition which must be lived with. However, if the envelope of 



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