In general, data concerning salinity distribution in the thermocline are 

 extremely scarce. Several random samples of salinity distribution in the 

 thermocline in the area around OWS CHARLIE show that the salinity gradient 

 is rather small, and that it probably does not vary much with time. As a 

 first step in this study, only the temperature gradient was considered to 

 be a stability factor ; salinity was assumed to be constant. Neglect of 

 salinity variation in time and space involves certain error; however, 

 other difficulties are also inherent to this problem. 



Temperature gradients obtained from BT data are only instantaneous 

 values, whereas considerable continuous variation of this gradient occurs 

 in the thermocline. Variation of the gradient is dependent mainly on 

 internal waves at the top and bottom of the thermocline, so that only a 

 mean value of the gradient can be practically applied for a given location 

 and for a given time period. Variance of several monthly means depends 

 on properties of internal waves which are more elusive and more difficult 

 to determine, rather than on the actual temperature distribution in the 

 thermocline. The mean value for a given location and for a given month 

 in one year can be quite different from that for the same location and 

 the same month in another year, because amplitudes and phases of internal 

 waves at the upper and lower boundaries of the thermocline are different 

 (Figures 2, 5> and 6). 



On the assumption that the vertical temperature difference is propor- 

 tional to the mean temperature gradient in the upper part of the thermo- 

 cline, the difference in the entire thermocline was adopted as the index 

 of stability instead of the temperature gradient in the upper part of the 

 thermocline. Since the temperature gradients in the mixed layer and below 

 the thermocline are small, computation of the temperature difference 

 between the surface and a level below the seasonal thermocline is more 

 convenient. The 500-foot level would probably be most convenient. BT's 

 usually reach a depth of only U00 feet; therefore, values at this level 

 were used for computing At = (T -T^qq). Mean temperature difference is 

 more easily determined than the mean temperature gradient in the thermo- 

 cline and apparently reflects more stability with respect to yearly oscil- 

 lations. 



Figure 13 shows frequency distributions, means, and standard deviations 

 of (1) temperature difference (At) between the surface and ^00 feet, (2) 

 temperature at ^00 feet, and (3) temperature gradient (G^.) per foot in the 

 thermocline computed from observations made at OWS CHARLIE between 7 a nd - 22 

 September i960. BT's were made every half hour when possible. Owing to 

 operational requirements of the observing ship, observations were inter- 

 rupted for one or more hours a few times each day; there were also a few 

 longer interruptions due to heavy storms. 



Figure ik shows frequency distributions and means of temperature dif- 

 ference (At) between the surface and ij-00 feet and temperature gradient per 

 foot in the thermocline computed from BT's taken every 5 minutes for eight 

 one-hour periods during the same period mentioned above. These observations 

 were not used in Figure 13. 



26 



