may be deeper than indicated by use of the initial stability index. If 

 the surface temperature can be measured during the mixing period, a A"t 

 value can be determined for use at the end of active mixing. The fore- 

 going statements apply to all h curves. 



Salinity Gradient 



The surface and the '400-foot depths have been discussed as being the 

 most convenient limiting levels for determination of the temperature dif- 

 ference between the surface and a level below the thermocline. A somewhat 

 deeper level would be more convenient, because the thermocline sometimes 

 extends below ^00 feet. This temperature difference is used instead of 

 the temperature gradient in the thermocline to represent stability; however, 

 stability is a function of both temperature and salinity gradients. Inter- 

 nal waves and possibly other factors cause fluctuation of these gradients. 



These fluctuations justify substitution of At for the mean temperature 

 gradient in the thermocline without any great loss of accuracy, because 

 At is considered proportional tc; the actual temperature gradient. When 

 At remains nearly constant, the temperature gradient in the thermocline 

 changes with decreasing thermocline and increasing mixed-layer thicknesses; 

 however, change of the temperature gradient is incorporated in the function 

 k(7;), and At efficiently substitutes for the actual temperature gradient 

 in the thermocline . 



Effect of the salinity gradient on stability has not been considered 

 in calculations to this point. If the salinity gradient is positive, the 

 maximum value is usually located somewhere in the deeper part of the thermo- 

 cline, at a point below which salinity decreases slowly. The maximum value 

 sometimes occurs below the thermocline. If the salinity gradient is nega- 

 tive its value usually decreases through the depth of the thermocline. 



In order to simplify the stability factor, salinity differences in 

 the thermocline are expressed as temperature differences and are designated 

 At'. Thus At' is an equivalent temperature difference that would produce 

 a density change equal to that produced by a given salinity change in the 

 thermocline. Temperature differences equivalent to salinity differences 

 have been computed for a wide range of mean temperatures in the thermo- 

 cline. Pressure and mean salinity effects in the thermocline are esti- 

 mated to be negligible and have therefore been disregarded in this study. 

 Increasing salinity with depth produces positive equivalent temperature 

 changes (At* ), and decreasing salinity with- depth results in negative 

 values (At'"). 



The result of 2k Kansen casts made in the thermocline at station CHARLIE 

 in September i960 is shown in Figure 26A. A~t' is O.89 F. Data for sa- 

 linity gradients in the thermocline during other months at this location 

 were not available; however, an additional ^3 observations from the same 



53 



