Corrections are computed in Table h for various salinity differences 

 for given mean temperatures in the thermocline. The above rules apply to 

 the correction values taken from the table. For example, with mean tempera- 

 ture of 55° F and salinity difference of 0.h2%o±n the thermocline the gen- 

 eral correction is 3°. If salinity decreases with depth ^° must be sub- 

 tracted from Tq-T^qo " fco obtain the corrected value of A~t; if salinity 

 increases 2° must be added to Tq-T^qq. 



Figure 27 shows the frequency distribution of At' in the North Equa- 

 torial Current, the Gulf Stream, and North Atlantic Current. Only 2 of 

 the 36 observations showed negative salinity gradients. The remainder, 

 which showed positive gradients, cover an extremely wide range of values. 

 Analysis indicates that conclusions concerning stability in the currents 

 based on temperature gradient alone have little meaning. For example, if 

 Tq-T^oo " 10 ° F and At' ■ 9° the actual stability index is 18° F, under 

 constant salinity conditions. Therefore, the k(^) curves in Figures 15 

 through 22 cannot be applied for prediction of thermocline thickness in 

 currents ,when salinity distribution in the thermocline is not known. 



Excluding areas of permanent currents, salinity gradient corrections 

 are required where salinity of surface water is increased by high evapora- 

 tion during the time of the seasonal thermocline. In the Mediterranean Sea 

 only k out of 25 observations showed a positive salinity gradient; the 

 remainder showed negative gradients. Stability seems to be lower in the 

 Mediterranean Sea for more time than the mean temperature difference 

 (T -T4oo) indicates. This is also probably true in all subtropical areas 

 of the North Atlantic. 



Weak Thermocline 



Within the mixed layer a small thermocline, termed "weak thermocline," 

 appears very often during decay of the mixed layer, when surface tempera- 

 ture rises and surface conditions supply only a small amount of mixing 

 energy. There is probably no essential difference in formation of the 

 weak thermocline, whether surface temperature increases are caused by advec- 

 tion or by other heating processes. 



Temperature difference in the weak thermocline seldom exceeds 2° F; 

 however, resulting stability is considerably stronger than normally ex- 

 pected. Figure 28 shows a plot of 21 k values determined for a weak thermo- 

 cline by Equation (6). The normal (central) curve for the At interval of 

 11° F (Figure 19) has been fitted to the points to indicate how well their 

 distribution agrees with the curve. In view of the small temperature dif- 

 ference in the weak thermocline, the point distribution would be expected 

 to correspond more closely to the normal k(^) curve for the At interval of 

 2° F (Figure 15) • Such a high stability index, with consequent strong 

 resistance to mixing by the weak thermocline, is difficult to explain. 

 One possible explanation is that salinity increases considerably in the 

 weak thermocline; however, the salinity increases could hardly account for 

 the increase of equivalent stability index by as much as 8° F. The steep 

 vertical temperat ire gradient in the weak thermocline may provide extra 

 stability until a critical level of mixing energy prevails. This critical 



57 



