TEMPER ATUREpF) 

 50 60 70 



u 1 



J 



1 



V . 



5*:: 















»«'" 



j* ^ 



r J 



[ 



St 



-. V 



P f 



h l 



uj 200 S 





"- £ 





x : t 



i_ 



£ ^ 













t 



t 



./ 



- - i 



t 



t .. 



















TEMPERATURE CF) 



50 60 70 



u 3 



1 



: J : 



j 



i 



r 



; t 



.. r 

























































Y 



t . 



r 



f 













i~i ' 



TEMPERATURE (°F) 



50 60 70 









































w 9n r> J- 



















o It X 



































FIGURE 7 INSTABILITY MIXING DUE TO ADVECTION 



overlies a warm water intrusion at depths between 30 and 110 feet (Figure 

 7-A). Three and one-half hours later (Figure 7B) the mixed layer increased 

 55 feet in thickness. Winds of Beaufort force h to 5 and sea state k 

 could perform slower mixing to approximately the same depth. During the 

 period of mixing more warm water intruded at depths between 110 and 300 

 feet, while a tongue of cold water intruded between 300 and 420 feet 

 (Figure 7B). Figure 7C shows the effect of 6 additional hours of insta- 

 bility mixing; the mixed-layer depth has increased to l6o feet. 



Prediction of thermocline depths could be made under these conditions 

 only when the motion and properties of the water masses are known. Detailed 

 observations for several days in the prediction area would be necessary to 

 permit a prediction to be made only a few days in advance. 



Instability Mixing Due to Density Increase 



Evaporation and/or cooling increase the density of surface water. 

 These are further sources of instability mixing, especially in autumn and 

 winter. If evaporation or cooling or both increase the density of surface 

 water during a certain period of time, the denser water sinks to a level 

 of equivalent density. The extent of. sinking in the thermocline depends 



on the vertical density gradient G 



M 

 di' 



A sharp gradient limits the 



sinking distance; a slight gradient permits considerable sinking. Less 

 dense water from the mixed layer and from the upper part of the thermo- 

 cline replaces the sinking water. The mechanism of density transformation 

 occurs within a thin surface stratum; density does not increase sharply, 

 because mixing is continuous. 



If no mixing process other than instability mixing occurs during a 

 ;iven time interval, the increase (Ah) of the mixed-layer thickness is 



12 



