Mixing Due to Permanent Convergence or Divergence 



In areas of permanent or semipermanent convergence or divergence, 

 ■water particles have vertical components of motion, thus fulfilling con- 

 ditions of mass interchange "between the surface and bottom of the mixed 

 layer. The processes involved in this type of mixing are not explicit. 

 A possible explanation is that downward motion of water in a convergence 

 zone creates turbulence at the top of the thermocline, and increases the 

 mixed-layer thickness to a certain extent. However, the downward motion 

 does not penetrate to the bottom of the thermocline; and, after the mixed- 

 layer thickness reaches a steady state, further circulation takes place 

 only within the layer. 



In case of divergence, upward movement of water causes denser thermo- 

 cline water to rise, resulting in reduction of the thickness. After a 

 balance is reached between the rate of divergence and reduction of the 

 mixed-layer thickness, further circulation is again limited to the layer 

 and does not cross the interface between the mixed layer and the thermo- 

 cline. 



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FIG 



URE 8 FREQUENCY DISTRIBUTION OF THERMOCLINE DEPTHS AT OCEAN WEATHER STATION ECHO 

 (35° N, 48° W) IN SEPTEMBER (6 YEARS' DATA, 1949-1954, 341 OBS) 



OWS ECHO appears to lie in a semipermanent convergence area during 

 late summer and autumn. Figure 8 shows the frequency distribution of 

 thermocline depths computed from 6 years of data at OWS ECHO for September. 

 The thermocline was between 80 and 150 feet 86 percent of the time, whereas 

 it was between 50 and 80 feet only 1.5 percent of the time. There were no 

 observed thermocline depths of less than 50 feet; 12.5 percent were between 



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