where |' =| -Al > a reduced mean vertical component of mixing length due 



"* z z to stability or a vertical distance along which a moving 

 particle is acted upon by the stability. 



The vertical component of the momentum transfer at a certain level will 

 decline to such an extent that the downward transfer of heat will not be 



9 dP 



possible because of the restoring stability force,- -g- -3— . The mixed 



layer will then be at a new steady state corresponding to existing surface 

 mixing conditions. 



Mixing forces all act concurrently; and formation of the mixed layer 

 results from interwoven, inseparable action of all factors. Swell can 

 also participate in mechanical mixing, if the swell is combined with cur- 

 rent and wind waves. 



Decay of the Mixed Layer 



A mixed surface layer of given age and depth has a limited area. Age 

 is defined as the number of days expiring after mechanical mixing ceases. 

 Assume that a mass of homogeneous water exists to a certain depth over a 

 certain area. Outside this area the surface mixed layer has different 

 properties. The homogeneous body of water floats within or above a system 

 of nonhomogeneous water which has inherent characteristics of motion. The 

 depth of the moving nonhomogeneous water is considerably greater than that 

 of the mixed layer. The thermocline is therefore a part of the surrounding 

 nonhomogeneous system. The water of the mixed surface system is not re- 

 quired to be completely homogeneous horizontally, because it can extend 

 over several nonhomogeneous systems. The mixed-layer system is charac- 

 terized mainly by pure wind current and wind waves that have acted to pro- 

 duce the mixed layer over the specific area of the wind field. 



In the absence of a mixed layer, the nonhomogeneous system may be 

 assumed to extend from the surface to the bottom or to a layer of no 

 motion as shown in Figure 11A. After a mixed layer has been formed over 

 a wind field, the mean flow of pure wind current in the mixed layer will 

 be approximately at right angles to the wind and at a certain angle to 

 the mean direction of flow of the nonhomogeneous system, as shown by 

 arrows in Figure 11B (when wind direction is toward the observer). 



After the mixing process ceases, the mixed layer possesses no kinetic 

 energy. The mixed layer will then be absorbed by the nonhomogeneous 

 system. The process of transformation is probably quite complex; however, 

 a few distinct partial processes may be assumed. Firstly, the whole mass 

 of mixed water should undergo deformation due to loss of dynamic rigidity 

 gained during mixing from independent horizontal flow and eddy viscosity. 

 The entire mixed layer becomes elongated and flattened during adaptation 

 to the permanent flow. In this process, velocity of currents in the 

 thermocline is gradually imparted to the mixed layer (Figure 12). Secondly, 

 the permanent flow in the upper part of the thermocline is not sufficient 

 to produce a complete mixed layer but is strong enough to accelerate tem- 

 perature conduction which extends the temperature gradient into the lower 



19 



