despite this current's vertical extent, which is more than tenfold that of 

 the mixed layer. Although the upper layers in a strong permanent current 

 are more subject to disruptions by advection of "foreign" water masses and 

 are usually less steady than those in a water mass with little or no perma- 

 nent flow, their formation and structure are similar to those of the latter 

 and follow the same pattern. 



It is hardly possible to locate an area of the ocean which does not 

 have horizontal flow extending vertically to a layer of no motion (except 

 at convergence and divergence lines), a level usually located many times 

 deeper than the surface layer of mixed water and the thermocline. For 

 all practical purposes, assumptions can be made that a mixed layer always 

 forms within a moving mass, and that "currents" are only areas or streaks 

 of exceptional velocity of horizontal flow. 



Thus, to summarize the factors participating in formation of turbulent 

 motion under given surface conditions at the depth £ in the mixed layer, 

 the momentum transfer (n) of a mass element in unit time will be the 

 resultant of several component momentum transfers: 



N - N + W-b + N wc + Np f (2) 



where W ■ component of momentum transfer due to orbital turbulence , 



Nfc, - component of momentum transfer due to breaking crests of 



surface waves, 



N v;c = component of momentum transfer due to pure wind current , and 



Npf * component of momentum transfer due to permanent flow . 



If the mixed layer is in a steady state and the energy produced by 

 surface conditions is insufficient to increase the thermocline depth, any 

 stirring in the mixed layer occurs under neutral conditions. The vertical 

 component of momentum transfer will then be 



n (C)=?v' T (3) 



where V z = mean vertical component of resultant turbulent velocity at 

 the depth £ and is a function of £ 

 T z = mean vertical component of the distance of mass transport at 

 the depth £ and is also a function of £, and p is the density. 



If surface conditions increase the mixing energy sufficiently to cause 

 an increase in the mixed-layer thickness, mixing occurs in the upper part 

 of the thermocline near the interface; this mixing gradually extends upward 

 to the surface. While stirring continues under nearly neutral conditions 

 in the former mixed layer, stability hampers mixing just below the inter- 

 face so that the vertical component of the momentum transfer will be 



/>v' 2 T 2 {k ) 



N * ,£, V<*f,7. 



" OZ i 



18 



