able to "balance the detrition and deformation forces. This balancing 

 requires considerably less energy than is required to overcome stability; 

 consequently, deepening of the thermocline can occur only with adequate 

 surface conditions, whereas the mixed-layer thickness can be maintained 

 by -weaker surface conditions. 



Since actual conditions in the oceans are generally similar to those 

 given in the second example, there are additional problems of determining 

 how much the mixing energy must decrease before decay can begin, and how 

 far and at what rate the decay will proceed once it begins. Convergence 

 must also be considered with respect to decay. On the convergent side of 

 the field of pure wind current, another current following the wind direc- 

 tion results from piling up of the water. This current will continue to 

 flow after the pure wind current ceases and will also tend to maintain 

 the already developed mixed layer, thus delaying its decay. 



In general, decay is quite rapid in strong permanent currents. Effects 

 may be observed in periods as small as one day. In areas of very slow 

 geostrophic flow, decay is also slow; and the age of a few days may reveal 

 no marked effects. 



All considerations lead to the conclusion that prediction of the 

 thermocline depth cannot always be based on the wind field in an area. 

 The history of surface conditions for approximately 15 to 20 preceding 

 days must be taken into consideration, and the prediction must be based 

 on the most recent and strongest mixing conditions. 



At present, historical data are inadequate for more efficient formu- 

 lation of the decay and time lag problems. Since the decay process is 

 closely related to motion below the interface between the mixed layer and 

 the thermocline, regional and seasonal peculiarities have great importance. 

 Personal experience of a forecaster may be greatly helpful in certain areas 

 during certain seasons. 



PROCEDURE 



Determination of the Mixing Parameter 



In the absence of mixing by instability or by permanent convergence 

 or divergence, existence of the mixed layer is attributable to a complex 

 process of mechanical mixing. Water particles are interchanged between 

 the surface and bottom of the mixed layer by energy supplied in varying 

 amounts from various origins. Turbulent motion of a particle is the 

 resultant of several simultaneous component forces acting in different 

 proportions. 



The main mechanical mixing factors are derived from wind stress on 

 the water. Either wind or a combination of wind waves and pure wind cur- 

 rent may be used to correlate the thermocline depth with its causes. The 



23 



