652 EIGHTH PACIFIC SCIENCE CONGRESS 



On further consideration, it is evident that in any case the source 

 (or sink) of the property may always be referred to the surface, since 

 the process of mixing is continuous at all depths, even though the rate 

 may vary. 



There is a temptation to associate each zone with a region of a 

 particular degree of turbulence. For example, the nearly homogeneous 

 zones might represent the region of internal turbulence in a water mass, 

 while the boundary zone would represent the extent of the added tur- 

 bulence induced by the shear between water masses. Hov/ever, this 

 would imply that the structure at a single position was an expression 

 of the energy characteristics of the system. The structure reveals the 

 mechanism, and is surely a result of the energy characteristics, but the 

 energy distribution can only be expressed by the difference between 

 structures. Furthermore, there is no quantity of time in the definition 

 of the structure (Equation 1) and finally it has been shown that the 

 slope of the gradient can be readily explained by the dimensions and 

 distribution of the water masses. 



It is concluded that each zone is an independent structure, be- 

 haves as if it were the only zone present, and as though its source or 

 sink were at the surface. This concept is implied in the evaluation of 

 the constant (c) in Equation 3. It is supposed that there is a surface 

 layer of unit thickness, and of constant properties (P) and that it is con- 

 tinually being renewed, so that it acts as an inexhaustible source, or 

 sink. 



In the simple case where the water masses are not affected by ex 

 ternal factors such as insolation or precipitation, the structure in each 

 zone tends towards homogeneity. But at the same time the conditions 

 at the limits of the zones are being renewed by transfer of water from 

 elsewhere. Each zone is a source and sink for its neighbours, so that 

 one zone cannot become homogeneous while the character of its neigh- 

 bours remains unchanged. Rather the gradients in the several zones 

 tend to become congruous, and then the whole structure tends to degen- 

 erate towards homogeneity. This process is well illustrated by the se- 

 quence of salinity observations shown in Figure 8. The step structure 

 is very apparent near the mouth of the Fraser River, but Vvith increasing 

 distance the steps become less and less apparent as the structure becomes 

 mature. 



The tendency towards homogeneity can only be opposed by the 

 introduction of new sources of properties. For example, the surface 

 water may be heated diurnally so that a gradient of temperature, as 

 shown in Figure 9A, is formed. During the night the surface is cooled, 

 or the zone is mixed to homogeneity by the wind so that a single step 

 structure is formed as in Figure 9B. The effect of further heating and 



