ESTUARINE MODEL OF SUB-ARCTIC PACIFIC OCEAN 427 



tides, winds, or current distribution. These shears provide the 

 energy for mixing. Sea water mixed upward through the shear 

 zone flows outward with the fresh water. Thus it is entrained and 

 eventually lost from the region. It is replaced by an inward flow, 

 toward the source region, below the shear zone. The density 

 distribution in the transition (halocline) zone adjusts to provide 

 the necessary inward pressure gradient force in the deeper water. 



Fresh or mixed water transferred downward through the shear 

 zone can never attain the salinity (density) of undiluted sea water 

 in the lower zone. Hence there is a limit of downward transfer of 

 fresh (mixed) water which must be near the lower limit of the 

 shear zone. This is defined in the structure (Fig. 1) by the lower 

 limit, L, of the halocline. In this position the mixed water must be 

 transported toward the source region, and be preferentially 

 available for upward mixing. 



Thus the entrainment process may be regarded as a unidirec- 

 tional upward transfer of sea water, coupled with a random mixing 

 process in the halocline. The lower limit, L, of the halocline is the 

 limit of the random mixing feature. At this limit the transfer of 

 sea water is unidirectional upward. 



The fresh water transport mechanism persists while there is a 

 difference of salinity (density) between the zones. The entrainment 

 process continually reduces this difference. It may be further 

 reduced by loss of fresh water, by evaporation, or by freezing. 

 When this difference vanishes, the structure is isohaline and the 

 mechanism fails. However, if the upper zone is progressively 

 refreshed, the halocline and the fresh water transport mechanism 

 will persist. 



Salinity Structure and Limits of Zonas 



From examination of extensive data, TuUy (1957) concluded 

 that the zone structure could be accurately delineated by plotting 

 salinity as a function of the logarithm of depth. In this presenta- 

 tion, as illustrated In the last diagram of Fig. 1, the salinity 

 distribution within each zone can be closely approximated by 

 straight line segments. The Intersections, D and L, of these seg- 

 ments define the limits of the zones. 



