442 BOUNDARIES OF THE SEA 



boundary in the Pacific Ocean. Therefore, a comprehensive 

 discussion of the oceanography of the region must include the 

 estuarine model. 



Some Features of the Model 



Origin of the Lower Zone 



The index salinity {S* = 33. S ±0.1%o) is remarkably constant 

 northward of the inner boundary proposed by Dodimead (1958a) 

 (Fig. 3). Further, the salinity gradient at the bottom of the 

 halocline is small and less variable in time and space than the 

 temperature gradient. These features were also noted in coastal 

 estuaries (Tully, 1949; Waldichuk, 1957). Evidently the intruding 

 lower zone is created by a mechanism which allows much less 

 variation in the salinity than in the temperature. 



It is suggested that the subhalocline waters in the eastern sub- 

 Arctic Pacific Ocean may originate in the isohaline region at the 

 outer limit of the sub-Arctic structure (Fig. 4). Northward 

 intrusion, below the halocline, from the direction of this boundary 

 has been observed by Bennett (1959) and Tully et al. (1959). The 

 salinity characteristics of subhalocline water intruding from this 

 source must be constant within small limits regardless of the depth 

 of its origin (at least to 600 m). However, there is a temperature 

 gradient in this boundary region; hence, the temperature of the 

 intruding water depends on the depth of its origin. 



Depth of Seasonal Effects 



Precipitation and evaporation, heating and cooling, and wind 

 mixing are all surface effects which vary with time. To the extent 

 that these variations penetrate the halocline, they affect the 

 constancy of the properties of the water there. 



The depth D (Figs. 1 and 2) of the near isohaline upper zone 

 coincides with the depth of the isopycnal layer in winter (Dodi- 

 mead, 1958a). In summer, when the winds are light, some small 

 haloclines appear in this upper zone, associated with the thermo- 

 cline. They vanish in winter along with the thermocline, when all 



