ESTUARIXE MODEL OF SUB-ARCTIC PACIFIC OCEAN 429 



Below this is a halocline extending to 200 ( + 50) m depth in which 

 the salinity increases to 3)3).^ ±0.1%o (the index salinity, 5*). Below 

 this is the lower zone in which the salinity increases gradually 

 with depth. 



This structure has been observed winter and summer throughout 

 most of the region northward of a sub- Arctic boundary located in 

 the Polar Front. Dodimead (1958a) (Fig. 3) shows that south- 

 ward of this boundary the salinity at the lower limit of the halocline 

 became greater than 33. 8%c), and a salinity minimum appeared 

 below it. The halocline structure degrades and finally vanishes 

 in a narrow band where the structure is nearly isohaline (Fig. 4). 

 This also could be regarded as the southern limit of the sub-Arctic 

 region (Tully and Dodimead, 1957). Pending further definition, 

 these may be designated the inner boundary (Fig. 3) and the outer 

 boundary (Fig. 4). 



Within the sub-Arctic region, in the vicinity of the central 

 Aleutian Islands, Dodimead (1958b) observed that the zone 

 structure degraded to an almost uniform gradient, to about 400 m 

 depth. This is attributed to extreme mixing in this locality. 



fresh Water 



Jacobs (1951) (Fig. 5) showed that precipitation exceeds 

 evaporation in the sub-Arctic Pacific Ocean and provides a con- 

 tinual, though irregular, source of fresh water in the region. The 

 low salinity in the upper zone and the halocline structure there 

 are consequences of this excess input of fresh water. The figure 

 defines the source region. 



Since the limit L of the halocline is the limit of downward 

 transfer of fresh water, and can be identified in a logarithmic plot 

 of the salinity structure, it is possible to compute the amount of 

 fresh water above this limit. At any position this may be expressed 

 as the fraction C of the depth L that would be occupied by the 

 fresh water if it were segregated from water of index salinity S*, 

 at the limit L of the halocline (Fig. 1). 



CL = S*L - 



L 



Sdz 







