Alaskan Stream Extension, which form a relatively intense westerly flow south of and 

 closely confined to the Aleutian Island Arc. A zone of easterly flow lies between 30° N 

 and 50° N. Dodimead, Favorite, and Hirano (in press) designate parts of this easterly 

 flow as the Subarctic Current, made up in large part of Oyashio waters: the Westwind 

 Drift, originating where parts of the Oyashio and Kuroshio meet and turn eastward: and 

 the North Pacific Current, formed by the main flow of the Kuroshio which turns easterly 

 at about 35° N. From 30° N to the southern limits of the present study area the flow is 

 weak, as would be expected near the center of the anticyclonic flow in the subtropical 

 north Pacific Ocean. A westerly flow persists at the southern limit (23°30'N) of the 

 present investigation. 



Strong gradients of temperature and salinity are found across the Westwind Drift. 

 This band of strong gradients, extending nearly across the north Pacific Ocean is called 

 the Polar Front. Fleming (1955, 1958), Uda (1963), and Dodimead, Favorite and Hirano 

 (in press) divide the North Pacific Ocean into natural regions based on the current systems 

 and the associated characteristics of temperature, salinity and dissolved oxygen. The 

 southern edge of the Polar Front is the boundary between the Subarctic Region in the north 

 and the Subtropic Region in the south. Dodimead and coworkers define the boundary 

 between the two regions as the zone where the 34°/ 00 isohaline is nearly vertical from the 

 surface to a depth of 300 to 500 meters (figs. 4, 7, 10, 13, and 16). In the present study area, 

 the boundary occurs at about 40° N. 



Above 1,000 meters, the density structure in the Subarctic Region is dominated by 

 salinity effects. Seasonal variations in the salinity structure are exhibited only in the zone 

 extending from the surface to about 100 meters. Below this zone a permanent halocline, 

 in which the salinity increases rapidly, occurs in the 100- to 200-meter depth interval. 

 Below the halocline, the salinity gradually increases with depth. Fleming (1958), and 

 Tully and Barber (1960) have described the salinity structure in detail. The negligible 

 seasonal variation in all but the upper 100 meters of the water column is explicit in their 

 descriptions. The temperature and salinity structures in the Subarctic Region are subject 

 to pronounced seasonal variations through the same depth interval. A temperature mini- 

 mum is often present in the depth interval of the halocline as shown in the northern end 

 of the sections (figs. 6, 9, and 12). The salinity distribution dominates the density struc- 

 ture and maintains stability despite the temperature inversion. 



The halocline becomes progressively less pronounced southward through the Polar 

 Front, and finally disappears. 



In the Subtropic Region there is no permanent halocline. Here the density structure 

 in the upper 1,000 meters is dominated by a strong thermocline extending to depths of 600 

 to 800 meters (figs. 3, 6, 9, 12, and 15). A salinity minimum occurs at depths of 500 to 

 600 meters (figs. 4, 7, 10, 13, and 16). Seasonal effects in the surface layers in the Sub- 

 tropic Region are less pronounced than in the Subarctic Region. 



The division of the North Pacific Ocean into regions on the basis of the temperature 

 and salinity distributions applies between the surface and 1,000 meters. Below 1,000 

 meters, temperature and salinity structures are similar throughout the area. 



The dissolved oxygen structure is characterized by a minimum occuring between 600 

 and 1,200 meters. It occurs at about 600 meters immediately south of the Aleutian Island 

 Arc, descends to about 1,200 meters in the region of the Polar Front, and ascends through 

 the Subtropic Region to about 800 meters near 23°30'N. A noteworthy feature of the 

 meridional sections of oxygen concentration (figs. 5, 8, 11, 14, and 17) is the thickening 

 of the zone of low oxygen values in the northern part of the area. From 46°N to 52°N, 

 the thickness of the oxygen zone, of concentrations less than 1 milliliter/liter, is twice 

 that south of 40°N. 



Figures 19 through 54 show the results of the dynamic computations for the sections 

 investigated in this study. These will be considered in the text which follows. 



4 



