in the density structure associated with the 

 eddies described previously may be seen in 

 the sections at the longitudes of the eddies. 



The depths of two sigma-t surfaces 

 associated with the region of the thermocline 

 are shown on horizontal plots (figs. 48 and 49); 

 the shallower, sigma-t = 25.2, comes to the 

 sea surface within the area of the cruise. 

 While sea surface influences determine the 

 northward extent of this layer, both surfaces 

 otherwise reflect the circulation indicated by 

 theplots of dynamic topography at these depths. 

 The complex region of eddies is seen in the 

 eastern part of the area, while the northward 

 shift with depth of the boundary of the westerly 

 North Equatorial Current is noticeable in the 

 southwest corner of the area on the deeper 

 (sigma-t = 26. 0) surface. In general, there 

 is more detail of the circulation evident on the 

 sigma-t surfaces than in the smoothed dynamic 

 topographies. 



The deeper portions of the 

 meridional sections (figs. 38-47) show a north- 

 ward 9hift with increasing depth of the trough 

 in the isopleths marking the transition from 

 easterly to westerly zonal flow. The topography 

 of the sigma-t = 26. 8 surface (fig. 50), which 

 corresponds closely to the position of the deep 

 layer of minimum salinity of northern origin, 

 again closely resembles the indicated geostro- 

 phic circulation at similar depths; the current 

 is slow, turning broadly back toward the west, 

 with southerly flow through a large part of the 

 area. 



Salinity 



In the surface layer (fig. 51) the 

 major features are the region of salinity maxi- 

 mum in the latitudes of the southern edge of 

 the cruise area and the rapid decrease in sali- 

 nity northward from this region, particularly 

 in the east. The maximum is formed by the 

 excess of evaporation over precipitation in 

 these latitudes and is distributed in a broad 

 zonal band by the currents (Jacobs 1951). The 

 water to the north of the cruise area is partly 

 of northern origin and is low in salinity; be- 

 tween is the area of transition characterized 

 by a large meridional salinity gradient. This 

 gradient is not smooth, but reflects the irre- 

 gularities (such as the eddies) in the circulation. 



Below the surface layer a deep 

 salinity minimum (400-600 m.) is evident on 

 all the sections (figs. 52-61), with a second 

 shallow minimum appearing in the northeast 

 part of the area. The deep minimum, 



characteristic of the greater part of the sub- 

 tropical and temperate Pacific, is probably 

 maintained by water formed in the northwest, in 

 the convergent region between the Kuroshio Ex- 

 tension and the Oyashio (Sverdrup et al. 1942). 



The shallow minimum occurring in 

 the northeastern part of the cruise area seems 

 to be formed by surface convergence. Figures 

 62 to 64 give the depth, salinity, and sigma-t at 

 the level of this minimum, and show its southern- 

 most extent. The flow is mostly parallel to the 

 isohalines, and the depths indicate that part of 

 the region of origin is close to or at the northern 

 edge of the cruise area. The widely varying 

 values of sigma-t suggest that considerable non- 

 isentropic mixing is occurring at these relatively 

 shallow depths, and also doubtless reflect ir- 

 regularities in the formation of the minimum at 

 the surface. This shallow minimum was men- 

 tioned by Sverdrup et al. (1942, fig. 202 and p. 

 723) and is apparently characteristic of this area. 

 The waters of the upper minimum turn toward the 

 south with the general circulation. More com- 

 plete station coverage, especially to the east of 

 the Smith cruise 25 area, is needed to describe 

 the formation and distribution of this minimum. 



The salinity distribution is shown on 

 the three sigma-t surfaces discussed in the sec- 

 tion on density (figs. 65-67). On the two shallower 

 surfaces the isohalines very roughly parallel the 

 geostrophic flow at these depths, with irregulari- 

 ties particularly noticeable in the eddy region. 

 The shallowest surface, sigma-t = 25.2, is still 

 somewhat within the direct influence of the sea 

 surface and shows the greatest variations in 

 salinity. On the intermediate surface, sigma-t 

 = 26. 0, the southward extension of low salinity in 

 the east is connected with the intrusion of the 

 upper salinity minimum, which is centered 

 slightly above this sigma-t surface (see fig. 64). 



On the deep surface, sigma-t = 26.8, 

 very nearly coinciding with the center of the deep 

 salinity minimum, the 34.00 /oo isohaline lies 

 across the flow. The flow here is very weak, 

 however, and the salinity changes are small; the 

 downstream salinity increase is the result of 

 vertical mixing and diffusion. 



Temperature -Salinity Relationships 



Although it is not within the scope or 

 intent of this report to enter into a detailed dis- 

 cussion of the temperature-salinity relationships 

 from the standpoint of the origin of the water, they 

 do help clarify the circulation. The curves for the 

 160°W., 155°W. , 147°W., andl41°W. transects 

 have been combined in single plots (figs. 68-71) 



