272 
FISHERY BULLETIN OF FISH AND WILDLIFE SERVICE 
tlierinocline depths, as read from the BT slides, 
liave been plotted and contoured in figure 15. 
On and near the Equator an objective determi¬ 
nation of tlie tlierinocline depth is generally diffi¬ 
cult since in this region there often is either more 
than one inflection of the curve or a continuous 
negative gradient from the surface to the maxi¬ 
mum depth of the BT trace (fig. 1C>). In both 
figures 15 and 1(1 this region has been indicated by 
shading, and the contours in figure 15 liave been 
terminated at their points of intersection with the 
nortliern and southern boundaries of this region. 
Til is was done because there is a question as to 
whether or not the two Avell-defined thermoclines, 
one south of the Equator and one to the north, are 
the result of the same physical processes. 
(^ertain inferences to the circulation features 
may be drawn from the spacing and configuration 
of the contours in figure 15. The northern bound- 
aiy of the Countercurrent was coincident with 
the ridge near 10° N. (<100 ft.) and the southern 
boundary was centered along the trough in the 
tlierinocline depth near 5° N. latitude. In the 
southeastern portion of the area, a second ridge is 
evident at about 4° S. latitude. This reflects the 
distribution of mass associated with easterly flow 
there (see the results of the geostrophic calcula¬ 
tions in fig. 3). 
Surface Salinity 
The lowest surface salinity values (fig. 17) were 
measured along the noifhern boundary of the 
C^ountercurrent, with the minimal value (33.6 
Voo) being near 10° N., 128° W. This distribution 
reflects the extension of low salinity water west¬ 
ward from the coast of Central America and is also 
coincident with the zonal band of high rainfall 
located near 10° N. (Schott 1935, plate XIX). 
As the Smith proceeded south across the Coun¬ 
tercurrent and into the South Equatorial Current, 
surface salinity generally increased, reaching the 
maximum observed values at the southern extremi¬ 
ties of the SmithSs tracks. Somewhat farther 
south (near 20° S. latitude) was the southeastern 
Pacific salinity maximum, with surface values of 
36.00 ®/oo or greater (Sverdrup et ah, 1942, chart 
VI). 
Xear the front, the configuration of the isoha- 
lines showed an abrupt discontinuity in salinity, 
with higher salinity water, reaching 34.9 ®/oo, on 
the northern side of the front and surface waters 
with lower salinity, 34.2 to 34.4 ®/oo, to the south 
Fi(;ri{K IT).— Depth of the thernioeliiie as detenniiied from the KT data. 
above.) 
(E^or explanation of shaded section, see text 
* 
f 
OCEANOGRAPHY OF EAST CENTRAL EQUATORIAL PACIFIC 
273 
65 70 75 65 70 75 65 70 75 65 70 75 65 70 75 65 70 75 65 70 75 65 70 75 
70 75 80 70 75 80 70 75 80 70 75 8070 75 80 70 75 80 70 75 80 70 75 8070 75 8070 75 8070 75 8C 
ETgi rk K).—Tempeiaitiire-depth traces for selected BT 
records between A-B and C-D, figure lo. Arrows denote 
the therino(‘line depth as plotted in figure 15; the BT 
nund)er is noted at bottom of each trace. 
of the frontal line. Knauss (1957, table 1) re¬ 
ported a somewhat dissimilar situation, with 
liigher salinities in the cooler surface waters to the 
south of the front (34.49 to 34.54) and a lower 
salinity to the north (34.46). 
Phosphate 
The concentration of inorganic phosphate 
(POj-P) in the surface waters was determined at 
are shown in figure 18. 
In the surface waters of tlie Countercurrent, the 
supply of this nutrient was relatively low. These 
are ‘‘old'' waters in the classification of Steemann 
Nielsen (1954)—they have been at the surface for 
a considerable period of time as they have moved 
from the west in a current in which stabilization 
is markedly developed and vertical mixing is lim¬ 
ited. East of 140° W., the surface values are be¬ 
low 0.5 fxg. at./L., a level regarded by Ketchum 
(1939) as limiting photosynthesis. These rela¬ 
tively low values (0.3 to 0.4 fig. at./L.) in the 
surface waters represent the balance in the mixed 
layer among utilization by the ])hytoplankton, bio¬ 
logical regenerative processes, and vertical dif¬ 
fusion. 
In the South Equatorial (\irrent, the phosphate 
concentrations generally peak at or near the Equa¬ 
tor with values of 1.0 fig. at./L. or more. The con¬ 
centrations decrease I’apidly to the north and 
south, and more gradually east to west. Along 
110° W. the highest concentration of phosphate 
was south of the Equator, peaking at about 4° S. 
This results from the proximity of the thermo- 
cline to the surface and wind mixing discussed 
in the section on surface temperature. The con- 
F'laiiKK 17.—Surface .salinity (°/oo) distribiitioii. 
The arrows (bmotf' cunaMit dircR-tion as dotorniined from geostrophic 
ealenlations. 
