264 
FISHERY BULLETIN OF FISH AND WILDLIFE SERVICE 
Figure 7— Vertical distribution of temperature (° F.) 
across the front observed by the Smith near 4° N. lati¬ 
tude, 120° AV. longitude. 
relation betAveen the position of the surface sa¬ 
linity maximum and the subsurface trajectory is 
reflected in the variations in the feature on the 
three longitudes (hu. 8) ; i.e., its northern extent 
ends abruptly near the Equator in the central and 
east-central Pacific. This vvas shown in previous 
POFI sections by CroniAvell (1954), Austin 
(1954a, 1954b), Stroup (1954), Montgomery 
(1954), and by Fleming et ah (1945, figs. 119 and 
143). Austin (1954b, fig. 21) has shown by the 
temperature-salinity characteristics that this 
abrupt termination of the salinity maximum at 
the Equator in the eastern Pacific is associated 
with the vertical mixing processes accompanying 
upwelling. In contrast, in the western Pacific the 
maximum penetrates into the Northern Hemi¬ 
sphere, reaching at least 5° N. {Mao and Yoshida, 
1955). Austin and Rinkel (in press) suggest that 
this is evidence for less-intensive upwelling in the 
western Pacific. 
Com])arison of the temperature/salinity (T/S) 
characteristics for the Eastropic stations along 
110° and 120° W. also reveals the longitudinal 
variation in the subsurface salinity maximum and 
the rather abrupt change near the Equator. 
Along 110° W. (fig. 9, A), the T/S curves south 
of the Equator all show a rapid subsurface de¬ 
crease in temperature with small change in salin¬ 
ity. Similar curves for the stations along 120° W. 
(fig. 9, B) show a configuration for those stations 
south of the Equator which is attributable to the 
subsurface maximum. On the T/S curve for the 
first station north of the Equator (station No. 21), 
1°06' N., 120° W., the Southern Hemisphere maxi¬ 
mum is no longer in evidence. 
Returning to figure 8, we note that the waters 
in the surface layer near the northern limits of 
the three sections, particularly those along 110° 
W. and 120° W., are characterized by low salini¬ 
ties. Along 110° W., the salinities in the surface 
layer are below 34.00 Voo (^ minimum of 3,3.49 Voo) 
at the northern two stations (07°06' N. and 05 52 
N.). Westward along similar latitudes, there is 
a gradual increase to 34.47 ®/oo od fbe 120° W. sec¬ 
tion and 34.72 Voo on 140° W. These low salinities 
are attributable to dilution by rainfall and in¬ 
fluence of waters moving in from the east and 
northeast. As shown by Schott (1935), a ridge of 
mean maximum rainfall is centered along 10° N., 
becoming increasingly prominent to the east and 
reaching a maximum in the Gulf of Panama. 
East of the Smith'^ survey area, as revealed by 
the Caf^egie data (Fleming et ah, 1945, fig. 222) 
and by data taken aboard the Scripps Institution 
of Oceanography vessels during Eastropic (Uni¬ 
versity of California, 1956), the salinities in the 
surface waters progressively decrease, reaching 
minimal values of 29.50 Voo to 30.00 Voo in the 
Gulf of Panama (3°-6° N. latitude). 
In the deeper waters sampled during expedition 
Eastropic, at 500-1,000 meters, the salinities were 
those characteristic of the Antarctic intermediate 
waters with the axis of the lowest values rising 
toward the north on each section (King et ah, 
1957, figs. 11, 16, and 21). 
Density 
The vertical distribution of density along each 
of the three sections, expressed as thermosteric 
anomaly in centiliters per ton (following Mont¬ 
gomery and Wooster, 1954), is shown in figure 10. 
The most striking feature on each of the three sec¬ 
tions is the rapid increase in density with depth 
(which we shall refer to as the stable layer) 
OCEANOGRAPHY OF EAST CENTRAL EQUATORIAL PACIFIC 
265 
Figure 8.—Vertical distribution of salinity (°/oo) alon.c: 110° W. (A), 120° AV, (B), and 140° AV. (C). 
0.2 °/oo (from King et al., 1957). Density isopleths in black (in centiliters per ton). Depths 
are shown by dots; station numbers are given along the top of each panel. 
Contour interval 
of observations 
