260 
FISHERY BULLETIN OF FISH AND WILDLIFE SERVICE 
Figure 3.— Station-to-station variations in the geopoten¬ 
tial topography (0/700-db. surface) for the Hugh M. 
110° W.. 120° W., and 140° W. sections. 
Uiifortiiiiately, the Smith was not equipped to 
make direct measurements of velocities for these 
transects. Surface currents measured by the GEK 
aboai-d the Horizon (University of California, 
1056), 5°()0' X. to 2°11' X. latitude, near 120° W. 
longitude (October 11 and 12, 1955), varied be¬ 
tween 0.86 knot (626° T.) at 2°48' X. and 2.0 
knots (624° T.) at 6°08' X., the latter value bein^ 
measui’ed near a marked temperature discontinu¬ 
ity (front). The contours in the region of the 
front (fig. 2), also encountered by W\^ Smith (near 
4° X., 120° W.), are rather closely spaced reflect¬ 
ing the comparatively swift westerly flow meas¬ 
ured by the Horizon. The Baird., near 115° W. 
longitude and between tlie same limits of latitude, 
observed velocities between 1.1 knots (066° T.) 
at 5°02' X. and 5.7 knots (288° T.) at 0°46' X. 
As can be seen from table 1, the calculated rela¬ 
tive velocities for the SmitJi^ survey did not ex¬ 
ceed 2.6 knots (6° X.-4° X. along 120° W.). 
deferring to figure 6, the break in slope near 
the northern limit of the 110° W. section simirests 
the northernmost station was in the Countercur¬ 
rent. The lack of a break in slope between the 
two northernmost stations of the 120° W. and 140° 
W. sections suggests these sections did not reach 
into the Countercurrent. In the southeastern por¬ 
tion of the survey area, the geostrophic considera¬ 
tions yield an easterly flow centered about 2° 8. 
latitude (fig. 6, 110° W. section). The trough 
normally centered at or very near the Equator is, 
in this instance, positioned at 4° 8. latitude. 
Indirect evidence of the easterly flow in the sur¬ 
face waters, 1° 8. to 4° 8., is to be found in the 
wire-angle data for the Xansen bottle casts. 
Along the 110° W. section, the angles near to and 
north of the Equator (to 4°60' X.) were large, 50° 
to 60° from the vertical. These large angles were 
the cumulative effects on the wire of the easterly 
winds on the vessel, the westerly surface flow and 
the decrease in velocities at the subsurface levels 
penetrated by the cast. Between 1° 8. and 4° 8., 
with 11- to 15-knot easterly winds, the angles fell 
to less than 10° (06° at 1°26' 8. and 06° at 2°54' 
8.). The easterly surface flow exerted a ^‘cancel¬ 
ing effect” on the vessel and the wire angles thus 
were very small. 
On both 110° W. and 120° W., there was an ap¬ 
preciable change in wire angle near the front, 
with smaller angles to the north of the front 
(10°) and larger angles to the south (50°). These 
differences in angle reflect the differences in veloc¬ 
ity of the surface waters. The two stations on 
120° W. nearest the front were station 22, approx¬ 
imately 90 miles to the south of the front, and 
station 26, approximately 60 miles to the north of 
the front. Beferring to velocity measurements 
made aboard the Horizon (University of Califor¬ 
nia, 1956), the speed of the surface flow decreased 
from near 4 knots in waters to the south of the 
front to 1 knot or less in those to the north. Con¬ 
sidering that most of the Xansen bottles and wire 
were in deeper waters of low velocities, the com¬ 
paratively swift surface currents south of the front 
would result in such large wire angles. 
VERTICAL DISTRIBUTION OF PROPERTIES 
Temperature 
Temperature sections along selected longitudes 
(120° W. to 160° W.), each crossing the main 
features of the equatorial zonal circulation in the 
central Pacific, have previously been published in 
POFI oceanographic and biological reports (i.e., 
(h'omwell 1954; 8troup 1954; Austin, 1954a and 
OCEANOGRAPHY OF EAST CENTRAL EQUATORIAL PACIFIC 
1954b; and Murphy and Shomiira, 1956). During 
Eastropic, no single leg of the SmithS track jiro- 
vided data for such a section. Thei'efore, in 
figure 4, two BT sections, one near 155° lY. from 
15° X. to 5° X. and one along 140° lY. between 5° 
X. and 8° 8. are used to illustrate tk.e noilh-south 
temperature-depth distribution. The (>0°, 7()°, and 
80° F. isotherms were drawn with the 72° and 74° 
F. isotherms included near the Eipiator. 
From north to south (right to left in fig. 4, A) 
in the Xorth Equatorial Current, the isotherms 
slope upward, reaching a minimum depth at the 
northern boundary of the Coimtercurrent, then 
slope downward to the southeiai boundary of the 
Countercurrent. This interpretation of the cur¬ 
rent boundaries is based on the assumption of 
geostrophic flow. In the next section (fig. 4, B), 
140° lY., 4° X. to 4° 8., both the upward trend of 
the isotherms toAvard the Equator and their 
deepening south of the Eipiator, compatible wdth 
westerly flow, are discernible. Various mixing 
processes at or near the Equator resulted in the 
“irregularities" which mask the ridge expected 
from the distribution of mass when a westerly flow 
is centered about the Equator. 
The mixing of the cooler subsurface waters Avith 
those at the suifface near the Equator is reflected 
in the configuration of the 74° F. isotherm, which 
intersects the surface to the north and to the south 
of the Equator. A trough in the isotherms cen¬ 
tered beneath the Equator is suggested in figure 4, 
but it is not as evident as generally found in meri¬ 
dional sections crossing the Equator in the central 
261 
Pacific (Austin 1954b; lYooster and (’romwell, 
1958). 
I he 60° and 70° F. isotherms at the southern 
limit of the 155° AY. section (fig. 4. A) are about 
50 meters deeper than those at the noi-thern limit 
of the 140° AY. section. This ditlerence results 
from the rapid east-west deepening of the thmano- 
cline between 125° AAh and 160° AY. longitude. 
To illustrate this deepening, tenqiei'ature-dejith 
data from BT's taken at eiglit })ositions along the 
Equator, 112° AA^. to 156° AA^. longitude, have been 
contoured in figure 5. Xear 155° AY., the top of 
the theianocline is at 150 metei’s, decreasing in 
depth to 50 meters near 125° AA^., then deepening 
slightly toward the eastern end of the section. 
Other features in the temperature-depth dis¬ 
tribution of the three meridional sections Avarrant 
attention. Of the sections that crossed the Eipia- 
tor (see fig. 1), three Avere made along coui'ses 
nearly normal to the Equator. The tenqieiutui'e- 
depth profile for each of these sections is shoAvn in 
figure 6, A (110° AAh), 6, B (120° AY.), and 6, C 
(140° AY.). In all three, the well-developed two- 
layer system, characteristic of tropical Avatei’S, is 
evident. The princi})al variation among the three 
sections is in tlie depth of the thermocline. 
Along the 110° AAh section (fig. 6, A), there is 
a gradual decrease in depth of the thermocline 
from 5° X. latitude south across the E(piator to 
4° 8. latitude. This suggests that between 0° and 
4° 8. there is a revei-sal in floAv Avith the suidace 
Avaters directed to the east, becoming Avesterly 
again south of 4° 8. Two centers of cold AA*ater at 
X 
I- 
Q. 
LU 
Q 
0 
50 
100 
150 
200 
250 
Figure 4. —Vertical temperature (°F) 
section from Eastropic BT records; composite of such records along 140° W. 
and 155° W. longitude. 
527056—60 
2 
