258 
FISHERY BULLETIN OF FISH AND WILDLIFE SERVICE 
Figure 1.— Track chart of vessels jiarticipating in expedition Eastropic. Positions of oceanographic stations shown 
only for Hugh M. Smith. 
October 10, 1955. The northern edge of the 
Countercurrent was followed east to 120° W. 
During this eastbound leg of the cruise, bathy- 
thermograpli (BT) lowerings were made at 2- 
hour intervals and surface samples were collected 
twice daily for inorganic phosphate and salinity 
determinations. The vessel then proceeded to 
Manzanillo, Mexico, for refueling and supplies. 
The Smith departed Mexico on October 24, 
1955, on a generally southwesterly course across 
the coastal currents, the Countercurrent, and to 
8° S. latitude in the South Equatorial Current. 
The westward passage was planned in order to 
study the equatorial divergence and the associated 
oceanographic and biological events. A series of 
standard oceanographic casts to 1,200 meters was 
made at approximately 90-mile intervals, between 
6° X. and 8° S. latitude near 110° W., along 120° 
M., and along 140° AY. longitude. The northern¬ 
most station of the 110° W. leg was in the Coun¬ 
tercurrent. The rest of the stations were in the 
westerly fiowung South Equatorial Current. 
Bathythermograph lowerings were made at each 
station and at 2-hour intervals between stations. 
Surface phosphate and salinity samples were 
taken at frequent intervals. 
The data resulting from the observations and 
collections taken by the Smith during expedition 
Eastropic have been published by King, Austin, 
and Doty (1957) ; those from the Baird and the 
Horizon by the University of California (1956). 
Both of these publications include tabulations of 
the observed data and a description of the 
sampling and analytical methods used aboard the 
vessels. The report of King et al. also includes 
oceanographic station curves, tabulations of the 
biological data and a description of the methods 
used in the shore-based processing and analyses. 
The purpose of this paper is, primarily, to 
describe the oceanic circulation features and the 
associated distribution of variables as determined 
from the data collected by the Smith.^ particularly 
those features having significant influence on the 
distribution and abundance of the biota. Data 
OCEANOGRAPHY OF EAST CENTRAL EQUATORIAL PACIFIC 
259 
from other participating vessels and those from 
previous surveys to the Eastern Central Pacific, 
both by POFI and other agencies, will be used as 
needed. 
RESULTS OF OCEANOGRAPHIC 
OBSERVATIONS 
We shall, in general, describe the results of the 
oceanographic program of the Smith in the fol¬ 
lowing order: The general circulation, the vertical 
distribution of variables, their horizontal distri¬ 
bution, and finally, a discussion of the significance 
of the circulation features to the distribution and 
abundance of the biota. In each section we shall 
d^cribe first the conditions in the Countercurrent, 
then those in the South Equatorial Current; the 
latter with particular attention to conditions 
along the Equator. 
CIRCULATION FEATURES 
As an introduction to the discussions of the ob¬ 
served vertical and horizontal distribution of the 
oceanographic properties, we shall first describe 
the general circulation features for the area and 
period of the Smith cruise. The direction of 
flow normal to the three oceanographic sections 
(henceforth referred to as 110° W., 120° W., and 
140° W.) was determined by means of geostrophic 
calculations. Inferences as to direction in' the 
region of the Countercurrent and elsewhere in the 
area where density data were lacking, were made 
from variations in the temperature-depth distri¬ 
bution. Information on variations in velocity was 
derived from changes in wire angles between suc¬ 
cessive Nansen-bottle casts. Frequent references 
will be made to the results of the GEK measure¬ 
ments made aboard the Baird and the Horizon. 
The geopotential anomalies were computed, 
with pressure terms neglected, directly from the 
oceanographic station graphs (see King et ah, 
1957, fig. 9). The average values of thermosteric 
anomaly for depth intervals of 100 meters in the 
deeper layers and 10 meters in the thermocline 
and mixed layer, were multiplied by the pressure 
interval in decibars and then were summed up¬ 
ward from the 700-db. level. The 700-db. surface 
was used as the reference level since the bottom 
bottle on several stations was down less than 800 
meters due to large wire angles. 
In figure 2, the heights of the sea surface in 
dynamic centimeters (as calculated from the sta¬ 
tions along 110°, 120°, and 140° W.) have been 
contoured relative to the 700-db. surface. The 
configurations of the isopleths for the geopotential 
anomaly between the three longitudes were drawn 
from consideration of the temperature-depth data. 
The inferred direction of flow is shoAvn by the 
heavy arrows. 
The station-to-station variation (meridional 
slope) of the 0/700-db. surface for each of the 
three sections is shown in figure 3. The slope, 
indicative of the speed of westerly flow, is steeper 
between the Equator and 5° N. on the 110° lY. and 
120° W. sections being, respectively, 0.28 and 0.42 
dynamic meters in 570 km. This is to be com¬ 
pared with 0.07 dynamic meter over the same dis¬ 
tance along 140° W. The surface velocities rela¬ 
tive to the 700-db. level (as well as the velocities 
for the remainder of the Smith sections) are given 
in the following table. As the geostrophic approx¬ 
imation is not considered applicable near the 
Equator, relative velocities between 3° X. and 3° 
S. have not been calculated. 
Latitude 
140° 
w. 
120° 
W. 
110° 
W. 
Speed 
(knot) 
Direc¬ 
tion 
Speed 
(knot) 
Direc¬ 
tion 
Speed 
(knot) 
Direc¬ 
tion 
7° N-6° N_ 
0.2 
E 
6° N-.5° N_ 
0.4 
W 
.4 
W 
5° N-4° N_ 
0.1 
W 
2.3 
W 
.7 
W 
4° N-3° N_ 
.2 
W 
1.8 
W 
.8 
W 
3° S-4° S_ 
.6 
W 
.2 
w 
.4 
E 
4° S-5° S_ 
1.0 
W 
.8 
W 
1. 1 
W 
5° S-6° S_ 
.3 
W 
.7 
W 
. 5 
E 
6° S-7° S_ 
.3 
w 
.6 
W 
Figure 2. —Variations in the geopotential topography, 
0/700-db. surface as calculated from Eastropic data. 
The arrows denote the direction of flow. (Contour in¬ 
terval, 5 dynamic centimeters.) 
