276 
FISHERY BULLETIN OF FISH AND WILDLIFE SERVICE 
isotherm, 80° W. to the 180th meridian are sliown. 
The hatched “envelope” shows the approximate 
range of variation in depth among these data. 
Part of this variation resnlts from seasonal fluc¬ 
tuations in the depth of tlie thermocline (Austin 
1058), and part from the ridgelike configuration 
of the thermocline between 2° N. and 2° S. lati¬ 
tude. Data obtained prior to Eastropic for the 
months October through December (Eastropic 
cruise period) are shown by the solid dots and 
those during Eastropic by the small circles. Com¬ 
parison of the mean (a curve through the center 
of the hatched band), the October-December data 
(dots), and Eastropic data (circles) demonstrates 
the comparatively steep slope of the thermocline 
during Eastropic. It is concluded that the com¬ 
paratively steep slopes of the sea surface and the 
thermocline are associated with a more than nor¬ 
mally active zonal circulation. 
In the section describing the vertical distribu¬ 
tion of temperature, we mentioned the 2-layer sys¬ 
tem which is typical of the tropics—the essentially 
homogeneous water from the surface down to the 
top of the thermocline, the thermocline (stable 
layer) through which the temperature decreases 
rapidly with increasing depth, and the extensive 
depth range beloAV the thermocline to the ocean 
floor through Avhicli there is but a comparatively 
slight further decrease in temperature. This sit¬ 
uation is typical of waters in the low latitudes 
throughout the seasons. In the middle and high 
latitudes, howeA^er, a deep mixed layer exists at 
the end of Avinter. As the season progresses, the 
depth of the mixed layer decreases to the mid¬ 
summer minimum and the depth of vertical turbu¬ 
lence is thus progressiA^ely restricted. 
The development in the spring of the seasonal 
thermocline is termed “stabilization” by Sverdrup 
(1958, p. 291). He demonstrated that the onset 
of stabilization folloAving the period of deep 
Avinter mixing played an important role in the 
ATrnal increase in biota. In the Ioav latitudes, 
there is no such period of stabilization—the verti¬ 
cal density structure during all months is char¬ 
acterized by a mixed layer beloAV AAdiich the density 
increases rapidly Avith deptli and turbulence is 
sup])ressed. Therefore, AATen considering geo- 
gra])hical and temporal variations in measure¬ 
ments of the biota in tropical oceans, AA^e must 
look for mechanisms that will affect the degree of 
stability or the depth of this stable layer in rela¬ 
tion to the compensation depth. Within the area 
studied from the ^^7iiifh during expedition 
Eastropic, Ave have mentioned several such 
mechanisms, including divergence of the surface 
Avaters and upAvelling at or near the Equator, the 
effects of sheer and associated mixing at the 
boundaries of opposing currents, and the shalloAV- 
ing of the stable layer to a depth that Avill bring it 
Avithin the euphotic zone, e.g., at the northern 
boundary of the Countercurrent. Although there 
may be seasonal Auiriations, these mechanisms are 
all primarily related to horizontal and Awtical 
transport features (as contrasted AAuth the spring 
Avarming and stabilization and the fall cooling and 
OA^erturn in the higher latitudes). 
In higher latitudes, folloAving stabilization, the 
nutrients in the mixed layer are quickly depleted 
by biological utilization and fallout of the organic 
material into or beneath the stable layer. Until 
the fall overturn and associated replenishment 
from beloAv, the nutrient concentration in the 
mixed layer represents, ])rimarily, a balance be- 
tAveen utilization by the phytoplankton and the 
biological regenerative processes Avithin this layer, 
with A^ertical diffusion playing a comparatWely 
minor role. This situation characterizes vast areas 
of the tropical oceans during all months of the 
year. In figure 22, the vertical distribution of 
A^arious properties is sIioaaui to illustrate conditions 
within the mixed, the stable, and the deeper layers 
at a position in the region of convergence north 
of the Equator (3° N.). It is to be noted that, at 
the particular station illustrated, Avithin a very 
limited range of depth (^50 meters), there is an 
abrupt change in the fields plotted. The tempera¬ 
ture decreases nearly 10° C., the thermosteric 
anomaly nearly 800 centiliters per ton (60 percent 
of total change, surface to 800 meters) and, of par¬ 
ticular biological importance, the phosphate sud¬ 
denly increases from 0.6 to 1.4 /xg.at./E. The other 
nonconservative property, oxygen, decreases from 
nearly 4 ml./L. to slightly less than 2.0 ml./L. 
Avithin the same depth increment. 
Nearer the Equator, hoAvever, the vertical dis¬ 
tribution of ])roperties differs someAvhat from that 
shown in fimire 22. In the discussion of the verti- 
cal distribution of oxygen, Ave described a feature 
Avith Avaters of comparatively high oxygen content 
that Avas positioned beneath the Equator. In each 
OCEANOGRAPHY OF EAST CENTRAL EQUATORIAL PACIFIC 
277 
PO 4 a O 2 —1.00 - 2.00 - 3.00 - 4.00 - 5.00 
DELTA-T-400 - 300-200-100-0 
SALINITY—34.20-34.40-34.60-34.80-35.00 
TEMP °C-9-13-17-21-25 
(/iS.at./L.) fraiii Hugh M. Smith station 4, ()3°13' N. 
latitude, W. longitude. 
of the four sections shoAvn in figure 11, this feature 
extended tertically from Avithin or immediately 
beloAv the therinocliue to a maximum depth of 
between 800 and 400 meters and was approxi¬ 
mately 200 miles wide. The oxygen content near 
its center Avas 2.8 to 8.0 ml./L. at 140° W., decreas¬ 
ing eastAvard to 2.5 to 2.7 near 100° W. 
Consideration of the possible causes for this 
geographically restricted feature leads to the con¬ 
clusion that it is the result of advection. In the 
first place, the deeper Avaters are essentially iso¬ 
lated from the oxygen-ricli Avaters of the mixed 
layer above the thermocline. Even thounrh isen- 
tropic principles may not apply near the Equator, 
the configuration of the density field shoAvn in 
figure 10 suggests that there is little mixing be- 
tAveen the waters in the surface layer and those 
beneath the thermocline. The oxygen content at 
depths betAA'een 100 and 800—400 meters decreases 
both to the north and to the south of the Equator. 
To emphasize the meridionally limited extent of 
this feature, the distribution of oxygen on a sur¬ 
face of constant density (180 centiliters per ton) is 
shoAvn in figure 28. In a narroAv band along the 
Equator, between 140° W. and 120° W., the oxygen 
values are 8.0 ml./L. or greater. BetAA^een 120° W. 
and 110° W., the values decrease someAvhat, then 
increase again farther to the east. If lateral mix¬ 
ing were significant across the Equator, the feature 
in question Avould be eliminated. 
Figure 23. — Distribution of oxygen (ml./L.) in red on density surface (IcSO cl./ton) in black. Depth of density surface 
in meters. 
