Physical Oceanography

in the

Marshall Islands Area

Bikini and Nearby Atolls, Marshall Islands

GEOLOGICAL SURVEY PROFESSIONAL PAPER 260-R
Physical Oceanography

in the

Marshall Islands Area

By HAN-LEE MAO and KOZO YOSHIDA

Bikini and Nearby Atolls, Marshall Islands

GEOLOGICAL SURVEY PROFESSIONAL PAPER 260-R

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1955
UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington 25, D. C. - Price 35 cents (paper cover)
CONTENTS

Abstract. _ __--_---.--_--_-_---__--_________________

Introduction.______________________________________

Acknowledgment- __________________________________

Water circulation.__________________________________

Latitudinal variation of isobaric topographies. _____

Horizontal current systems at various depths___--_- 

Current at surface or near surface___________

Current at middepth levels_________________

Currents at deeper layers.___________________

Comparison of geostrophic flow and direct 

measurement of current---....-----_______

Dynamics of large horizontal eddies.______________

Generation.---_ _-_-_-__-_-_____-______-____

Maintenance.______________________________

Mass transport.--__-__--___---_-___--___-__-___ 

Distribution of temperature and salinity.______________

Vertical distribution..-.-_--__--_-_______________

Page

645

645

647

647

647

649

649

649

649

653

653

653

654

655

656

657

Distribution of temperature and salinity—Continued 

Horizontal distribution.___-_-__-______-_____-.

At the surface______---__-_--_--_-_--_-__.

At the 100-meter level_____-_____-___-__.

At the 250- and 300-meter levels________..

At the 500-meter level___________________

Concluding remarks.______________________

Seasonal variations__________________________

Distribution of density._______-_____--___--__-_--.

At the surface___-------------_---------------

At the 100-meter level_______-____-_--_-___.-

At the 250-meter level____----_________________

At the 500-meter level_________________________

Temperature-salinity ( T—S) relationship_____________

General relationship.______-____-__-_-_-_______

In the southern region_______________________

In the central region._____---___________-___-_

In the northern region_______________________

Remarks on the distribution of oxygen._____--____-_

References and literature cited._____------__--_---_

Page 

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663 

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669 

669 

669

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674

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677

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679

681

684

ILLUSTRATIONS

Page

FIGURE 179. Station plans. A, U. S. Operation Crossroads, March 11 to August 10, 1946. B, Japanese Hydrographic Investigations, 1933-41___________________________________________________________________ 646

180. Vertical distribution of isobaric topography relative to the 1,000-dbar surface.____________________________ 648

181-183. Horizontal distribution of dynamic-height anomalies.

181. At surface________________________________________________________________________ 650

182. A, At 250 meters. B, C, At 300 meters______._____________________________________ 651

183. At 500 meters_____________________________________.__--____.____________________ 652

184-186. Schematic representations.

184. Horizontal eddies produced in the boundary shear zone_____-_-___-___-_-___-_-___-_-_______-____-_ 654

185. Horizontal eddy caused by a barrier_____________________________________________________________ 654

186. Forces acting in a horizontal eddy___-______________________-_______-_____________-_________-____ 654

187-190. Latitude dependence.

187. Mean wind stress________________________________________-___-_________---____________________ 655

188. Integrated pressure gradient__________________________________________________________________ 656

189. East-west component of mass transport.-.________________________________________________________ 657

190. North-south component of mass transport______________________________________________________ 657

191-193. Vertical distribution of—

191. Temperature, salinity, and <rt values at station B-39_____________________________________________ 658

192. Temperature___________________________________________________________________________ 659

193. Salinity__._____________-__________________________________---________-______._-_-__--_-.___ 661

194-201. Horizontal distribution of—

194. Temperature at surface__--_____-_-_______-___-_____---_____-_-_--_-__-_-_---__-______-__--_-_- 662

195. Salinity at surface___________________________________________________________________________ 664

196. Temperature at 100 meters___________________________________________________________________ 665

197. Salinity at 100 meters_______________________________________________________________ 666

198. Temperature. A, At 250 meters. B, C, At 300 meters_____-______-____________-__----___.__ 667

199. Salinity. A, At 250 meters. B, C, At 300 meters_______________._________________-______-_____ 668

200. Temperature, at 500 meters________________________________________________________________ 670

201. Salinity, at 500 meters________________________________________________.-___.-_-_____ 671

in
IV CONTENTS

FIGURE

202-203. Seasonal variation in the upper mixed layer. Page

202. Temperature______________________________________________________________________________ 672

203. Salinity_--_---..--_-_--_-______________________________-________-______--___--_----_-______-__ 673

204. Vertical distribution of density_______________________________________________________________________ 674

205. Horizontal distribution of density. A, At surface. B, At 100 meters. C, At 250 meters. D, At 500 meters. _ 676 

206-212. Temperature-salinity diagrams.

206. 4° S.-2° S., Crossroads data__--_____-_____-________-___--__--___-___--__----._-_---__________ 677

207. 4° S.-2° S., Japanese data_________________________________________________________.____ 677

208. 0°-2° N., Crossroads data___________________________________________________________________.__ 677

209. 0°-2° N., Japanese data____________________________________________________________._________ 678

210. 11° N.-120 N., Crossroads data-_-_____-__--__---__-__.-__-__-_--_--._-__ —— ._--. — --- — . — .__ 678

211. 10° N.-120 N., Japanese data, summer____________________________________-__-___---_--_-___-__ 678

212. 10° N.-120 N., Japanese data, winter_____________________-___-___--__-______-_----------_-_--_-_ 679

213. Stratification of water masses in the Marshall Islands area______--______-__ _____________________________ 679

214-216. Temperature-salinity diagrams.

214. 17° N.-200 N_____________-_____--_._________________________________________________________ 679

215. 18° N.-200 N., Japanese data, summer_________________..________________-____-_--__-----_-____ 679

216. 18° N.-200 N., Japanese data, winter_____________________. __________________________________ 680

'217. Vertical distribution of oxygen concentration____________________. _____________________________________ 682

218. Horizontal distribution of oxygen concentration._________________-_____-__-_-_---_--_-_-_---_----_-_-- 68S
BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

PHYSICAL OCEANOGRAPHY IN THE MARSHALL ISLANDS AREA

By HAN-LEE MAO and Kozo YOSHIDA

ABSTRACT

The major features of physical oceanography in the Marshall 

Islands area are described in separate sections. Owing to the 

important role played by the system of water circulation in the 

shaping of other features, discussion of the circulation system is 

somewhat stressed. In addition to the flow patterns derived 

from dynamic anomalies, some directly measured current data 

are included, the large-size horizontal eddies which appear in the 

boundary zone between the North Equatorial Current and the 

Equatorial Countei current are discussed, and computations of 

mass transport are made. Distribution of temperature, salinity, 

density, and temperature-salinity (T-S) relationships are discussed mainly in light of the circulation system and the characteristics and movement of water masses. Only some general remarks are made on the distribution of oxygen because of the 

scarcity of data available.

An outstanding feature which appears to characterize physical 

oceanography in this area is the unique distribution of all the 

major properties in the areas of large eddies. It is to be noted 

that particular attention is always placed on the descriptions and 

interpretations of these outstanding features in this area of 

eddies.

INTRODUCTION

The primary purpose of this work is to present the 

general features of physical oceanography, except waves 

and tides, in the Marshall Islands area. The geographical area covered by this report extends from latitude 

5° S. to 20° N. and from longitude 155° E. to 175° E. 

However, owing to the scarcity of data—especially, 

more recent data—south of the Equator, conclusions 

drawn as to the Southern Hemisphere are tentative.

General distribution of oceanographic conditions in 

this area is shown on horizontal and vertical charts. 

]ti this report only the major features are described, 

and reasonable explanations are given. Particular emphasis is always placed on the interrelated phenomena 

in various aspects.

The data which we used here are derived from (1) 

United States Operation Crossroads from 11 March to

i Contribution from the Scripps Institution of Oceanography, New Series, No. 644. 

NOTE.—This work is partly supported by the Office of Naval Research.

10 August 1946 (fig. 179A) and (2) Japanese Hydrographic Investigations from 1933 to 1941 (fig. 1795). 

For convenience, we shall refer to these two sets of data 

as the Crossroads data and the Japanese data. The 

value of the Crossroads data, which were taken more 

recently and with more modern instruments, is offset 

by the limited number of stations occupied, particularly 

since most of the stations were concentrated around 

Bikini Atoll. Very few stations were occupied south 

of latitude 10° N., and therefore available information 

in the Equatorial Countercurrent area is far from sufficient. On the other hand, the Japanese data have 

merit because many stations (670) were used and because there was a rather even distribution of stations. 

These advantages are partly offset by the inhomogeneity of the data. This inhomogeneity may be real, 

in the sense that it represents the actual short- or longperiod variations of the oceanic conditions during a 

period of almost 10 years. However, it may also be due 

partly to progress in the use of instruments and the 

consequent accuracy of observation in oceanographic 

investigation.

In view of this, the two sets of data are charted 

separately, and only major features which appeared in 

them are discussed. To reveal some characteristics of 

the seasonal variations in hydrographic conditions, the 

Japanese data were divided, wherever possible, into two 

seasons: summer, from April to September, and winter, 

from October to March. This division is mainly in 

accord with the study by Robinson (1954) of bathythermograph temperatures in the same area. However, it must be remarked that the monthly distribution 

of data is very uneven.

It is unfortunate that, owing to the lack of reliable 

data on wind and humidity covering a sufficiently long 

period, the energy budget, one of the most interesting 

topics in physical oceanographj7 , cannot be pursued in 

this study.

645
155" 160°

~1——

170° 

FIGURE 179.—Station plans. A, U. S. Operation Crossroads, March 11 to August 10,1946. B, Japanese Hydrographic Investigations, 193
PHYSICAL OCEANOGEAPHY IN THE MAESHALL ISLANDS AEEA 647

ACKNOWLEDGMENT

The authors wish to express their sincere thanks to 

Prof. C. E. Palmer, Institute of Geophysics, University 

of California at Los Angeles, who kindly furnished some 

unpublished original wind data; and to Mr. Daitaro 

Shoji, Japanese Hydrographic Office, who supplied 

valuable suggestions for analyzing Japanese data. The 

permission to use and publish the Japanese hydrographic data was kindly granted by Dr. Kanji Suda, 

Director of the Japanese Hydrographic Office.

WATER CIRCULATION

Apart from water circulation in the upper mixed 

layer, which has been separately discussed in another 

study by the writers and Paul L. Horrer (Yoshida 

others, 1953), the water circulation in this investigation is that delineated by geostrophic flow only. 

Some direct current measurements from the Japanese 

Hydrographic Office (Japanese Hydrographic Department, 1933; Japanese Hydrographic Bureau, 1948; 

and Japanese Maritime Safety Agency, 1950) are included for comparison. In the dynamic computations 

a depth of 1,000 m is customarily adopted as the level 

of no motion.

For brevity, the major features of geostrophic circulation in the Marshall Islands area are presented in 

four parts. First, the presentation of vertical profiles 

of latitudinal variations of isobaric topography is 

intended to give a brief view of current systems. 

Second, the general flow pattern at various levels is 

illustrated by charts of horizontal currents. Third, 

dynamics of large-scale, semipermanent horizontal 

eddies, which appear outstandingly in the boundary 

between the Countercurrent system and the North 

Equatorial Current system, call for closer attention 

and are discussed separately. Fourth, mass transport 

from the surface to the depth of no motion is calculated.

LATITUDINAL VARIATION OF ISOBARIC TOPOGRAPHIES

Three sections of vertical profile of isobaric topography at equigeopotential surfaces, representing, 

respectively, the Crossroads data, the Japanese data 

for summer, and the Japanese data for winter are 

shown as figures 180. These sections are selected in 

such a way that each passes through approximately the 

central part of the area and trends approximately 

northward. In order to include a maximum number 

of stations in each section, those stations which do not 

fall along the north-trending line exactly, but within 

the distance of )£° of longitude, are also included in the 

vertical profile. Thus, we have selected lines running 

approximately along lontitude 166° E. and 164° E for

the Crossroads and the Japanese sections, respectively.

Some of the most pronounced features from these 

sections of vertical profiles are summarized as follows:

At the Equator and in its vicinity the isobaric 

topography of the Crossroads and the Japanese sections present opposite features. In the Crossroads 

section (fig. 180A), all isobaric surfaces reach maximum 

values: in the Japanese sections (fig. 180B, C) all isobaric surfaces attain minimum values. Figure 180A 

indicates that for the Crossroads sections the current 

flows toward the East near the Equator in both hemispheres; that is to say, the Countercurrent system 

extends to the Southern Hemisphere, whereas figure 

1805, C shows the South Equatorial Current system 

extending to a latitude of about 2°-3° N. Although 

it is generally recognized that the Countercurrent 

system is always found in the Northern Hemisphere, 

the extension of the system into the Southern Hemisphere is, of course, dynamically possible. Dynamic 

conditions require only that all isobaric surfaces must 

reach their extreme values at the Equator or near it, 

regardless of whether the value is a maximum or a minimum. In fact, Sverdrup (Sverdrup and Fleming, 1944) 

noted in the Western Pacific that the Carnegie results 

showed the Countercurrent extending from latitude 6°20' 

N. to 3°20' S. This northern limit coincides very well 

with our figure 180A; the southern limit of the Countercurrent is not clear in the Crossroads sections. The 

temporal and spatial variations of the position of the 

Countercurrent have been mentioned in Robinson's 

recent work in the analysis of bathythermograph data 

of the same area (Robinson, 1954). It is unfortunate 

that owing to the heterogeneity of the Japanese data 

no definite conclusion can be drawn as to the nature of 

seasonal variations of the current systems at or near 

the Equator.

From the southern boundary of the Countercurrent 

system, isobaric surfaces in all three sections dip to the 

north, and reach their minimal values—that is, the 

northern boundary of the Countercurrent system, at 

latitudes about 6° N. and 9° N. respectively for the 

Crossroads and the Japanese sections. Comparing 

figure 180A -and figure 1805, C, it is immediately seen 

that the positions of current systems from the Crossroads sections shift about 3° of latitude southward 

from those of the Japanese sections. Positions of 

three Equatorial current systems of the Japanese 

sections agree fairly well with others. The nature of 

the "southerly shift" of the Crossroads sections requires further investigation, and it will be discussed in 

the section on mass transport.

Commencing at the northern boundary of the 

Countercurrent, we enter into the system of the 

Northern Equatorial Current; in figure 180A, B, C, all
ju a. 

5 F 

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*~t j2 

&< %. 

METERS DYNAMIC 

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b in b 01 O 

•

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O! <f> OD 

88 8 . 8 1 o o 

999

i

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METERS DYNAMIC 

01
PHYSICAL OCEANOGRAPHY IN THE MARSHALL ISLANDS AREA 649

isobaric surfaces north of the boundary tilt up toward 

the north, meaning that the currents flow due west. 

The northern boundary of the Northern Equatorial 

Current system is probably out of the area considered 

in this study.

Another unusual feature in the Crossroads section is 

the appearance between latitudes 16°-18° N. of a relatively sharp "hump" in the isobaric topography in 

figure 180 A, meaning that a narrow band of strong 

easterly current exists there. Owing to the scarcity 

of observations there and the fact that no similar 

phenomenon occurs in the Japanese sections or other 

data, it is questionable whether this "hump" does 

mean a true easterly current. It should, however, be 

mentioned that a similar "hump" had been noted by 

Cromwell (1950)—at similar latitudes (18°~20° N.), 

though at a very different longitude (approximately 

along longitude 158° W.)—in one section of the isobaric 

topography in his study of the oceanography of the 

Mid-Pacific.

• HORIZONTAL CURRENT SYSTEMS AT VARIOUS 

DEPTHS

Three levels—that is, at the surface and at 250 m 

and 500 m—have been chosen as representative of 

surface (and near surface), middepth, and deeper layers. 

Since there shall be no discussion of the current features 

which do not appear in the charts of geostrophic flow, 

readers who are especially interested in the characteristics of surface flow in this area are referred to another 

paper (Yoshida and others, 1953).

CURRENT AT SURFACE OR NEAR SURFACE

[Fig. 181]

One of the most important features which appears in 

all these figures is the cyclonic eddy (or several eddies) 

at the northern boundary of the Countercurrent 

system. Its location is about latitude 6°-8° N. in 

the Crossroads data (fig. 181A) and latitude 8°-10° N. 

in the Japanese data (figs. 1815, C). Because of the 

outstanding nature of these eddies and their important 

bearing on the other aspects of physical oceanography, 

they will be discussed in detail immediately following 

this section.

At the surface, velocity of the Countercurrent is 

fairly high, though it varies considerably within the 

system. The highest velocity, about 2 knots, generally 

appears at the southern edge of the eddies; that is, at 

about latitude 2°-4° N. in figure 181A and at latitude 

4°-6° N. in figure 18l£, C. Since the validity of the 

geostrophic flow at very low latitudes is open to question, too much emphasis should not be placed on the 

numerical values of current velocity calculated from 

dynamic computations.

315915—55———2

The over-all mean of current velocity, from dynamic 

computation of the North Equatorial Current system 

in this region, is about % knot. The highest velocity 

reaches 1 knot in some localities at the northern edge 

of the eddies, at about latitude 10°-12° N. and latitude 

12°-14° N. for the Crossroads and the Japanese restdts, 

respectively. The "looping" of the current in the 

northern side of Bikini Atoll is probably due to the 

damping effect of the island on the flow.

The narrow band of the (uncertain) easterly flow 

which appears at latitude 16°-18° N. from the Crossroads results was presented in the chart only tentatively. The velocity of this flow is relatively high, 

amounting to about 0.8 to 1.0 knot.

CURRENT AT MIDDEPTH LEVELS

[Fig. 182]

The characteristic features of the geostrophic flow 

patterns at middepths are quite similar to those at 

the upper levels. The intensity of the horizontal 

eddies at the northern boundary of the Countercurrent 

becomes very weak at these levels, as shown in figure 

182,4, B, but remains practically unchanged in figure 

182C. The size of these eddies seems to increase with 

depth, and a large part of the territory of the Countercurrent at upper levels is occupied by horizontal eddies 

at middepth. This agrees with Sverdrup's suggestion 

that the Equatorial Countercurrent is the flow of a 

relatively thin layer. However, it does not necessarily 

mean that the Southern Equatorial Current system 

underlies the Countercurrent system as Cromwell (1950) 

remarked. In fact, both theoretical results (Yoshida 

and others, 1953) and direct measurements 2 have 

shown that the opposite might be true. A reasonable 

interpretation might be that the Countercurrent velocities decrease very suddenly below the intersurface of 

discontinuity or the thermocline. This renders the 

velocities of flow there almost immeasurable, a fact 

noted by the Shellback Expedition and conforming to 

figure 182A, B.

CURRENTS AT DEEPER LAYERS

[Fig. 183]

Except for the continuing decrease of velocities, the 

geostrophic flow pattern at a depth of 500 m retains 

characteristics quite similar to those of upper levels. 

The horizontal eddies at this depth now become so 

large that the whole system of the Countercurrent is 

nearly merged into the eddies, but the direction is still 

due east. As depth increases the damping effect of 

the islands on the eddies becomes more pronounced, 

because (1) the cross-sectional areas of the islands

* Wooster, W. S., 1952, Preliminary report, Shellback Expedition, May 17 to August 

28,1952, Univ. Calif. Scripps Inst. Oceanography, SIO Ref. 52-47 (mimeographed).
650 BIKINI AND NBAEBY ATOLLS, MARSHALL ISLANDS

Cm/sec O IO 2O 3O 4O 5O 6O 7O 80 9O IOO 

.4 .6 .8 I.O 1,2 1.4 1.6 1.8 

CURRENT SPEED

155° 160° 165° 170° 175°

FIGTJEE 181.—Horizontal distribution of Dynamic-height anomalies at Surface. A, From Crossroads data, March to August 1946. B, From Japanese data, 1933-41, summer

season. C, From Japanese data, 1933-41, winter season.
160° 

0° —

J65L

PHYSICAL OCEANOGRAPHY IN THE MARSHALL ISLANDS AREA

170° 175° 1»° 160" ........ 165"

651

20°

085-—.___

10°

Cm/sec 0 10 20 30 40 50 60 70 80 90 100 5° 

Knots 0 .2 .4 .6 .8 1.0 I.Z 1.4 1.6 1.8

CURRENT SPEEO

155° 160° 165° 170° 175°

FIGUKE 182.—Horizontal distribution of dynamic-height anomalies. A, At 260 meters, from Crossroads data, March to August 1946. B, At 300 meters, from Japanese data,

1933-41, summer season. C, At 300 meters, from Japanese data, 1933-41, winter season.
652 BIKINI AND NEARBY ATOLLS. MARSHALL ISLANDS

Cm/ttK 0 IO 20 30 40 50 60 70 80 90 100 

Knots: 6 .2 .4 .6 .8 1.0 1.2 J.4 1.6 1.8 

CURRENT SPEED

055

\

155" 160° 165° 170° 175°

FIGURE 183.—Horizontal distribution of dynamic-height anomalies at 500 meters. A, From Crossroads data, March to August 1946. B, From Japanese data, 1933-41, summer

season. C, From Japanese data, 1933-41, winter season.
PHYSICAL OCEANOGRAPHY IN THE MARSHALL ISLANDS AREA 653

increase with increasing depth, and (2) the eddies become so weak, and hence unstable, that they are very 

easy to split. It is clearly seen from figure 183(7 that 

the eddies are split into smaller ones.

It is also interesting to note that the narrow band 

of easterly flow at latitude 16°-18° N. in figure 183A 

maintains an unusually high velocity at such depth. 

This suggests it may be apparent, owing to tidal effect, 

rather than a reality.

COMPARISON OF GEOSTROPHIC FLOW AND DIRECT 

MEASUREMENT OF CURRENT

During those years when the Japanese Hydrographic 

Office was surveying this area, certain direct measurements of current were taken by two methods: by use 

of the floating buoy and by use of the Ekman current 

meter, which was lowered to a preassigned level from 

an anchored or drifting ship. When the floating ship 

is on the open sea, to determine the drift of the ship 

to an acceptable degree of accuracy is not easy. The 

method which the Japanese Hydrographic Office (1933) 

adopted is to assume a level of no motion, thus allowing the drift of the ship and consequently the measured "relative current" with reference to the fixed level 

of no motion to be determined. For the area of the 

tropical Pacific, a depth of 400 m was accepted as the 

level of no motion.

It is to be noted that the accuracy of the results of 

direct measurement of current is affected by (1) the 

instrumental accuracy, (2) effect of the tidal current, 

and (3) temporal and local variations, especially in the 

upper wind-stirred layer. The Japanese Hydrographic 

Office (1933) stated that in general they believed the 

accuracy of their current meters was within 10 degrees 

in direction and 0.2 knot (10 cm/sec) in magnitude. 

For the whole area of concern the maximum magnitude 

of tidal current was calculated about 5 cm/sec, if we 

assume that the mean depth is 2,000 m and that the 

maximum amplitude of the tides in the open sea is 

50 cm. Thus, the probable error due to instrumental 

and tidal effects amounts to 15 cm/sec, not including 

such temporal variations as wind effect.

It is immediately seen that when the currents are 

weak, the results of the direct measurement of currents 

are not very reliable because the actual magnitude of 

current velocity might be smaller than the probable 

error. However, comparing the results of the direct 

measurements—which for the purpose of convenience 

are entered as arrows in the horizontal current charts 

at the surface and at 300 m, shown as figures 181.5 and 

1825—with geostrophic flow at corresponding depth, 

the agreement of their major features is fairly satisfactory. Some of them can be stated as follows:

In moderate and strong current, the direct measurements provide sufficient and reliable information of the 

magnitude as well as direction of the current velocities.

The positions of the Equatorial Current systems, 

derived from geostrophic flow and direct measurement, 

generally agree with each other.

The results of direct measurement appear to indicate 

the existence of eddies at the northern boundary of the 

Countercurrent, where the arrows of measured current 

seem to have a counterclockwise rotation.

DYNAMICS OF LARGE HORIZONTAL EDDIES 

GENERATION

The charts of dynamic topography appear to indicate 

that there exist large-scale horizontal eddies between 

4° N. and 10° N., corresponding to the northern boundary of the Countercurrent. Although it is not necessarily evident whether the occurrence of these eddies is 

real or apparent, some of the recent results from direct 

measurements of currents appear to support the existence of these eddies (Cromwell, 1950) .3

It is to be remarked that each of the eddies found in 

our data has an approximate radius of several hundred 

kilometers and that no eddy is found in the southern 

boundary of the Countercurrent.

Although the cause of these eddies is not yet clear, 

several possible interp ^ations are proposed, as follows:

First, it is consider- JL very probable that these eddies 

are of a similar nature to those found in the outer area 

of the Gulf Stream. The system which involves the 

Countercurrent flowing in the opposite direction to the 

surrounding currents may be analogous to the (Grulf 

Stream system. The eddies are expected to be generated 

by cut-off from the meandering current system; this 

can be produced in the boundary zone between two 

different current systems and actually is found to exist. 

(Haurwitz and Panofsky, 1950.)

It is desirable to examine the question of whether this 

consideration can account for the wavelength of the 

meandering or the dimension of the eddy and also for 

the fact that no eddy can be found in the southern 

boundary of the Countercurrent. A theoretical investigation will be made in the near future. In the present 

report our discussion is confined to possibilities.

The second possibility is related to turbulence in this 

boundary zone between two different systems. Since 

in such an area the current shear is very large, we can 

expect considerable intensive horizontal mixing, and 

under this circumstance some external disturbances can 

produce eddies in this region (fig. 184). Such disturbances may be caused by bottom topography, islands, or 

steady or unsteady meteorological conditions. So far 

as we have examined, the predominance of the effects

s Also Wooster, W. S., op cit.
654 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

of islands is considered most probable, judging from the 

location of the eddies. Furthermore, the existence of 

islands may effect the flow in such a manner that the 

intensity of turbulence increases owing to friction. 

Since most of the islands in this area are located from 

5° N. to 12° N. and from 165° E. to 173° E. this 

consideration might lead to an interpretation of the 

particular localities and dimensions of the eddies.

FIGURE 184.—A schematic representation of horizontal eddies produced in the 

boundary shear zone.

The barrier effect of islands may be interpreted in 

another way, which provides the third possibility: the 

islands which constitute the Ratak Chain and Ralik 

Chain are assumed to be some kind of barrier. Even 

where there is no significant turbulent motion or 

meandering it may be possible to expect the eddies, as 

illustrated in figure 185.

Which of these possibilities is predominant is not 

easy to say. Actually, it is most probable that these 

factors act together, and in this connection it is desirable 

that further observations and theoretical considerations 

be made.

In the theoretical study of the equatorial currents in 

the upper mixed layer by the present writers and Paul 

L. Horrer (Yoshida and others, 1953), effects of lateral 

mixing were discussed. Some considerations of lateral 

mixing in connection with the dynamics of eddies will 

be presented later.

MAINTENANCE

It is seen that the counterclockwise circulation associates two forces, both of which are directed outward: 

Coriolis and centrifugal. To maintain a steady state, 

radial pressure gradient is required to be directed 

inward. In the northern boundary of the Countercurrent this condition is actually fulfilled (fig. 186^4). On 

the other hand, the major pressure field near the 

southern boundary of the Countercurrent appears to 

indicate that no clockwise eddy can be established 

(fig. 1865). The minimum of integrated pressure P 

is found at about 6° N., from the Crossroads data, and 

at about 8° to 10° N., from the Japanese data, and these 

minima coincide with the centers of the boundary 

zones or horizontal eddies.

It may thus be suggested that lateral mixing takes 

place at the southern boundary of the Countercurrent 

and that momentum is exchanged by small, variable, 

irregular eddies between two current systems. On the 

other hand, in the northern boundary zone the effect of 

mixing is considered a little differently. First, owing

FIGURE 185.—A schematic representation of horizontal eddy caused by a barrier.

to the effects of the islands as well as the large current 

shear associated with the major current field, the 

turbulence may be more intensive, and small, irregular 

eddies are easily generated. Thus, to establish a 

steady-state distribution of current, lateral mixing may

A B

FIGUEE 186.—A schematics! representation of forces acting in a horizontal eddy. 

A, Northern boundary of Equatorial Countercurrent. B, Southern boundary of 

Equatorial Countercurrent. Explanation of symbols: Single straight arrow represents a Coriolis-force vector; double straight arrow represents a centrifugal-force 

vector; wavy single arrow represents a pressure-gradient-force vector.

play an important role. This current field in the large 

eddy should also associate the corresponding radial 

pressure gradient. Since the velocity field as well as 

the pressure gradient field is not completely free from 

those in the surrounding systems it may be expected 

that such forces as Coriolis, centrifugal, pressuregradient, and horizontal mixing are acting simultaneously and tending to bring the field towards a steady 

state. Vertical mixing might also play a part in such 

adjustments, but it is supposedly less significant. If 

this stationary state is established, mixing effects will 

become less. Any temporary or local deviation from 

the stable configuration can be cancelled owing to these
PHYSICAL OCEANOGRAPHY IN THE MARSHALL ISLANDS AREA 655

mutual arrangements. Thus, in this region horizontal 

mixing contributes to maintaining the boundary eddies 

instead of causing direct exchange of momentum 

between the £wo major current systems.

MASS TRANSPORT

The advancement of mass-transport theories by 

Sverdrup (1947), Reid (1948a), Munk (1950) and 

many others marks an important step towards our 

understanding of water circulation in the oceans. 

Reid's result of mass transport, based on the statistical 

mean values of wind stress, provides us with a general 

feature for the Equatorial Eastern Pacific. It is interesting to see whether Reid's major features also apply 

for the Marshall Islands area. Furthermore, in the 

previous sections we mentioned that the current systems obtained from dynamic computations of the Crossroads data are apparently shifting about 3° of latitude 

southward in comparison with those obtained from 

the Japanese data. Does the same phenomenon also 

appear in the picture of mass transport? If so, what 

is the possible nature of such a shift? The intent is to 

investigate these questions in this section.

In order to compute the mass transport for a specific 

region such as the one considered in this study, trustworthy local wind data covering a sufficiently long 

period are required; It is unfortunate that the wind 

data from Operation Crossroads did not cover a period 

long enough so that we can have sufficient confidence

to use them to compute the mass transport of a steady 

flow. The wind data from Japanese cruises are apparently too low, because of the old-type wind vane then 

used.

Under these circumstances, to compute the mass 

transport of this area it seems desirable to supplement 

the wind data from Operation Crossroads with data 

from Operation Greenhouse, which was kindly put at 

our disposal by Dr. C. E. Palmer,4 Institute of Geophysics, University of California at Los Angeles. This 

combination is, of course, a sheer necessity from the 

statistical point of view. It is to be noted that the 

major features of thesa two sets of data are so similar 

that the combination is considered justifiable. The 

table below shows the number of wind observations of 

the two operations.

Stations

Bikini...... ...........

Truk. __ .............

BikatL.. ..............

Tarawa... ... .........

Latitude

19°18' N.

il°21' N. 

8°43' N. 

7°15' N. 

7°06' N. 

6°50' N. 

5°20' N. 

3°0? N. 

1°21' N. 0°33' S.

Longitude

166°37' E.

162°3i'E. 

167°44' E. 

151°51' E. 

171°24' E. 

158°12' E. 

163°05' E. 

172°50/ E. 

172°56' E. 

166°55' E.

Number of wind observationsOperation Crossroads

232 

151 

232 

230 

0 

195 

0 

0 

0 

221 

0

Operation Greenhouse

273 

0 

335 

333 

224 

281 

233 

275 

281 

0 

272

Total

505 

151 

567 

563 

224 

476 

233 

275 

281 

221 

272

* Palmer, C. E., 1951, The tropical Pacific project. Thirteenth Report: Institute 

of Geophysics, Univ. Calif. Los Angeles (mimeographed).

12 13 14 15 16 17 18 19 20

L AT ITUDE 

3456789 10

WAKE

BIKINI

FIGURE; 187.—Latitude dependence of mean wind stress (**), from Crossroads data, March to August 1946, and from Greenhouse data.
656 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

In the computations of wind stress, Reid's method 

has been adopted. A smooth curve of best fit for the 

latitudinal variation was drawn from the computational results. Owing to the relatively small extension 

of the area east and west, only variations north and 

south are concerned. For those stations within 1° of 

latitude, such as Bikini and Eniwetok Atolls, Truk, 

Majuro, and Ponape, the mean value was taken by 

averaging the number of stations considered.

The computational results show that for most of the 

stations the north-trending component of the wind 

stress is normal to the zero mean, or nearly so. Therefore only the east-trending component should be considered. The latitudinal dependence of the easttrending component of the mean stress TX, taken from 

the wind data of Operation Crossroads and Operation 

Greenhouse, is shown in figure 187.

Values of the first and the second derivatives of TX 

with respect to latitudinal distance—that is, 8Tx/dy and 

tfTz/Sy2—are the finite differences taken from the curve 

in figure 187. The vertically integrated pressure P is 

evaluated from the numerical integration of the 

expression

cd

J= pdz, 

Jo

where p is the hydrostatic pressure and integration is 

performed from the sea surface to the depth of no 

motion, assumed a level of 1,000 m. The horizontal 

gradient of P north and south—that is dP/8y from the 

Crossroads data along a section running approximately 

along 166° E. (shown in figure 180A) is calculated 

and entered in figure 188.

Reid shows that mass transport can be computed by 

two independent sets of solutions: one in terms of wind 

stress—for which Reid's solutions have been used— 

and the other based mainly on oceanographic data— 

for these data except for the region in the vicinity of 

the Equator, our solutions, as developed in another 

paper (Yoshida and others 1953), are essentially the 

same as those of Reid's. This is especially true when the 

wind component trends predominantly east and west 

and the horizontal gradient of P east and west is smaller 

than that north and south. Numerical computations 

by means of the second set of solutions show that the 

contributions of "drift current" to the mass transport 

is negligibly small. In other words, for practical purposes the second set of solutions is only an integration 

of geostrophic flow. Therefore, in the region of very 

low latitude only the first set of solutions (those in terms 

of wind stress) should be used. In the calculation of 

Mx by means of this set of solutions the quantity 

|Xi—JLQ| (the longitudinal distance between the eastern 

boundary and the western path) is assumed as 8X108

cm, that is, the approximate distance between the 

outer boundary of the California Current and the mean 

path of the area considered. Because of the small 

coverage of the area east and west, no reliable mean 

value of SP/8x can be evaluated; therefore, in the calculations of My, only the solution in terms of wind

20'N

(dynes-cm )

FIGUBE 188.—Latitude dependence of integrated pressure gradient (SP/Sy), from 

Crossroads data, March to August 1946.

stress has been employed. The numerical results of 

longitudinal mass transport Mx as well as latitudinal 

mass transport Mv are entered in figures 189 and 190. 

The agreement of the results of Mx from two independent sets of solutions above latitude 6° N. in figure 

189 is considered fairly satisfactory. This suggests 

that the wind data obtained from Operation Crossroads 

and Operation Greenhouse are able to account for the 

current systems shown by the oceanographic results 

of Operation Crossroads. Comparing our results of 

mass transport with Reid's, the southwardly shift is 

obvious. To determine whether this shift is semipermanent in this area demands additional wind 

observations.

DIS1RIBUTION OF TEMPERATURE AND SALINITY

Values of temperature entering into this discussion 

were taken only from reversing-thermometer tempera-
PHYSICAL OCEANOGRAPHY IN THE MARSHALL ISLANDS AREA 657

<—— WESTWARD TRANSPORT EASTWARD TRANSPORT

FIGURE 189.—Latitude dependence of east-west component of mass transport (Mx~).

tures of the Crossroads data and the Japanese data. 

A separate study of bathythermograph temperatures 

for the upper 400 ft has already been made by Robinson 

(1954). Since a much larger number of bathythermograph 

temperatures than reversing-thermometer temperatures 

were available in the Marshall Islands area and furthermore, since the bathythermograph data were comparatively recently taken, their reliability is expected to be 

higher than some of the reversing-thermometer data, 

which were taken almost 20 years ago. Therefore, the 

reader who is especially interested in the detailed 

temperature distribution and its seasonal variations 

within the upper 400 ft should refer to Robinson's work.

VERTICAL DISTRIBUTION

To present a typical example, the vertical distribution 

of temperature and salinity (and its computed a t value) 

of Station B-39 (12°00' N., 168°01' E.; data taken on 

12 July 1946) of the Crossroads results is shown in 

figure 191.

In the temperature profile, it is readily seen that the 

curve can be unmistakably divided into three parts: 

the topmost isothermal layer, the thermocline layer, 

and the layer of weak gradient. The topmost isothermal 

layer is also referred to as the homogeneous layer 

(Ekman), Zone of Agitation (Storungs Zone, Defant),

315915—55———3

convection layer (Sverdrup), upper mixed layer (Munk), 

wind-stirred layer (Munk) and others. These various 

names suggest that the uniformity of temperature 

within this topmost layer is mainly caused by (1) 

thorough mixing of water in this layer by ceaseless and 

vigorous mechanical stirring or agitation, and (2) by 

thermohaline convection near the surface.

In the salinity profile, the division of the curve into 

the topmost layer, the upper layer (characterized by 

salinity maximum), the intermediate layer (characterized by salinity minimum), and the deeper layer 

(characterized by weak-salinity gradient) is distinct.

The mirror image of the <r, profile to that of temperature profile actually tells nothing more than the wellknown fact that the density of sea water is, in general, 

predominantly determined by the water temperature 

and that it reflects it.

Figure 192 is given to present the characteristic 

features of latitude dependence of temperature at 

various depths. It should be noted that the longitudinal positions of these sections are the same as those 

of isobaric topographies—that is, for the Crossroads 

section (fig. 192^4) the line running approximately 

along 164° E. and for the Japanese sections (fig. 

1925, (7) the line running along approximately 166° E.

To avoid the overcrowding of isotherms in the upper

• My =

6 -4 -2 Jf

'•

—— SOUTHWARD TRANSPORT

<> 4 6

0

8

[

1

NORTHWARD TRANSPORT ——— » '

10

metric ton ~j

m-s<c- J

FIGURE 190.—Latitude dependence of north-south component of mass transport (Mu).
658 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

500

1000

TEMPERATURE (°C) 

15 25

/

/ T

3450

SALINITY (%o)

35.00

22.00

cr.

26.00

FIGURE 191.—Vertical distribution of temperature, salinity, and <rt values at station B-39 (12°00' N., 168°01' E.), from Crossroads data, July 12,1946.

layer, unequal isotherm intervals have to be chosen 

and depths of thermocline have to be omitted. However, in this area the substitution of the 28° C isotherm 

for the thermocline is satisfactory, at least for the low 

latitudes (lower than 10°-12° N.). Original temperature profiles show that for this region the thermocline 

is almost invariably at 28° C. For latitudes higher 

than 12° N. the thermocline is less distinctive, although 

as an average the 26° C isotherm is a good approximation.

Bearing these facts in mind, it is obvious from these 

figures that the thickness of the isothermal layer in this 

region generally lies between 50 m and 100 m, with an 

average thickness of about 75 m. Figure 1925, C, 

reveals that the isothermal layer is somewhat thicker 

in winter than in summer. This is especially true in 

the region between 8° N. and 12° N., where the summer 

isothermal layer almost reaches the surface and the 

winter isothermal layer is much thicker. The characteristic features of the latitude dependence of the 

isothermal layer in this area agree fairly well with 

Reid's density model (Reid, 1948a, b). Near the 

Equator the isothermal layer has a thickness of about 

100 m or more where it tends to tilt northward, first 

gradually, then rather sharply, reaching the shallowest 

depth at about 10°-12° N. in the summer months and 

at about S^IO^N. in the winter months. If we accept 

the generally recognized theory that the shallowest 

isothermal layer coincides with the northern boundary 

of the Equatorial Countercurrent, then the thermal 

boundary lies somewhat more to the north than the

boundary of our wind structure and, hence, the boundary of our computed current system (fig. 189). The 

difference in the positions of the shallowest isothermal 

layer for winter and summer sections from the Japanese 

results probably indicates the seasonal shift of the 

Countercurrent system. Commencing at latitude 10°- 

12° N., the isothermal layer dips rather sharply to the 

north, which means a deeper layer in the North 

Equatorial system than in the Countercurrent system.

Sverdrup and others (1946, p. 708) also noted that 

the thickness of the isothermal layer in the North 

Pacific increases from east to west. Owing to the 

relatively small east-to-west coverage of the area under 

study, however, no longitudinal variation was detected 

in this area.

The thermocline layer, which was indicated by the 

large gradient in the temperature profile in figure 191 

and by the crowding of isotherms in figure 192, lies 

in general between 75 m and 300 m, an average of 

about 200 m for the area as a whole. The general 

tendency of isotherms in this layer is to lie nearly 

parallel to the isotherms in the isothermal layer. 

However, the position of the shallowest thermocline 

layer apparently shifts to the south; it appears, at 

about 5°-8° N. in the Crossroads section and at about 

6°-9° N. in the Japanese sections. These positions 

correspond fairly well with the northern boundary of 

the Countercurrent as established by dynamic computations. The hump-shaped isotherms which indicate 

a lower temperature at each relevant depth, in the 

northern boundary of the Countercurrent, probably
PHYSICAL OCEANOGRAPHY IN THE MARSHALL ISLANDS AREA 659

16° 18° 20°

FIGURE 192.—Vertical distribution of temperature. A, From Crossroads data, March to August 1946. B, From Japanese data, 1933-41, summer season. C, From Japanese

data, 1933-41, winter season.
660 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

suggest that the vertical motions which were treated 

by Defant (1936) and Yoshida and others (1953) are 

not limited to the upper mixed layer only but might 

penetrate quite deeply into the thermocline layer. 

This phenomenon of lowering temperature in the 

northern boundary of the Equatorial Countercurrent 

is a characteristic feature which we shall discuss further 

with the horizontal distributions of temperature.

Another noticeable feature deserves mention in discussion of the thermocline layer. In the Crossroads 

section (fig. 192A) a sharp dip appears in the isotherms 

of 12° C and 10° C near the Equator, thus causing the 

thermocline layer to be much thicker there. This 

feature was explained by Barnes and others (1948) 

as an intrusion of the South Pacific Water. A similar, 

yet less distinctive, feature also appears in the Japanese 

sections, but the positions of the dip and the isotherms 

in which the dip appears are somewhat different; this 

probably suggests that the intrusion of the South 

Pacific Water varies considerably in its extent and in 

its depth.

Descending from a depth of about 300 m, the vertical 

temperature gradient is generally so slight that the 

isotherms in the vertical sections become farther and 

farther apart. The characteristic features of isotherms 

in the two upper layers generally disappear at this 

layer. Some quite drastic changes of temperature in 

the neighboring stations are thought to be of a local 

nature and therefore of no general significance. The 

quite wavelike trend in the deeper isotherms can be 

largely attributed to the fact that below the thermocline the vertical temperature gradient is so slight that 

it is very difficult to be sure of the exact depth of a 

scaled value of temperature. Neglecting their wavelike variations, the isotherms of this layer are largely 

horizontal, which means that latitude does not play too 

strong a role in temperature distributions. A temperature of 3° C was observed in both the Crossroads and 

the Japanese investigations.

It may be of some interest to note that near the 

Equator the dips of isotherms appearing in the thermocline layer extend even to this depth. In the Crossroads 

section (fig. 192A), it is clearly seen that this dip appears 

distinctly at a depth as great as that of the 6° C isotherm (about 800 m) and in the Japanese sections as 

deep as the 7 ° C isotherm (about 700 m). Furthermore, 

the position of this dip shifts more and more to the 

north with increasing depth. This probably suggests 

that the component of the intrusive South Pacific 

Water becomes ever larger with increasing depth. 

As we shall see later, owing to the higher salinity of 

South Pacific Water, its density is naturally higher; 

therefore, it should underlie the North Pacific Water.

Three profiles of vertical salinity distribution from 

the Crossroads data and from the Japanese data along 

the same longitudes as for the temperature are shown 

as figure 193. In an attempt to explain the^profound 

characteristic features in these figures, a brief mention 

of the salt concentration associated with the major 

water masses in this area is necessary. From a first 

glance at these three sections, it is immediately seen 

that the vertical salinity distribution is characterized by 

a pair of high-salinity tongues and a pair of low-salinity 

tongues, both coming from north and south. A detailed 

discussion of the properties of water masses could not be 

properly presented until the temperature-salinity (T-S) 

relationships are duly analyzed, but it should suffice to 

say here that the two tongues of high salinity and the 

two tongues of low salinity represent the saline Tropic 

Water and fresh Intermediate Water, respectively.

The Tropic Waters (Upper Water masses) acquire 

their characteristic properties at the air-sea boundary in 

the tropical area, where the evaporation is usually high 

and hence where salinity is high. Since Tropic Water 

is warm, its density is not great, and therefore only the 

warm and fresh Equatorial Water may overlie it. The 

depth of the salinity maximum, which roughly corresponds to the central axis of the two saline Tropic 

Waters, lies at a depth of about 150 m and is nearly 

horizontal. Comparing the northern and the southern 

tongues, the southern is the more saline. The maximum salinity reaches 35.60%0 to 35.80%0 in the 

southern tongue, while in the northern tongue it rarely 

exceeds 35.30%0 . The vertical salinity gradient from 

the surface to salinity maximum is exceedingly high in 

some places; for instance, the Crossroads section (fig. 

193A) shows that in the vicinity of the Equator salinity 

increases more than 1.4%o at an interval of about 

100 m (from 34.20%o at about 50 m to more than 

35.60%0 at about 150 m), which is a very large gradient of salinity in the open ocean.

In contrast to the characteristic properties of the 

Tropic Waters, the Intermediate Waters are characterized by their low temperature and low salinity. These 

properties were acquired at their sources (North and 

South Subpolar areas) where the air temperature is low, 

precipitation high, and the salinity therefore low. In 

spite of their low salinities, their densities are high 

because of their low temperatures. Naturally they 

underlie the Tropic Waters and occupy the intermediate 

depth, whence their name. The depth of the central 

axis of minimum salinity varies greatly with latitude. 

As we have previously noted, the southern tongue is the 

deeper. Near the Equator, the salinity minimum may 

appear as deep as 800 to 1000 m, and the value of 

salinity is almost uniformly close to 34.50%0 . The 

inclination with latitude of the central axis of the
PHYSICAL OCEANOGRAPHY IN THE MARSHALL ISLANDS AREA 661

2-5 8° 10° 

LATITUDE

FIQUEE 193.—Vertical distribution of salinity. A, From Crossroads data, March to August 1946. B, From Japanese data, 1933-41, summer season. C, From Japanese data,

1933-41, winter season.

315915—56
602 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

FIGTJEE 194.—Horizontal distribution of temperature at surface. A, From Crossroads data, March to August 1946. B, From Japanese data, 1933-41,

summer season. C, From Japanese data, 1933-41, winter season.
PHYSICAL OCEANOGRAPHY IN THE MARSHALL ISLANDS AREA 663

Southern Intermediate Water is slight yet perceptible. 

On the contrary, the inclination of the central axis of 

the Northern Intermediate Water is tremendous, lying 

at a depth as great as 500 to 600 m at 20° N. and rising 

almost linearly to a depth of 150 to 200 m at 8°-10° N. 

This inclination of axis probably suggests the ascending 

of the North Intermediate Water with decreasing latitude because of the fact that the density in this water 

mass decreases owing to the "warming-up" of the water 

mass through mixing with the surrounding water masses 

on its long journey. The associated salinity of this 

water mass is also somewhat increased from 34.00%o34.20%0 at 20° N. to 34.50%0 at 8°-10° N.

The region at about 6°-10° N. roughly corresponds to 

the eddy area which we mentioned in the discussion of 

water circulation, acting as a "baffle" or a "transitional" 

zone between the northern and southern tongues. No 

appreciable salinity maxima and minima appear there. 

Consequently the salinity gradient is relatively weak.

HORIZONTAL DISTRIBUTION 

AT THE SURFACE

The horizontal distribution of the surface reversingthermometer temperatures from the Crossroads data 

and the Japanese data are shown as figure 194. It is to 

be noted that at the sea surface as well as in the upper 

mixed layer, water temperature is primarily affected by 

such ever-varying local boundary conditions as heating, 

cooling, evaporation, and local wind. The characteristic property of the surface-temperature distribution is 

not so clear as that in deeper layers where water temperature is essentially associated with water-mass properties and water circulations. However, some noticeable features which can be derived from these figures 

and need to be briefly mentioned are:

1. In general, the surface temperature in this area 

decreases slowly with increasing latitude, though the 

horizontal gradient is very slight.

2. Intensive up welling associated with Equatorial 

divergence is thought to occur at and near the Equator; 

thus the surface temperature there should be somewhat 

lowered. This feature is clearly seen in the two figures 

(fig. 194^4, B) for the summer months. In the winter 

season, from the Japanese results, this feature is not 

evident.

3. A tongue of cool water, roughly corresponding to 

the cool-water tongue from the northeast mentioned by 

Kobinson (1954) appears in all three figures in somewhat 

different latitudes. In an attempt to explain the origin 

of the cool-water tongue from the northeast, Robinson 

suggested it might be associated with the ascending 

motion in the northern boundary of the Countercurrent. 

The temporal and spatial variations of its position

might be due to the change of such external boundary 

conditions as winds.

The characteristic features of surface-salinity distribution as shown in figure 195 are much more distinctive than those of surface temperature. Bands of 

low salinity appear generally in the same area of large 

eddies. Their respective positions are at about 4°-6° 

N. from the Crossroads data (fig. 195^4.) and at about 

6°-8° N. from the Japanese data (fig. 1955, O). From 

both sides, waters of higher salinity push toward this 

band of low salinity. The larger gradient on the 

southern side probably suggests that the strongest 

invasion comes from the southwest sector—from the 

Southern Hemisphere.

AT THE 100-METER LEVEL

The horizontal distributions of temperature and 

salinity at the 100 m level are shown as figures 196 

and 197, respectively. The temperature distributions 

at this level differ fundamentally from those at the 

surface. Whereas at the surface there exists no close 

connection between the distributions of these two 

properties, an obvious resemblance now appears at 

this level. Comparing figures 196 and 197, it is easily 

seen that in the area of large eddies distinctive bands 

of low temperature and salinity occupy nearly the same 

area. Recalling the fact that the density of sea water 

is, in general, a mirror image of the water temperature, 

then the band of "dynamic low" (the high density) in 

the geostrophic flow may be simply a reflection of the 

band of low temperature. To determine the reality of 

the horizontal large eddies in this region, more direct 

measurements of the current are needed.

AT THE 250- AND 300-METER LEVELS

For the representation of the intermediate level, 

different depths were used. This is due to the fact 

that in the processed data of the Crossroads and the 

Japanese results, values of temperature, salinity, and 

pertinent factors were scaled at these different "standard depths." To avoid the tedious and time-consuming 

work of reprocessing, it is found necessary to use different depths. However, this representation has no 

appreciable effect on our conclusions since the major 

features of temperature and salinity distributions at 

these depths are essentially the same.

With the exception of a few minor changes, the characteristic features of the distribution of temperature 

(fig. 198) and salinity (fig. 199) at these depths remain 

practically unchanged from those at the 100 m level. 

A noticeable feature is that the band of low temperature is enlarged at this level in comparison to the 100 m 

level. This agrees with our earlier statement that the 

size of the large eddies increases with increasing depth.
664 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

FIGURE 195.—Horizontal distribution of salinity at surface. A, From Crossroads data, March to August 1946. B, From Japanese data, 1933-41, summer

season. C, From Japanese data, 1933-41, winter season.
PHYSICAL OCEANOGRAPHY IN THE MARSHALL ISLANDS AREA 665

——120°

FIQUEE 196.—Horizontal distribution of temperature at 100 meters. A, From Crossroads data, March to August 1946. B, From Japanese data, 1933-41,

summer season. C, From Japanese data, 1933-41, winter season.
666 BIKINI AND NEAEBY ATOLLS, MAESHALL ISLANDS

FIGURE 197.—Horizontal distribution of salinity at 100 meters. A, From Crossroads data, March to August 1946. B, From

C, From Japanese data, 1933-41, winter season.

170" 175°

data, 1933-41, summer season.
PHYSICAL OCEANOGRAPHY IN THE MARSHALL ISLANDS AREA 667

FIGURE 198.—Horizontal distribution of temperature. A, At 250 meters, from Crossroads data, March to August 1946. B, At 300 meters, from Japanese data. 1933-41,

summer season. C, At 300 meters, from Japanese data, 1933-41, winter season.
668 BIKINI AND NEAEBY ATOLLS, MARSHALL ISLANDS

T:

35 XX)——- 

35.00——— 

34.90-—— 

34.80 -"—— 

34.80-——

5'bj_

FIGURE 199.—Horizontal distribution of salinity. A, At 250 meters, from Crossroads data, March to August 1946. B, At 300 meters, from Japanese data, 

1933-41, summer season. C, At 300 meters, from Japanese data, 1933-41, winter season.
PHYSICAL OCEANOGRAPHY IN THE MARSHALL ISLANDS AREA 669

In the distribution of salinity it is noticeable from 

both the Crossroads and the Japanese data that the 

band of low salinity which appears at about 6°-8° N. 

at 100 m, now shifts to the north of 10° N. at this lower 

level. Recalling our previous statement as to the 

inclination of the axis of salinity minimum (of Intermediate Water), the shift is easily understood. From 

figure 1935 we can immediately see that at the 250 

m-300 m level, the area at about 10°-14° N. is right 

at the axis of the salinity minimum, whereas the area 

south of it is above and the area north of it is below the 

salinity minimum at this depth.

AT THE 500-METER LEVEL

Two features characterize the horizontal distribution 

of temperature at the 500 m level (fig. 200): (1) the 

band of low temperature at the area of the large eddies 

is still somewhat recognizable, but very faint, and (2) 

the axis of lowest temperature at this level moves 

northward to about 13° N., from the Crossroads data, 

and to about 15° N., from the Japanese data.

The main feature of the distribution of salinity at 

this level (fig. 201) is that the center of low salinity 

moves farther northward. The northern region, which 

was an area of high salinity in the upper levels, now 

becomes the center of low salinity.

These features in the distribution of both temperature and salinity can be similarly attributed to the fact 

that the axis of the Intermediate Water is inclined. 

It is clearly seen from figure 193A that as depth increases the area of southern latitudes is already out of 

the center of the Intermediate Water, only the northern area being within its central axis.

CONCLUDING* REMARKS

Concluding the discussion of the spatial (vertical as 

well as horizontal) distributions of temperature and 

salinity of the Marshall Islands area, it is noted that

1. With the exception of the surface temperature, the 

most outstanding feature of the distribution of temperature and salinity for the upper 250 m-300 m layer is 

the appearance of a band of low temperature and 

low salinity in the area at about 5°-10° N., or approximately the area of the large eddies.

2. The primary causes of this outstanding feature are 

thought to be

a. High precipitation.

b. Absence of saline Tropical Water tongue from 

both north and south.

c. Ascending of the North Intermediate Water mass 

with decreasing latitude, thus enabling the cool 

and fresh water associated with this water mass 

to come up to a level which in all other areas is

occupied by warm and saline (northern and 

southern) Tropical Water masses, 

d. Upwelling associated with mean current (Defant,

1936; Yoshida and others, 1953). 

e. Upwelling associated with cyclonic eddies (Uda,

1940; 1949; 1952).

It is reasonable to assume that both types of upwelling originate at some intermediate depth, thus 

bringing cool and fresh water into the upper levels. 

The pushing and ascending of both the northern and 

southern Intermediate Water masses may further 

strengthen this upwelling.

SEASONAL VARIATIONS

A detailed discussion of seasonal variations of temperature and salinity from month to month is not possible 

in this study. For several months no observational 

data were available from either the Crossroads or the 

Japanese data, and in some other months the number 

of data available was so small that their statistical 

value is questionable. Therefore the phrase, "seasonal 

variation," which we adopted here according to usage, 

in actuality only means the difference of average 

temperature or salinity between the summer months 

(April to September) and the winter months (October 

to March). All values of temperature and salinity 

within 2° of latitude interval were averaged together 

for each season. No account is taken for the longitudinal difference. For the sake of brevity, the area 

with an average value of temperature or salinity in the 

summer season larger than that in the winter season 

is named the "positive area"; the area where the average 

value of temperature or salinity is smaller in summer 

than in winter is termed the "negative area."

Figures 202 and 203 present the major features in 

seasonal variations of temperature and salinity in the 

upper 100 m level. Since the seasonal variation of 

both quantities below the 100 m level is rather confusing, no definite features can be traced; therefore, we 

shall not mention them here. In an attempt to 

interpret the profound characteristic features in figures 

202 and 203, the following suggestions are proposed:

The striking parallelism of both temperature and 

salinity variations for the upper 50 m level and the 

remarkable difference between the 100 m curve and 

other curves for both temperature and salinity strongly 

indicate the influence of a discontinuity surface—that 

is, the thermocline or the interface of the upper mixed 

layer and the underlying layer, exerted on the vertical 

distribution of temperature and salinity above and 

below this surface. Our previous statements as to 

depths of this discontinuous surface—that it varies 

with latitude considerably and that it averages about 

75 m for the whole area considered—are further con-
670 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

FIGUEE 200.—Horizontal distribution of temperature at 500 meters. A, From Crossroads data, March to August 1946. B, From Japanese data, 1933-41,

summer season. C, From Japanese data, 1933-41, winter season.
PHYSICAL OCEANOGRAPHY IN THE MARSHALL ISLANDS AREA 671

FIGURE 201.—Horizontal distribution of salinity at 500 meters. A, From Crossroads data, March to August 1946. B, From Japanese data, 1933-41,

summer season. C, From Japanese data, 1933-41, winter season.
672 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

1.5

1.0

-5

0

-.1

-.2 

.-.3

-4.

-.5

-1.0

Meters 

0 —

10 ————

25 ————— 

50 ————

100

3 5 7 9 II 13 15 17

LATITUDE (°N)

FIGURE 202.—Seasonal variation of temperature in the upper mixed layer, from Japanese data, 1933-41.

19 21

firmed by the trend of the curves in these two figures.

The approximately linear increase of temperature 

in the upper mixed layer with increasing latitude 

probably reflects the latitudinal dependence of seasonal 

variations in radiation.

The salinity positive area—that is, the area where 

summer salinity is higher than winter salinity—at about 

7°-14° N. in figure 203 cannot possibly be due to the 

seasonal variation of evaporation because, in general, 

the evaporation at the sea surface is higher in winter 

than in summer. A rough calculation of seasonal variation of evaporation at these latitudes shows that the 

evaporation in winter is about twice as high as that in 

summer. It should, however, be noted that the absolute values of evaporation at these latitudes are comparatively low even in the winter season (Jacobs, 1951). 

This suggests that a negative area—that is, an area 

where summer salinity is lower than winter salinity— 

instead of a positive area should appear there if evaporation is an important factor. A possible seasonal 

shift of the position of the Countercurrent system may

likewise have an unfavorable effect. As we have shown 

in the previous sections, the northern boundary of the 

Equatorial Countercurrent is an area of fresh water; a 

shift to the north of that fresh water in the summer 

would also reduce the summer salinity. Thus a negative area would appear in the seasonal variation of 

salinity.

It is thought that the appearance of a salinity negative area (at about 1 °-7° N.) and a salinity positive area 

(at about 7°-14° N.) side by side in figure 203 might 

be associated with the seasonal variation of the vertical 

motion and especially the upwelling. It seems reasonable from Kobinson's work (1954) that both upwelling in 

the northern boundary and descending motion in the 

southern boundary of Equatorial Countercurrent are 

more intense in winter than in summer. Uda (1940,. 

1949, 1952) in his study of cold water in the Japan Sea 

also pointed out that the type of upwelling associated 

with cyclonic eddies is also stronger in winter when the 

stability of sea water is slight. As we previously men-
PHYSICAL OCEANOGRAPHY IN THE MARSHALL ISLANDS AREA 673

9 II 13 

LATITUDE (°N)

15 17 19

FIGUEE 203.—Seasonal variation of salinity in the upper mixed layer, from Japanese data, 1933-41.

tioned, both types of upwelling were thought to originate at an intermediate depth where both temperature 

and salinity are low. The stronger upwelling of both 

types in winter means that more cool and fresh water 

is brought up to the topmost layer in that season, thus 

reducing the winter salinity and causing a positive area 

in the northern boundary of the Countercurrent. The 

intense descending motion in the southern boundary of 

the Countercurrent in a winter has a reverse effect generally and therefore may cause a negative area in the 

lower latitude.

It is thought that the cooling and warming effects 

which are exerted on the seasonal variation of temperature in the topmost layer by respective upwelling 

and descending motion is largely covered or masked by 

the seasonal variation of radiation.

DISTRIBUTION OF DENSITY

In discussing the distribution of density, only the 

Crossroads data are available, because the values of <r t 

have not been included in the computations from the

Japanese data. Consequently the prepared figures of 

density distributions are not detailed enough to be discussed at length.

The distribution of density of sea water, customarily 

represented by values of <r t, is solely determined by the 

distribution of temperature and salinity, which have 

been discussed in the previous sections. This section, 

therefore, is intended only to draw attention to the 

general features of the density distributions in this area.

In dealing with the distribution of density, it should 

be of particular interest to keep the interrelationship 

between mass and current in mind. It is expected that 

the general rule for mass-current relationship can be 

satisfied as far as our computed current system is concerned. Although, especially at the levels near the 

surface, the actual current may differ somewhat from 

such geostrophic flow, knowledge of vertical distribution of density is still essential to the discussion of the 

currents. In this connection it may be desirable that 

the vertical profile of density from our data (fig. 204) 

be compared with Reid's density model (Reid, 1948b).
674 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

1.——^" -' '. —'——•".—?Rnn_.——•r*~~**i *^-———————•——.,_!

FIGURE 204.—•Vertical distribution of density (<r« values), from Crossroads data, March to August 1946. In order to avoid overcrowding of contours in the shallower layers

unequal contour intervals have been used in this figure.

Only a rough comparison with Keid's model indicates 

a somewhat essential difference from the Carnegie data. 

It is interesting to note that the vertical profile at 

5° N. from our data is similar to that of the Carnegie 

at 10°-12° N., corresponding to the southward shift 

of the wind characteristics.

As was mentioned before, it is seen that isopycnic 

lines largely follow isotherms in the relevant section. 

A detailed discussion of the depth of thermocline was 

given in the section on temperature distribution and 

is not entered into in this section.

AT THE SURFACE

[flg. 205,41

The most prominent feature of the density distribution at the surface is the appearance of an area of low 

density at about 5° N. This feature seems to be inconsistent with the general rule for mass-current relationship. According to the general rule an area of 

high density instead of low should appear in this area 

of the northern boundary of the Countercurrent. (See 

p. 647.) However, beneath the surface in this area, 

water of the highest density exists, as will be mentioned 

later. This is in good accordance with the Carnegie 

data. Actually, however, in the surface layers the 

relationship between current and density is different 

from that inferred from the rule for geostrophic flow.

Comparing figure 205A with the chart of the horizontal distribution of salinity at the surface (fig. 195A) 

it is interesting to note that the agreement between 

distribution of salinity and that of density at the above 

area is remarkably good. This appears to suggest that 

the salinity distribution is the dominating factor in the 

distribution of the density at the very surface, mainly 

owing to the uniform horizontal distribution of the 

temperature in this area. This tendency is especially 

outstanding in the southern part of the area where the

horizontal temperature gradient at the surface is almost 

negligible.

The density of water increases northward from the 

center of low density arid the shape of isopycnic lines 

closely resembles that of isohalines. At about 14° N., 

northeast of Bikini Atoll, the density reaches its highest 

value, the natural consequence of the low temperature 

and high salinity there.

AT THE 100-METER LEVEL

[fig. 2C5JB]

The horizontal distribution of density at the 100 m 

level is essentially changed from that at the very surface. This can be understood by the fact that the 

thickness of the surface isothermal layer is generally 

less than 100 m, as was mentioned in the previous 

section.

The area between 4° and 7° N. which corresponds to 

a region of low density at the surface is an area of 

high density at the 100 m level. This feature is similar 

to that indicated in the Carnegie data (Sverdrup and 

Fleming, 1944). The distribution of the major wind 

system in this area is south of the average location for 

the whole Equatorial Pacific. It may be of particular 

interest to pay attention to these features of density 

distribution in connection with those of wind distribution. An area of low density appears to be found in 

the area between 10° and 12° N., with local high density at about 13° N.

The agreement of the reverse relationship between 

the distribution of density and that of temperature is 

extraordinary. Areas of low density coincide with 

those of high temperature, while regions of high density 

are almost identical with those of low temperature. 

The predominant influence of temperature on the 

distribution of density is principally due to the high 

gradient of temperature at this level, which in most
PHYSICAL OCEANOGRAPHY IN THE MARSHALL ISLANDS AREA 675

cases lies within the thermocline layer. (See fig. 192A) 

With the exception of the southernmost area, where an 

area of low density is located and directed in exactly 

the same fashion as that of the low salinity, the effect 

of salinity distribution on that of density is almost 

negligible. In this southernmost area, temperature 

gradient is not so large as in the northern parts.

AT THE 2 50- METER LEVEL

[fig. 205(7]

An extensive area of high density, which follows 

exactly the area of low temperature between 2° and 

7° N., constitutes the main feature of the distribution 

of density at this level. This feature appears to be 

ascribed to the Intermediate Water which extends 

above this level in this region and also to be related to 

the enlarged regions covered by horizontal eddies.

Density decreases rather rapidly northward from 

this high-density area. The lowest density is found at 

about 16° to 17° N. The density distribution in the 

southernmost part is unfortunately obscure because 

of the lack of salinity data.

AT THE 500-METER LEVEL

[fig. 205J>]

At this level the distribution of density in the 

southern area (below 10° N.) is remarkably uniform; 

from 2° S. to 12° N., a t values at most stations lie in 

the neighborhood of 27.0. Commencing at 12° N., 

where the local high density is found, density decreases 

with increasing latitude. This may be essentially due 

to the regular northward decrease of salinity while the 

temperature distribution remains quite uniform (7.5° C) 

at this level in this area. The appreciable decrease of 

salinity in the northern area may be ascribed to the 

Intermediate Water, the core of which exists at this 

level in this area.

TEMPERATURE-SALINITY (T-S) RELATIONSHIP 

GENERAL RELATIONSHIP

In previous sections we discussed the distributions of 

temperature and salinity separately, partly in connection with character of currents. The discussion in the 

present section is intended to make the interpretation 

of those distributions more clearly understood by use 

of the temperature-salinity (T-S) relationships, which 

may characterize water masses.

Observed temperature and salinity data are combined into groups for every 2° of latitude (in the area 

between 17° and 20° N., from the Crossroads data, the 

values over 4° of latitude have been grouped together

because of scarcity of observations in those areas). T-S 

diagrams have been prepared respectively for each 

group from the Crossroads data and the Japanese data. 

In the diagrams, the mean curves of Western South 

Pacific Water (WSP), Equatorial Pacific Water (EP), 

and Western North Pacific Water (WNP) are reproduced as the reference curves after Sverdrup and others 

(1946, figs. 195, 196, and 199).

Three very distinct types of curves can be readily 

recognized from these groups of T-S diagrams. This 

situation is common to both the Crossroads and the 

Japanese data. Accordingly the area under study is 

divided into three regions for convenience of discussion.

Since the T-S relationship in the upper mixed layer 

is not considered conservative, discussion of it above 

100 m is excluded here. Below about the 150-200 m 

level, generally well-defined water masses are present in 

all latitudes concerned, although the depth of the core 

of each water mass varies with latitude. It is noted that 

a transitional layer—corresponding approximately to 

the thermocline layer—appears between the surfacewater mass and well-defined water masses below 200 m. 

A layer of salinity maximum, caused by the subsurface 

waters from tropical convergences, is present within 

the thermocline layer, and is shallowest at 8°-10° N., 

where the thermocline layer is also found to be the 

shallowest. It can be said that Western North Pacific 

Water covers almost all levels in the northern region, 

Equatorial Pacific Water covers almost all levels in 

the southern region, and in the central region stratification of these two waters is found.

As will be mentioned later, it appears that an 

Intermediate Water of northern origin is present at 

300-500 m levels in the northern and central regions, 

with another Intermediate Water of southern origin at 

the 800 m level mostly in the southern and central 

regions. Thus, in the central region two maxima and 

minima of salinity are found, while in the northern and 

southern regions only one maximum and one minimum 

are seen.

Hence, the division of the area into these three regions may provide adequate information on the characteristics of the waters in this area. In the Carnegie 

data, the T-S curves divided the areas into two groups, 

one which almost corresponds to our northern region 

and the other to our central and southern. Using the 

Carnegie data, division into such regions appeared to 

obscure the characteristic pictures of T-S curves which 

indicate the presence of a maximum and minimum at 

200 m and 500 m, respectively, found just in the 

boundary area between these two groups, at about 

8°-12° N.
676 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

175° 

— 20°

—22.4

-2J.2

10° 10°——

0° ——

175° 

20°

B

160°"

175° 

— 20°

10° 10°——„

1653"

15° ——

170° J2S°

160°

-27.0

-27.0

FIGTJEE 205.—Horizontal distribution of density, from Crossroads data, March to August 1946. A, At surface. B, At 100

meters. C, At 250 meters. D, At 500 meters.
PHYSICAL OCEANOGRAPHY IN THE MARSHALL ISLANDS AREA 

IN THE SOUTHERN REGION

[figs. 206, 207, 208, 209]

677

As was mentioned above, almost the whole water 

column in this region (between 200 m and 1,000 m) is 

covered by Equatorial Pacific Water.

Lot. 4e -2.«S

.**

34.00 .1 .2 9 3500 .1

.8 .9 35.00

4«-2°S

FIGURE 206— Temperature-salinity (T-S) diagram for 4° S.-2° S., from Crossroad? 

data, March to August 1946.

30

28 " O ° O

26

24

22

20

18

16

FIGURE 208.—Temperature-salinity (T-S) diagram for 0°-2° N., from Crossroad 

data, March to August 1946.

In the northern part of this region, at 0°-2° N., some 

scatter of the values is seen at the 200 m level. However, this scatter does not seem to be of irregular dis6 tribution, but indicates that at the 200 m level the 

transition takes place from Equatorial Pacific Water to 

Western North Pacific Water, more and more of the

O o O

o o

00

34.00 .1 .2 .3 .4 .5 .6 .7 .8 .9 35.00 .1 .2 .3 .4 .5 .6 .7 .8 .9

S C7«)

FIGURE 207.—Temperature-salinity (T-S) diagram for 4° S.-2* S., from Japanese data, summer season, 1933-41,
678 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

0°-2°N

DEPTH 

•6- 0-100 

O 150-200 

o 300-500 

$ 800-1000

S CM

FIGURE 209.—Temperature-salinity (T-S) diagram for 0°-2° N., from Japanese data, 

winter season, 1933-41.

latter becoming mixed with Equatorial Pacific Water 

as the latitude increases until Western North Pacific 

Water completely replaces Equatorial Pacific Water 

at about latitude 10° N. It is found from the original 

data that at 0°-2° N. some of those points which are 

close to Western North Pacific Water belong to those 

near 2° N.

IN THE CENTRAL REGION

[figs. 210, 211, 212]

Whereas the Equatorial Pacific Water covers the 

whole column in the southern region, the typical characteristic in the central region is that Equatorial Pacific 

Water underlies Western North Pacific Water which 

covers the upper part of the water column. As was 

mentioned before, an Intermediate Water of northern 

origin becomes obscure at the southern part of this 

region. For this reason, the presence of double maxima 

and minima is most distinct in the northern part of 

the region. (See figs. 210, 211, 212).

Below a depth of salinity maximum (100-150 m), 

Western North Pacific Water exists as a comparatively 

thin layer, a thickness of about 50 m to 100 m, and 

below the 300 m level (lower than 6° N.) to the 500 m 

level (higher than 10° N.), Equatorial Pacific Water 

exists. The transitional layer has a thickness of about

30

28

26

24

22

20

18

16

£ .4

1-

12

10

8

6

4

II0 -I20 N

34.00 .1

FIGURE 210.— Temperature-salinity (T-S) diagram for 11° N.-120 N., from Cross 

roads data, March to August, 1946.

I0«-I2e N

DEPTH 

•» 0-100 

O 150-200 

O 300-500 

$ 800-1000

34.09

100 m to 300 m here and Equatorial Pacific Water

ftxtPTirk TTinrP wir1«l v TVm« Koai^a fl^ »T,TM a «l^;fw FIGURE 211.-TemPeraturc-salinit5(r-S) diagram for 10«N.-12°N., from Japanese extends more widely. Thus, besides the upper salinity data, 1933-41, summer season.
PHYSICAL OCEANOGRAPHY IN THE MARSHALL ISLANDS AREA 679

maximum of 150 m there are another salinity maximum 

and one or two salinity minima. Typical stratification 

of those water masses is illustrated in figure 213.

Generally in this region, the water of the 200 m level is 

a transition between Western North Pacific Water and 

Equatorial Pacific Water. In the northernmost part, 

•at about 10°-12° N., this transitional layer is replaced 

by 300 m to 400 m layers.

30

28

26

24

22

20

18

16

,4

12

10

8

6

4

2

10" - 12° N

34.00 .1 .2 .3 4 .5 .6 .7 .8 .9 35.00 .1 .2 .3

FIGURE 212.— Temperature-salinity (T-S) diagram fer 10° N.-120 N., from Japanese 

data, 1933-41, winter season.

10 12 14 16 18 20

100

200

300

? 400

V

^» 

E

x ^

t

Ul

o 600

700

800

900

1000

E P

s"

mm

SOUTHERN 

REGION

CENTRAL 

REGION

NORTHERN 

REGION

IN THE NORTHERN REGION

[figs. 214, 215, 216]

The water in this region is essentially characterized by 

Western North Pacific Water which covers almost all 

depths. The thermocline layer is not well defined. A

170 _ 20" N

34.00 .1 .2 .3 .4 .5 .6 .7 .8 .9 36.00 .1 .2 .5 .4 

S(%o)

FIOURE 214.—Temperature-salinity (T-S) diagram for 17° N.-200 N., from Crossroads data, March to August 1946.

DEPTH 

•O- 0-100 

O 150-200 

o 250-500 

O 800-1000

FIGURE 213.—Stratification of water masses in the Marshall Islands area.

34.00 .1 .2 .3 .4 .5 6 .7 .8 9 35.00 .1 .2 .3 .4 

S (7oo)

FIOURE 215.—Temperature-salinity (T- 8) diagram for 18° N.-200 N., from Japanese 

data, 1933-41, summer season.
680 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

DEPTH 

•o 0-100 

O 150-200 

O 300-500 

0 800-1000

3400 .1 2 .3 4 5 .6 7 .8 .9 3500 .1 .2 .3 .4

S (%„}

FIGURE 216.—Temperature-salinity (T -S) diagram for 18° N.-200 N., from Japanese 

data, J933-41, winter season.

salinity minimum, is found at the 500 m level in the 

T-S diagrams for this region and becomes shallower 

with decreasing latitude. This layer of salinity minimum is considered to be associated with the core of the 

Intermediate Water of northern origin, and it continuously ascends to 200 m level in the central region. 

It has been mentioned in previous sections that the 

decrease of depth of salinity-minimum level with 

decreasing latitude is due to a slow ascending motion of 

the Intermediate Water of northern origin. The

salinity minimum found at a depth of 200 m to 500 m 

is considered of this nature. The fact that this minimum 

level becomes less and less distinct towards the Equator 

lower than 8° N. is a good indication of the southward 

travel of this water.

These investigations appear to indicate that another 

Intermediate Water travels northward at deeper 

levels from the south. The second minimum of salinity 

found at about 800 m in low latitudes may correspond 

to the center of the Intermediate Water of southern 

origin.

Temperature at the first salinity-minimum level 

decreases with latitude from 15° C at latitude 2°-4° N 

to 7° C at latitude 18°-20° N. The corresponding 

values of salinity and <r t are from 34.70°/00 to 34.20%o 

and from 26.2 to 26.7. Since it appears that the 

Intermediate Water of the northern origin is characterized by <r,=26.7, T=6.5° C, and /S=34.0°/00 in the 

source of the origin (Koenuma, 1939), the above may 

indicate the ascending southward travel of this Intermediate Water. It is seen that in the northern region 

the identity of this Intermediate Water is preserved,, 

while in the central and southern region, this Intermediate Water is greatly mixed with the overlying 

water. In the second salinity-minimum level, values 

of T, S, and at in our area remain almost constant— 

T=5.5°, S=34.55%0 , and <r t=27.2. The Intermediate Water of southern origin is characterized by 

<r,=27.2-27.4 (Sverdrup and others, 1946, p. 744).

For an easy comprehension of the gradual changes of 

water masses in this area, the table below shows the 

respective depths and salinity readings of the first 

and the second salinity maxima and minima for an 

interval of every 2 degrees of latitude.

Salinity maxima and minima in the southern, central, and northern regions 

[The values in the parentheses are not well defined]

Region

Southern _ ....

Central... __ ... . .

Northern _ ..-..

Lat. (°)

4S 

2S 

0 

2N 

4N 

6N 

8N 

ION 

12N 

14N 

16N 

18N 

20N

Salinity maxima and minima

1st maximum

Depth 

(m)

150 

150 

125 

125 

125 

125 

100 

125 

125 

130 

140 

140

Salinity

(°/oo)

35.5 

35.5 

35.3 

35.2 

35.0 

34.9 

34.9 

35.0 

35.1 

35.2 

35.25 

35.25

1st minimum

Depth 

(m)

(200) 

(200) 

200 

200 

200 

250 

300 

400 

500 

500

Salinity

(°/oc)

(34.6) 

(34.6) 

34.55 

34.55 

34.45 

34.4 

34.4 

34.4 

34.25 

34.2

2d maximum

Depth 

(m)

300 

300 

350 

(400) 

(400)

Salinity

(°/oo)

34.72 

34.65 

34.65 

34.65 

34.58

2d minimum

Depth 

(m)

800 

800 

800 

800 

800 

800 

800 

800

Salinity 

CYeo)

34.55 

34.55 

34.56 

34.55 

34.55 

34.55 

34.55
PHYSICAL OCEANOGRAPHY IN THE MARSHALL ISLANDS AREA 681

REMARKS ON THE DISTRIBUTION OF OXYGEN

Only a few remarks on the characteristic features of 

oxygen distribution can be made here, because the 

number of oxygen data available for the Marshall 

Islands area is small and because very little is known 

about the consumption of oxygen through biological 

activities in this area. Attention is directed to the two 

vertical sections of oxygen distribution (fig. 217) from 

the Crossroads and the Japanese data for the summer 

season, respectively. Winter data are not detailed 

enough to plot a reasonable section. Four charts of 

horizontal distributions of oxygen at the surface, and 

at the 100 m, 250 m, and 500 m levels from the Crossroads data were prepared as usual. However, since 

those charts do not add to our knowledge materially, 

they will not be discussed separately.

The major features of oxygen distribution in the 

area of concern are:

1. A triple division of the vertical distribution of 

oxygen in sea water is (1) an oxygen-saturated layer at 

and near the sea surface, (2) an oxygen-minimum layer 

at the intermediate depth, and (3) an oxygen-rich 

layer at the greater depth. (Sweiwell, 1935.) This 

division is distinctively shown in both sections. At 

and very near the surface, oxygen concentration is invariably, from both the Crossroads and the Japanese 

results, around 4.0 ml-4.5 ml/1 at all latitudes concerned.

2. In the Japanese section, a subsurface layer (presumably a layer of oxygen supersaturation) of oxygen 

concentration higher than the surface value clearly 

appears. This feature is thought to be probably 

•caused by one (or both) of the following:

The intrusion of the northern Tropical Water 

mass. Favorable evidence for this argument is the 

exceptional coincidence of the location and the direction of the 4.75 ml/1 oxygen-concentration isoline

(fig. 2175) and the salinity-maximum tongue (fig. 

1935) which was presumably considered as the axis 

of the northern Tropical Water mass.

The high production of oxygen by plants in this 

layer. Favorable arguments in this respect are that 

(1) a subsurface layer of oxygen supersaturation is 

quite a common feature in the summer season because of the high production of oxygen by plants 

and (2) the complete absence of a similar supersaturated layer in the Crossroads section casts some doubt 

as to the first alternative.

3. The inclinations of isolines of oxygen concentration 

and the axis of the oxygen-minimum layer (fig. 2175) 

from high latitudes toward the area of large eddies 

(at about 6°-10° N.) resembles very much the inclination of a salinity-minimum tongue in figure 215. The 

parallelism of these two features strongly suggests that 

they are most probably associated with the same 

phenomenon—that is, the ascending of the Intermediate 

Water mass from higher to lower latitudes, reaching its 

shallowest depth at the area of large-size eddies (see 

sections on salinity distribution and T-S relationship).

4. In the area of large eddies, the distribution of

oxygen is characterized by remarkably horizontal

isolines of oxygen concentration and exceptionally large

vertical gradient. These account for the fact that in

horizontal distribution of oxygen in a layer at about

100 m-300 m, oxygen concentration at each relevant

depth is phenomenally low (fig. 2185, (7). The main

causes of these characteristic features are thought to be:

The high stability at the bottom of the isothermal

layer in these latitudes prevents an effective transport

of oxygen below the interface of upper mixed layer

and underlying layer. *

The aged water masses which appear at these 

latitudes contain less oxygen.

The strong upwelling which occurs there brings up 

water of low oxygen from some intermediate depth.
1400-

1600

2°S 0° 2°N 4° 6° 8° 10° 12° 14° 16° 

LATITUDE

FJGUEE 217,—VertioaJ distribution of oxygen concentration (ml/1). A, From Crossroads data, March to August 1946. B, From Japanese data, 1933-41, su
PHYSICAL OCEANOGRAPHY IN THE MARSHALL ISLANDS AREA 683

160" 

20° —

10° 10° ——

160°

FIGURE 218.—Horizontal distribution of oxygen concentration (ml/1), from Crossroads data, March to August 1946. 

surface. B, At 100 meters. C, At 260 meters. D, At 500 meters.
684 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

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O