THE STRUCTURE AND CIRCULATION

OF DEEP AND BOTTOM WATERS

IN THE ANTARCTIC OCEAN

Jeffrey Edwin Callahan

THE STRUCTURE AND CIRCULATION
OF
DEEP AND BOTTOM WATERS
IN
THE ANTARCTIC OCEAN

by
Jeffrey Edwin Callahan

A dissertation submitted to The Johns
Hopkins University in conformity with
the requirements for the degree of
Doctor of Philosophy.

Baltimore, Maryland
1971 •

T /4c ?yy

LIBRARY

KATAL POSTGRADUATE S65008

MONTEREY, CALIF. 93940

ABSTRACT

The circulation of deep and bottom waters in the Antarctic Ocean
is studied "by analyzing the distributions of heat, salt, and dis-
solved oxygen in the ocean south of U0°S. The discussion of the
deep-water flow is based on maps showing the depth, salinity,
and dissolved-oxygen content of the surfaces where potential
steric anomaly equals 50 and 30 centiliters per ton (potential
density equals 27-60 and 27.81 grams per liter). The 50-cl/t
surface is close to the vertical minimum of dissolved oxygen
in the antarctic. This feature is shown to originate in the east-
ern South Pacific Ocean. The 30-cl/t surface nearly coincides
with the deep salinity maximum, which originates in the North
Atlantic Ocean.

Maps showing the potential temperature, salinity, and dis-
solved oxygen in the bottom layer (depth > 3500 m) of the Antarc-
tic Ocean are also included in the report. Bottom water is shown
to be produced in both the Ross and Weddell seas. Ross Bottom
Water is characterized by high salinities relative to Weddell
Bottom Water. The distributions in the bottom layer indicate
that the Weddell Sea is a much stronger source of bottom water
than is the Ross Sea.

The velocity field south of Australia is examined through
geostrophic computations and direct current measurements. The
eastward Antarctic Circumpolar Current is shown to be divided into
two distinct zones separated by weak westward flow. The narrow

westward countercurrent is found near the crest of the Indian-
Antarctic Ridge. The net eastward flux between Australia and
Antarctica is 233 x 10 m3/s, which is nearly equal to the flux
through Drake Passage.

ACKNOWLEDGEMENTS

In preparing this dissertation, I have teen assisted by many
people. Foremost among them are Professor Raymond Montgomery of
The Johns Hopkins University and Mr. Joseph Reid, Jr. of the
Scripps Institution of Oceanography. Professor Montgomery, in
his capacity as dissertation advisor, has paid careful attention
to this project and has made numerous suggestions for improving
the final product. The method of analysis upon which much of
the dissertation rests is itself based primarily on the research
of Professor Montgomery.

Mr. Joseph Reid has, over the past few years, frequently en-
couraged and assisted me in my study of oceanography. The oppor-
tunities which he provided to participate in oceanographic cruises
in the antarctic have had an obvious impact on my choice of re-
search topics. I have greatly enjoyed my associations with both
of these gentlemen.

I have benefited from discussions with members of The Johns
Hopkins University Department of Earth and Planetary Sciences.
The advice of Professors Owen Phillips and Francis Bretherton and
of Associate Professors James Carpenter and Blair Kinsman (now
at the University of Delaware) has been particularly helpful.

I have received expert technical assistance from the staff
of the Chesapeake Bay Institute. Mr. Michael Karweit and Mrs.
Carol Feister helped write the computer programs required for
data processing. Mr. William Wilson and Mrs. Dean Pendleton pre-

ACKNOWLEDGEMENTS ( continued)

pared the many figures included in the dissertation. Mrs. Arlene
Sullivan has done all the typing.

I am grateful to the United States Navy for financial support
provided under the Burke Scholar Program. The National Science
Foundation supported my work on Eltanin Cruise Ul.

Finally, I wish to record my gratitude to my wife for the
steady support she has given me during my studies .

TABLE OF CONTENTS

Page

I. INTRODUCTION 1

A. Purpose and Significance 1

B. Previous Work 3

C. Method of Analysis . 12

D. Data Processing and Quality Control 19
II. THE 50-cl/t SURFACE 27

A. Depth 28

B. Salinity 33

C. Dissolved Oxygen 37
III. THE 30-cl/t SURFACE Ul

A. Depth Ul

B. Salinity 1+1+

C. Dissolved Oxygen 1+9
IV. THE BOTTOM WATER 51

A. Potential Temperature 51

B. Salinity 57

C. Dissolved Oxygen 63

V. FLUX AND VELOCITY STRUCTURE OF THE

ANTARCTIC CIRCUMPOLAR CURRENT 66

VI. CONCLUSIONS AND REMARKS 79

FIGURES 86

REFERENCES 122

VITA

TABLE OF FIGURES

Page

Figure 1. Major topographic features of the

Antarctic Ocean 87

Figure 2. Stations for the 50- and 30-cl/t maps 88

Figure 3. The 50-cl/t surface: depth 89

Figure h. The 50-cl/t surface: salinity 90

Figure 5. The 50-cl/t surface: dissolved oxygen 91

Figure 6. The 30-cl/t surface: depth 92

Figure 7. The 30-cl/t surface: salinity 93

Figure 8. The 30-cl/t surface: dissolved oxygen 9^

Figure 9- Stations for the bottom-water maps 95

Figure 10. The bottom water: potential temperature 96

Figure 11. The bottom water: salinity 97

Figure 12. The bottom water: dissolved oxygen 98

Figure 13. Drake Passage: salinity section 100

Figure lU. Drake Passage: dissolved-oxygen section 101

Figure 15- Greenwich Meridian: potential-temperature

section 102

Figure 16. Greenwich Meridian: dissolved-oxygen

section 103

Figure 17. 20 °E: salinity section 10*+

Figure 18. 132°E: salinity section 105

Figure 19. 132°E: dissolved-oxygen section 10o

Figure 20. 170°W: salinity section 107

Figure 21. 170°W: dissolved-oxygen section 108

Figure 22. 65°S: salinity section . 109

TABLE OF FIGURES (continued)

Page

Figure 23. 65°S: dissolved- oxygen section 110

Figure 2k. 132°E: zonal component of relative

geostrophic velocity 111

Figure 25. 132°E: zonal component of absolute

geostrophic velocity 112

Figure 26. 115°E: zonal component of relative

geostrophic velocity 113

Figure 2J . 117°E: zonal component of relative

geostrophic velocity 11^

Figure 28. 128°E: zonal component of relative

geostrophic velocity 115

Figure 29. lU0°E: zonal component of relative

geostrophic velocity 116

Figure 30. lh6°E: zonal component of relative

geostrophic velocity 117

Figure 31. 20°E: zonal component of relative

geostrophic velocity 11°

Figure 32. Discovery dissolved-oxygen correction 119

Figure 33. Station curves for the Ross Sea and

the Weddell Sea 120

I. INTRODUCTION

I. A. Purpose and Significance

The Antarctic or Southern Ocean, which I will take to be the re-
gion between U0°S and the Antarctic continent, plays a central
role in the deep and bottom circulation of the world ocean.
There are two fundamental reasons for its importance in these
phenomena: (l) The Antarctic Ocean is by far the largest con-
nection among the Atlantic, Indian, and Pacific oceans. Enormous
amounts of water, heat, salt, and nutrients are exchanged among
the oceans. The exchange helps maintain the observed tempera-
ture, salinity, and dissolved-oxygen fields in the world ocean,
thereby affecting the earth's heat budget and the distributions
of marine organisms. (2) The extreme southern reaches of the
Antarctic Ocean are among the few regions of the world where
climatic conditions may lead to navifacial1 water becoming suf-
ficiently dense to overturn and replenish the deep and bottom
water. These sources of bottom water are critical elements in
the thermohaline circulation of the ocean.

Insight into the various dynamic and thermodynamic processes
which determine the antarctic circulation may be gained from tak-
ing several points of view. Theoretical attacks on the problem

l

The term "naviface" (Montgomery, 19&9) will be used to distin-
guish the air-sea interface from the several other surfaces
discussed in this paper.

have so far centered on modeling the quasi-zonal Antarctic Cir-
cumpolar Current, which is the dominant flow in the region.
These studies have been reviewed in the author's masters essay
(1968, unpublished).

The present paper takes a different approach, an essentially
empirical one. The horizontal and vertical distributions of
temperature, salinity, and dissolved oxygen in the abyssal lay-
ers are examined in detail. Within the limitations of certain
simplifying assumptions, to be discussed later, conclusions are
then reached concerning the large-scale structure of the circu-
lation.

Similar studies have been undertaken previously. Most of
them were based on data gathered prior to World War II. Within
the last decade a large amount of hydrographic work has been
carried out in the antarctic. Much of this work has been done
in areas that were previously very poorly sampled (e.g.3 the
Weddell Sea) or in areas where old measuring techniques were not
sufficiently sensitive to detect differences in water character-
istics [e.g., the Pacific Ocean). Further, the recent stations
include samples taken closer to the sea floor than on the early
stations. Although the new data have been discussed in relation
to specific sections of antarctic waters, notably by A. L. Gordon,
the present paper represents the first attempt to incorporate
these data into a general analysis of the circumpolar region.

In part, this study was prompted by the availability of a
large body of new data and a desire to find out what new features

might "be revealed in the deep and bottom waters of the antarctic.
The investigation was also motivated by an interest in some spe-
cific aspects of the antarctic circulation which are not yet well
understood. These include the structure of the velocity field
and the transport of the Antarctic Circumpolar Current, the merid-
ional exchange of deep water between the antarctic and the oceans
to the north, and the importance of the Ross Sea as a source of
bottom water.

I.B. Previous Work

Coir present understanding of the large-scale structure and cir-
culation of antarctic waters has been pieced together from the
efforts of many individuals . Most of this work has been carried
out within the last forty years, and my review will therefore
focus on this period. However, it is interesting to note that at
least a partial appreciation of the role of the antarctic in the
deep oceanic circulation was evident somewhat earlier. Buchan
(1895, p. 8) concluded from the deep-sea temperature measurements
made by the Challenger (1872-76) that, "There can be no doubt
that these very low deep-sea temperatures have their origin in
the Southern or Antarctic Ocean, the icy-cold waters of which are
propagated northwards... ." Buchan (p. 27) also deduced the ex-
istence of a warm deep current flowing from the North Atlantic to
the South Atlantic. This water mass, now known as North Atlantis
Deep Water, is another major component of the abyssal circulation
in the antarctic.

During the 1920' s, subs t ant ial improvements were made in
the hydrographic coverage of the South Atlantic, Indian, and
Pacific oceans. For the first time, oceanographers could begin
to understand the character of the general circulation. On the
basis of Carnegie (1928-29) stations taken in the Pacific Ocean,
Sverdrup (1931) was able to show that the deep circulation of the
Pacific must be qualitatively different from that of the Atlantic
and Indian oceans. He concluded that the Pacific deep water is a
mixture of deep water from the Atlantic and Indian oceans with
bottom water from the antarctic. Owing to a scarcity of data,
Sverdrup was unable to venture beyond these general conclusions,
and some of the details of the flow he deduced were later proved
incorrect. Nonetheless, the concept of inter-ocean exchange which
he formulated is basic to our present understanding of the abyssal
circulation.

In 1933, three papers dealing with the characteristics and
circulation of Atlantic -Ant arctic waters were published. Wiist
(1933) discussed at length the distributions of potential temper-
ature and salinity near the sea floor in water deeper than U000 m.
His maps, which were based on all data available at that time,
showed in a very striking fashion the propagation of cold bottom
water out of the Weddell Sea, northward along the western trough
of the Atlantic and eastward into the Indian Ocean. Wust's maps
have become the classic piece of evidence for the importance of
the Weddell Sea as a source of bottom water, although there is
some ambiguity in Wust's salinity map.

A more general description of the hydrography of the western
Atlantic-Antarctic was written by Deacon (1933). His study was
based on the early cruises of the Discovery and Disoovevy II. In
a brief section on the bottom water, Deacon reviewed several pro-
posed mechanisms for bottom-water formation. He concluded that
the bottom water is probably formed over the continental shelf
of the Weddell Sea, as Brennecke (1921, referenced by Deacon,
1933) had proposed somewhat earlier.

The question of exchange between the Atlantic and Pacific
oceans via Drake Passage was examined by Clowes (1933) in his
study of the deep water of the Scotia Sea. Noting that the deep
water of the Scotia Sea is somewhat less saline than that farther
to the east, he concluded that most of this water must come from
the Pacific. Clowes computed the geostrophic transport relative
to 3500 db through Drake Passage and arrived at a net eastward
flux of 110 x 106 m3/s. This was the first quantitative estimate
of the exchange of water through Drake Passage.

A broad account of the hydrography of the Atlantic-Antarctic
sector south of 55°S, between U0°W and U0°E, was given by Mosby
(193*0. A large part of his data were from the Norvegia Expedi-
tions of 1927-28, 1929-30, and 1930-31, but Deutschland (1911-12),
Meteor (1925-26), and William Sooresby (1931) stations were also
included. In addition to describing and classifying the water
masses of the region, much as Deacon (1933) had done, Mosby dis-
cussed two dynamical features of interest:

(1) His maps of dynamic topography (e.g.3 his Figure 28)
show clearly a weak, elongated cyclonic gyre centered in the
northeastern Weddell Sea and extending as far as 15°E. Moshy
pointed out that this flow pattern is consistent with the drift
of the Deutschlandj which was icebound in the Weddell Sea during
the winter of 1912. Noting that the winds in this region also
have a roughly cyclonic pattern, shifting from westerlies north
of 60°S to easterlies along the antarctic coast, he concluded
that the Weddell gyre is driven by the wind.

The existence of a cyclonic gyre in the Weddell Sea is also
suggested by Mosby's map of maximum sub-surface temperatures, on
which a westward intrusion of relatively warm water along the
antarctic coast can be seen. According to Mosby, this feature
had been observed in the Deutsehland data by Brennecke (1921),
who theorized that the warm water comes directly from the Indian
Ocean. Mosby, however, recognized the warm water as the uniform
deep water found elsewhere around Antarctica, and ruled out a
direct Indian Ocean influence.

(2) Mosby devoted considerable space to the question of
bottom-water formation. After reviewing the theories of bottom-
water formation, he, like Deacon, concluded that Brennecke' s
(1921) explanation is correct. Brennecke had proposed that bottom
water is formed in the Weddell Sea from navifacial water which is
overturned during winter. The postulated vertical convection,
which takes place in shallow water, is driven by cooling and the

addition of salt at the naviface during the formation of sea ice.
Dense, uniform shelf water then flows down the slope, mixes with
warmer, saltier deep water, and forms a dense mixture which sinks
to the bottom. In accepting this mechanism for bottom-water
formation, Mosby pointed out that to produce bottom water with
the observed characteristics, the shelf water (assumed to be at
the freezing point) must have a salinity of at. least 3^.62/?c .

Mosby rejected Wust's (1933) suggestion that the mechanism
proposed by Brennecke is supplemented by deep vertical convection
occurring "at the edge of the convergence area created by pack
ice..." (Wust, 1933, p. Vf) • Wust's theory seemed highly unlikely
because (l) no such convergence has been detected (in fact, the
prevailing wind field would suggest the existence of a region of
divergence), and (2) the observed vertical temperature and salin-
ity distributions are inconsistent with deep overturning.

The first study of the circumpolar characteristics of the
Antarctic Ocean was by Deacon (1937) • The principal source of
data for Deacon's work was the circumpolar cruise of the Dis-
covery II in 1932-33, during which nineteen sections were made
in the antarctic zone.

Several highly significant features of the deep and bottom
flow around Antarctica were revealed in Deacon's paper. It be-
came clear for the first time that bottom water is not formed
all around the antarctic coast, as proposed by Sverdrup (1931),
but is instead formed primarily in the Weddell Sea. Deacon also
showed that the bottom water formed in the Weddell Sea flows

8

around Antarctica towards the east, not, as Sverdrup had deduced,
towards the west. (These conclusions were later confirmed by
Sverdrup (19AO) on the basis of data taken in the Indian -Ant arctic
Ocean during the BANZAR Expedition2 ( 1929-31) . ) Another valuable
result of the Discovery data was to confirm the extreme uniform-
ity of the deep water around Antarctica, which Sverdrup (1931)
had predicted from the limited data available to him.

In his BANZARE Report Sverdrup acknowledged the defects in
his earlier theory of the deep circulation, and he proposed a
revised version. He again emphasized the importance of the
Atlantic Ocean as the primary source of deep water in the world
ocean and the role of the Antarctic Circumpolar Current as the
agent by which Atlantic deep water is transported to the Indian
and Pacific oceans. Sverdrup computed the eastward water trans-
port relative to 3000 db across a section from Tasmania to
Antarctica (lU5°E). His result, ikk x 106 m3/s, was the first
estimate of the flux of water from the Indian to the Pacific
ocean.

Another significant aspect of the circumpolar circulation
was pointed out by Sverdrup in his discussion of the general cir-
culation in The Oceans (Sverdrup et al.3 19^2) . Sverdrup noted
a systematic deflection of the predominantly zonal transport lines
by major topographic features (see his Figure 163) . The direction

2

British, Australian & New Zealand Antarctic Research Expedition

of the deflection (equatorward on approaching a ridge; poleward
after crossing over the ridge) is in agreement with qualitative
predictions made by assuming conservation of potential vorticity.
One implication of these deflections is that the Circumpolar Cur-
rent must extend to great depths.

During the period from World War II through the mid-1950's
little oceanographic research was carried out in the antarctic,
and few papers were published on the subject. One paper from
this period which is notable in the present context is Fofonoff 's
(1956) study of bottom-water formation. Using temperature and
salinity data collected in the northern Weddell Sea, Fofonoff
gave quantitative support to Brennecke's model of bottom-water
formation.

The International Geophysical Year (1957-58) brought about
a resurgence in all phases of antarctic research including ocean-
ography. Over the past decade, research ships from the United
States and the Soviet Union have been very active in the antarc-
tic, and many hydrographic stations have been occupied. The sub-
stantial increase in field activity has resulted in numerous pub-
lished reports on antarctic oceanography.

Ishino (1963) presented circumpolar plots of temperature,
salinity, and density at several depths from 50 m to 3000 m,
using mainly Discovery (1925-37) and Ob ' (1956-58) stations. He
attempted to discuss seasonal variations in characteristics down
to 2000 m, but this effort was severely limited by the lack of
winter data. Perhaps the strongest conclusion that may be drawn

10

from Ishino's maps is that no systematic seasonal variations in
temperature and salinity are evident below 200 m, at least in
those areas where sufficient data are available for comparison.
Ishino (p. 166) apparently believed that large amounts of bottom
water are produced in shallow regions along the Indian Ocean sec-
tor of the antarctic coast. This conclusion conflicts with the
evidence presented by Sverdrup (19^+0), Deacon (1937) » and the
present work.

The most recent descriptive studies of the Antarctic Ocean
are those by Gordon (1966, 1967b, 1970) and Reid and Nowlin (1971).
The first two papers by Gordon are analyses of the sector from
20°W to 170 °W, utilizing the early Eltanin data. Gordon (1970)
has extended his study of bottom water into the eastern Indian
Ocean. Gordon and Goldberg (1970) have prepared a folio contain-
ing maps of temperature, salinity, and dissolved oxygen at con-
stant depths and sections showing the general structure of ant-
arctic waters. Pertinent aspects of these studies will be dis-
cussed in later sections.

Reid and Nowlin (1971) discuss the deep circulation in the
Drake Passage region. By using deep current measurements made
in the Passage as reference values for geostrophic computations,
they have determined that the net eastward mass flux through Drake
Passage is well over 200 x 106 m3/s (their value is 237 x 106 m3/s)
Most previous estimates were on the order of 100 x 106 m3/s.

The results of the above studies may be summarized in the
following brief description of deep and bottom waters in the

11

antarctic: Circumpolar Deep Water (Sverdrup, 19^+0) is a thick,
uniform layer of relatively warm and salty water found in all
longitudes around Antarctica. It has the following characteris-
tics: 8 = 1.0 - 2.0°C, S = 3^.70 - 3U.76 t ,0 = U.O - U.6 ml/1.

2

Circumpolar Deep Water flows in a broad, quasi-zonal current east-
ward around the continent at speeds of roughly 5 cm/s , and partic-
ipates in a much slower meridional exchange with the oceans to the
north. The mean zonal transport of the Antarctic Circumpolar Cur-
rent is probably in excess of 200 x 106 m3/s. Permanent large-
scale meanders in the zonal flow are found over major topographic
features .

A major constituent of Circumpolar Deep Water is North At-
lantic Deep Water, which moves south and mixes into the circumpolar
flow in the Atlantic and Indian ocean. The magnitude of the south-
ward flux of North Atlantic Deep Water is difficult to determine,
but calculations by Wright (1969) and Sverdrup et at. (19^2) in-
dicate that it is on the order of 20 x 106 m3/s at 30°S. As the
Circumpolar Deep Water flows around Antarctica, its properties
are further modified by lateral and vertical mixing, ultimately
producing the extremely uniform deep water (Common Water, Mont-
gomery, 1958) of the Pacific Ocean.

The second major water mass found in the abyssal depths of
the antarctic zone is Antarctic Bottom Water. (For the present,
Antarctic Bottom Water will be referred to as a single water mass.
Later I will show that two distinctive water types can be recog-
nized within this mass.) Antarctic Bottom Water has the char-

12
acteristics: 9 = -0.8 - 0°C, S = 3^.65 - 3U.71 %* , 0 = 5.U -

2

7.0 ml/1.

The primary source of bottom water is the Weddell Sea, al-
though the Ross Sea probably does produce limited quantities
(Gordon, 1966, 1970). The bottom water formed in the Weddell
Sea is a mixture of very cold, moderately salty shelf water and
warmer, more saline deep water. The dense shelf water is supposed
to be produced during winter when sea ice forms. This mechanism
is only inferred from summer observations , since no observations
of shelf water have ever been made in winter.

I.C. Method of Analysis

In this paper the circulation is studied by means of maps and
sections showing the distributions of salt and dissolved oxygen
in the deep and bottom waters . These substances act as tracers
with which the mean flow field can be deduced using a model based
on the diffusion equation.

Several assumptions and approximations have been made in order
to simplify the interpretation of the maps. The first of these is
that the large-scale ( > 102km) distributions of heat, salt, and
dissolved oxygen are in a steady state below the shallow navifacial
layer. In view of the demonstrated reproducibility of temperature
and salinity measurements below the naviface (compare Wiist , 1936,
Teil A and Fuglister, i960) this appears to be a reasonable approx-
imation, although the period over which such measurements have been

13

made is admittedly short in comparison with the residence time of
water in the deep ocean.

Both salt and dissolved oxygen are treated as conservative
properties, meaning that there are no sources or sinks below the
naviface. In the case of dissolved oxygen this is obviously not
strictly true. Phytoplankton concentrated in the upper 100-200 m
provide a source of oxygen, and decaying organic matter through-
out the water column acts as a distributed sink. Below the
euphotic zone the net effect is equivalent to a weak sink, the
strength of which is not well known. Wright (1969) quoted deep

0 -depletion rates of U.3 - 53.0 x 10 ml/1 per year; data col-

2

lected on the 19&9 GE0SECS test station suggest an average rate
of 25 x 10_I+ ml/1 per year (Craig and Weiss, 1970). The time
scale of the deep circumpolar flow is roughly 20 years. At an
average depletion rate of 25 x 10 ml/1 per year, changes in
oxygen-concentration due to the sink term would amount to 0.05
ml/1 over 20 years. This change is small compared with the prob-
able error of the oxygen data.

The third fundamental assumption behind the analysis is that
large-scale flow in the ocean tends to take place along surfaces
of uniform potential density or potential steric anomaly. Because
of geostrophy, such surfaces often have a small ( < 1:103) slope
relative to level {i.e., uniform-depth) surfaces. Over oceanic
scales, these small slopes may be significant, and it is there-
fore important to distinguish between surfaces of uniform depth
and surfaces of uniform potential steric anomaly. Processes oc-

lU

curring along the latter surfaces will be designated as "lateral"
processes {e.g.3 lateral mixing) as opposed to "horizontal" pro-
cesses occurring on surfaces of uniform depth.

The degree to which the above assumption is valid cannot be
stated quantitatively. If there were no diffusion or mixing in
the ocean, this approximation would necessarily be an exact state-
ment, since the potential steric anomaly of a fluid element is de-
fined by its salinity and potential temperature, which are conser-
vative properties in the sense stated previously. It does seem
reasonable to assume that in a stratified system (such as the
ocean) fluid elements are constrained somewhat to flow along,
rather than across, surfaces of uniform potential steric anomaly.
Ultimately, however, use of the assumption rests on empirical
grounds {e.g., Reid, 19&5, Figure 3) and on the proven value of
this model.

As a consequence of the third assumption, vertical advection
is no longer a separate term in the diffusion equation, having
been absorbed in the lateral advection terms. At this stage the
model has been reduced to a balance between lateral advection and
diffusion. In the large-scale physical processes of the ocean,
molecular diffusion is negligible compared with turbulent diffu-
sion, or mixing, which represents the gross effect of the fluc-
tuating advective terms on the mean field. Turbulent mixing is
customarily decomposed into vertical and lateral components.

In studies like the present one it is often assumed (at least
tacitly) that vertical mixing is of secondary importance compared

15

with lateral mixing. This assumption is probably due to the
heavy emphasis placed on lateral mixing by Rossby (1936, 1937) »
Montgomery (1938), and others who developed the method used here.
In most cases , such an assumption is not inconsistent with the
distributions of properties in the open ocean. On the other hand,
even in those studies where lateral mixing is assumed to predom-
inate there may be regions where the observed patterns can be ex-
plained only in terms of vertical mixing. That is, on the par-
ticular surface in question, there is no source of water with the
characteristics necessary to produce the observed distributions.

Theoretical studies are of little assistance in deciding
whether lateral or vertical mixing should predominate. The large-
scale oceanic distributions are matched equally well by models
containing (l) only vertical mixing (Robinson and Stommel, 1959) »
(2) only lateral mixing (Sverdrup, 1939) » or (3) no mixing at all
(Welander, 1959)- Some of the confusion is no doubt due to our
imperfect understanding of the mechanisms involved in turbulent
mixing, and the consequent use of rather arbitrary mathematical
functions to represent the mixing terms. In the present work I
am not concerned with the analytical form of the mixing terms
but only with the qualitative effects of mixing. The relative
importance of vertical and lateral mixing is determined from the
observed distributions. Except in a few cases where vertical
mixing is clearly important, the distributions are assumed to
represent a balance between lateral mixing and lateral advection,

16

with the mean flow taking place along the isopleths .

Three sets of maps have been drawn for this study. The
first two sets show the salinity, concentration of dissolved
oxygen, and depth of the surfaces where potential steric anomaly
equals 50 and 30 centiliters per ton3 (abbreviated cl/t). Poten-
tial steric anomaly, 6Q, is defined by the following equation:

(1) 6e = as,e,o " a35,o,o

where a „ - = specific volume of sea water of salinity

s, potential temperature 8, at sea pres-
sure 0 db ; potential temperature is the
temperature a sea water sample would
have after being raised adiabatically to
the naviface (p = 0) ;

and a = specific volume of sea water of salinity

->s jU »U

¥i%e, potential temperature 0°C, sea
pressure 0 db,
= 0.9726U ml/g.
Surfaces of uniform potential steric anomaly will be called isan-
osteric surfaces .

C.-G. Rossby appears to have been the first to use the dis-
tribution of a conservative property along a surface of uniform
potential density to deduce geophysical circulation patterns.

3

These surfaces correspond to potential density surfaces
aQ = 27.60 and 27.81 g/1, respectively.

IT

The method was applied to the atmosphere by Rossby and collabora-
tors (1937) > who used specific humidity as a tracer on surfaces
of constant potential temperature (isentropic surfaces).

Because of the transient nature of even the large-scale
atmospheric motion, isentropic analysis, as it was called, has
not found much application in synoptic meteorology. However, the
concept has proven valuable in studies of the ocean, where, as I
noted earlier, the large-scale circulation is reasonably steady.

The earliest oceanographic applications of Rossby' s technique
were by Parr (1938) and Montgomery (1938). Parr discussed the
distribution of salinity on several surfaces of uniform a
( ^ potential density) south of Newfoundland. Montgomery made
a detailed study of the circulation in the tropical North Atlantic
using the distributions of salinity and dissolved oxygen on six
Q -surf aces . Montgomery chose to call the method isentropic anal-
ysis in conformity with meteorological usage. This terminology
is still commonly seen. In view of the fact that isanosteric
surfaces (or surfaces of uniform O ) are not in general isentropic
surfaces, the expression "isanosteric analysis" suggested by
Tsuchiya (1968) may be more appropriate.

Notable applications of isanosteric analysis have been made
in recent years by Taft (19&3), whose maps of salinity and dis-
solved oxygen at intermediate depths cover the southern hemisphere;
Reid (1965) » who examined the intermediate waters of the Pacific;
Tsuchiya (1968), who studied the tropical Pacific; and Barkley

18

(1968), who has prepared an atlas of the Pacific Ocean.

All the above studies have used isanosteric surfaces which
are confined to the shallow and intermediate layers of the ocean.
When surfaces which extend to great depth (as in the present work)
are used, an additional complication is introduced. The problem
is caused by a rather subtle effect pointed out by Montgomery
(1937) some time ago, but generally ignored until quite recently
(Lynn and Reid, 1968) . The compressibility of sea water is a
function of its temperature, cold water being more compressible
than warm. As a consequence, two fluid elements of equal steric
anomaly, but different temperatures, at the naviface will have
different steric anomalies if both are moved adiabatically to
some depth, say 2000 m. In other words, fluid elements which
could mix laterally at the naviface could not do so at 2000 m.
The implication is that a single isanosteric surface does not
define a surface of lateral advection and mixing at all depths.

Necessary conditions for the above effect to be important
are that the temperature range on the isanosteric surface be
significant (2-3°C) and that the isanosteric surface undergo
large variations in depth ( > 2000 m) . In the case of the 30-
cl/t surface, the depth range is large ( ^ 2500 m) , but the
temperature range is small ( < 1°C) over the region where most
of the variation in depth occurs. The largest temperature gra-
dients are found near Antarctica, where the 30-cl/t surface is
shallow and pressure effects are not significant. Similar condi-
tions obtain on the 50-cl/t surface. It therefore appears that

19

the errors introduced by using a single isanosteric surface to
represent a surface of lateral advection and mixing are not ser-
ious in this particular study.

In the antarctic, the water "below the 30-cl/t surface is
extremely uniform with respect to density. Small measurement
errors or interpolation errors lead to substantial errors in
computing the characteristics of deeper isanosteric surfaces.
Therefore, isanosteric analysis is not useful for studying the
water near the bottom.

The third set of maps prepared for this paper shows the
distributions of potential temperature, salinity, and dissolved
oxygen near the sea floor. Data were plotted only in regions
where the depth of water exceeds 3500 m. With few exceptions
the samples are within 300 m of the bottom; the great majority
are within 200 m of the bottom.

Deacon (1937) and Wust (1938) have prepared circumpolar maps
of bottom potential temperature in depths greater than U000 m.
These maps were based on a much sparser data grid than is now
available. Gordon (1966) plotted maps of bottom potential temper-
ature and dissolved oxygen in depths greater than 3000 m for the
sector 20 °W to 170 °W. The bottom salinity and dissolved oxygen
maps presented in the present paper are the first such maps to
be drawn for the entire circumpolar zone.

I.D. Data Processing and Quality Control

The bulk of the data used in this study was acquired from the

20

National Oceanographic Data Center (NODC) , Washington, D. C.
Additional data have been acquired from various sources shown
in Table 1, which lists the data by ship in chronological order.
Over three-quarters of the stations were taken by three ships :
Eltanin (kl%) 9 Discovery (27%), and Ob' (9%). All but three of
the stations are Nansen casts, the exceptions being Eltanin
S-T-D stations.

Figure 2 shows the geographical distribution of stations
for the isanosteric maps, and Figure 9 shows the distribution of
stations for the bottom maps. Most, but not all, of the bottom
stations were used in the isanosteric maps too. Note the rather
sparse coverage of the central Indian Ocean.

Approximately 75% of the stations were occupied during the
six months November-April, with 50% of the total being occupied
in the three months January -Mar ch , the austral summer.

The hydrographic data were processed on The Johns Hopkins
University Computing Center IBM 709^. Potential temperature and
potential steric anomaly were computed using equations from
Fofonoff (1962a). Linear interpolation was used to determine
the values of parameters on the 50- and 30-cl/t surfaces . All
data were plotted and contoured by hand on polar sterographic
maps of scale 1:15 x 106. Bottom topography, Figure 1, was taken
from U. S. Naval Hydrographic Office charts H.O. l6, 892-1 (1959)
and H.O. 1262A and from the Soviet Atlas Antarktiki (Tolstikov,
1966) . In order to avoid crowding on the smaller-scale maps

21

TABLE 1. Sources of data.

SHIP

NATION

TOTAL BOTTOM DATA
YEARS STATS. STATS. SOURCE

Meteor

Germany

1925-26

32

11

1

Discovery

Gt. Brit.

1931-39
1951

240

39

1

Thorshaven

Norway

1934

1

0

1

Galathea

Sweden

1952

1

1

1

Ob'

USSR

1956-63
1965-66

78

8

1

Capitan
Canepa

Argen.

1957-59
1962

10

4

1

Horizon

US

1957

5

1

1

San Martin

Argen.

1958

1

0

1

Operation Deep
Freeze: various
Navy & Coast
Guard icebrkrs

US

1957-61

1964

1967

22

0

1

Atlantis

US

1959

6

k

2

Vema

US

1959-60

6

3

1

Argo

us

1960-61
1962

5

k

1

Diamantina

Austral.

1960-62

15

4

1

Gas coyne

Austral.

1961

6

0

1

Umitaka-Maru

Japan

1961-62

21

0

1

Africana II

S. Africa

1962-64

8

0

1

Eltanin

us

1962-67
1967
1968
1970

362

208

1
3
4
5

22

TABLE 1. (continued)

SHIP

NATION

YEARS

TOTAL BOTTOM DATA
STATS. STATS. SOURCE

Anton Bruun

US

196 k

2

0

1

Glacier
(IWSOE)

US

1968-69

22

5

6

Hakuho-Maru

Japan

1969

18

Ik

7

T. Washington

US

1969

21

Ik

8

882

320

1. National Oceanographic Data Center

2. Fuglister (i960)

3. Cruises 28, 29 (Scorpio Expedition) : SIO/WHOI (1969)
h. Cruises 32-36: Jacobs et at. (1970)

5. Cruise kl: author (unpublished data)

6. International Weddell Sea Oceanographic Expedition:
Hufford & Tennyson (1970)

7. Tokyo University Ocean Research Inst. (1970)

8. Piquero Expedition: SIO (unpublished data)

23

presented here, only station positions have been plotted.

Data quality and quality control are extremely important
considerations in a study of this type, for they determine the
resolution and reliability of the contours. The precision of
routine shipboard temperature and salinity measurements has been
well established by accumulated experience and by statistical
analyses of results (see, for example, Wooster and Taft , 1958).
Reversing-thermometer measurements are accurate to better than
0.02°C. Salinities determined by chlorinity titration are gen-
erally good to within 0.02 %a , although greater precision can
be achieved by taking special precautions, as in the case of
Meteor salinities. Salinities measured with electrical conduc-
tivity salinometers are significantly more accurate than those
done by titration. Cox (1965) states a precision of 0.002 %> f or
these instruments. In routine use the reliability is probably
somewhat less, perhaps 0.00 if %0 .

The reliability of the diss olved-oxy gen determinations is
difficult to estimate. Nearly all methods for determining the
concentration of dissolved oxygen in sea water are based on the
Winkler method. Under optimum conditions this method may achieve
an accuracy of 0.1$ (Carpenter, 1965) . However, as Carritt and
Carpenter (1966) have demonstrated, numerous variations in ana-
lytical technique among oceanographic institutions and individual
chemists make it impossible to establish the routine reliability

Virtually all the salinity data since 1962 (about half of the
total) were measured by this method.

2U

of oxygen data. Wide variations in quality may be expected.

Of particular significance to this study are the oxygen
measurements made during the Disaovery cruises . Several authors
(Wooster and Volkmann, i960 ; Gordon, 1966; Carritt and Carpenter,
1966) have noted that these measurements are consistently lover
than those made by other ships in the same vicinity. The dis-
crepancy has been reported to be on the order of -0.5 ml/1 by
Gordon (1966) and Carritt and Carpenter (1966).

The present author has made numerous comparisons between the
Discovery dissolved-oxygen data and those of other ships at the
same depth and approximate geographical location. The consistent
difference observed by others was confirmed. The possibility of
a long-term increase in dissolved-oxygen concentration was ruled
out by the fact that Disaovery values were also lower than Meteor
(1925-26) data, and that no trend in oxygen concentration was ob-
served in the Discovery data, which were taken over a ten-year
period.

Unfortunately, the raw Discovery data were destroyed in
World War II, and it is not possible to determine and correct
the error properly. I decided to devise a correction curve by
comparing the measured navifacial oxygen concentration with the
saturation value appropriate to the measured navifacial tempera-
ture and salinity. The resulting linear best-fit shown in Figure
32 was used to correct all Discovery oxygen data. The correction
is proportional to the dissolved oxygen concentration, ranging
from +0.2 ml/1 at a measured value of U.O ml/1 to + 0.U ml/1 at
7.0 ml/1.

25

A similar trend (increasing error with concentration) was
observed from the comparison of deep Discovery oxygen measure-
ments with those of other ships. However, the corrections de-
rived by extrapolating those comparisons are somewhat larger
than those found from the navifacial -concent ration/saturation-
value method. The differences between the two methods might be
partly accounted for by the fact that antarctic navifacial waters
are commonly supersaturated (Jacobs and Amos, 1967) » leading to
an underestimate of the navi face -measurement error. On the whole,
it was felt that the navi face-comparison method was a more reli-
able correction procedure, since it was based on a wider range of
data points. It is likely, however, that the corrected Discovery
oxygen data are still, on the average, lower than neighboring
points from other ships .

Aside from the systematic correction applied to all Discovery
dissolved-oxygen data, no corrections were made to the original
data. Questionable points were evaluated, and occasionally elim-
inated, on the basis of the following objective and subjective
criteria:

(1) agreement with neighboring points on the same surface,
and consistency of vertical profile;

(2) method of making measurement;

(3) general reliability of data from the particular ship
or cruise in question.

The "general reliability" test was based on repeated compar-
isons of stations from several ships in close proximity to each

26

other. As a rule, these comparisons showed good agreement among
the data of different ships. The notable exception (in addition
to the Discovery oxygens) was the Ob ' salinity measurements,
which contain more scatter than those of other ships. Since most
of the Ob * stations used are close to the Antarctic continent,
where the lateral gradients are relatively strong, this scatter
is not serious .

27
II. THE 50-cl/t SURFACE

The 50-cl/t isanosteric surface was chosen for analysis because
it forms what might be considered the upper boundary of the deep
water mass being studied here. It is recognized that the defini-
tion of such a boundary must be somewhat arbitrary, being based
on gradients in the characteristics which distinguish each water
mass .

Perhaps the most fundamental and unambiguous of these char-
acteristics is salinity. South of about 55°S, the 50-cl/t sur-
face lies within the sharp halocline which marks the transition
from the fresh upper waters of the antarctic to the more saline
deep water. North of about 50°S (the "Antarctic Convergence"),
the 50-cl/t surface is found between the salinity minimum (Ant-
arctic Intermediate Water) at 80 cl/t and the deep salinity max-
imum (Circumpolar Deep Water) at 30 cl/t (see Figures 18, 20).

Over much of the antarctic region (indeed, over a large part
of the southern hemisphere) , the 50-cl/t surface coincides with
the vertical minimum of dissolved oxygen (see Figures l6, 19, 2l) .
This relationship persists from the subtropics to about 60°S, ex-
cept in the eastern South Atlantic, where the two coincide between
roughly 30°S and 50°S. In this respect the 50-cl/t surface again
provides a convenient boundary between the upper and intermediate
waters and the deep water, all of which have higher dissolved-
oxygen concentrations.

28

II. A. Depth

The depth of the 50-cl/t surface (Figure 3) ranges from less
than 100 m near Antarctica to more than 1600 m at about U0°S.
During the summer season, this isanosteric surface lies below
the naviface everywhere except in some parts of the Ross and
Weddell seas. The data used in this study show the 50-cl/t sur-
face intersecting the naviface at several stations along the
western side of the Ross Sea and at two stations very close to
the western and southern boundaries of the Weddell Sea. Only
two years' (1968, 19&9) summer data are available for the Weddell
Sea, and it is not known how representative they are of general
conditions there. In the Ross Sea, on the other hand, numerous
stations have been occupied by U. S. icebreakers over the past
fifteen years. These data show that navifacial steric anomaly
values less than 50 cl/t are not uncommon in the southeastern
and western portions of the Ross Sea. It appears that the 50-cl/t
surface frequently intersects the naviface in the interior of the
Ross Sea during summer. In view of the similarity of climatic
conditions in the Ross and Weddell seas, it is not unlikely that
this situation occurs intermittently in the extreme reaches of
the Weddell Sea too.

The only winter observations from either of these areas
were taken by the Deuts ohland in the Weddell Sea in 1912. These
stations show navifacial steric anomaly values well below 50 cl/t
(but above 30 cl/t) occurring throughout the central Weddell Sea.

29

Indirect evidence of winter conditions is provided by the very
cold water (Winter Water, Mosby, 193^) which is observed just
below the thin navifacial mixed layer near the Antarctic contin-
ent in summer. This water, which is the remnant of the homoge-
neous navifacial layer formed in winter, has the following char-
acteristics: 9 < -1.0°C, S ^ 3U.3 X , 6. < 50 cl/t. Therefore,

y

it is highly probable that in winter the 50-cl/t surface inter-
sects the naviface several degrees from the coast all around
Antarctica.

The depth contours of an isanosteric surface provide a
qualitative representation of the large-scale geostrophic circu-
lation. The dominant pattern evident in the contours of the 50-
cl/t surface is the quasi-zonal, eastward-flowing Antarctic Cir-
cumpolar Current. In most longitudes the zonal flow occupies a
band of latitude some 10° - 15° in width, as defined by the dis-
tance between the 200-m contour and the lUOO-m contour in the
Atlantic and the l600-m contour in the Indian and Pacific oceans.
However, in several places the flow constricts tightly to a
width of about 5°> indicating significantly increased velocities
in these areas. Such constrictions occur in Drake Passage,
across the Macquarie Rise, and north of the Ross Sea at lU5°W.

The first two of these constrictions are obviously forced by
contractions of the lateral boundaries of the flow, but the con-
striction north of the Ross Sea cannot be explained in terms of
lateral boundary effects. Gordon (1967b) has stated that the
flow pattern here is caused by the "channelling" effect of the ■

30

fracture zone (called by Gordon the USARP fracture zone) which
cuts across the Pacific-Antarctic Ridge at lUo°W. Although the
fracture zone may play some part in constricting and turning the
flow north of the Ross Sea, it is difficult to "believe that such
a striking change in the flow could be caused by this relatively
small topographic feature.

In all of the places where constrictions of the flow occur,
and over the Scotia Ridge too, wave-like distortions of the zonal
flow are observed. These features are in approximately the same
positions as those noted by Sverdrup et at. (19^2).

Close to the antarctic coast there is a tendency for the
50-cl/t surface to deepen slightly toward the south. The reverse
slope, which is most pronounced from 0° to lU0°E, is indicative of
westward flow along the coast. Deacon (1937) gave the name "East
Wind Drift" to the shallow coastal current. The name suggests
the probable cause of the westward flow near the coast: easterly
winds prevail south of about 65°S. Barcilon (1966) has shown that
such a current might be driven by the discharge of melt water from
the antarctic ice cap. Even if this is not the case, significant
seasonal variations in the coastal current due to changes in the
sea-ice cover and melt-water runoff might be expected. For ob-
vious reasons there are no observations to support or contradict
such conjecture.

North of the eastward Circumpolar Current there is evidence
of the anticyclonic subtropical gyres which dominate the low- and

31

middle-latitude circulation. The gyre stands out particularly
well in the western South Pacific, where the 50-cl/t surface
reaches its maximum depth of nearly 2000 m. The trough which
marks the axis of the gyre runs along U9°S approximately. Strong
southward flow along the eastern edge of the New Zealand Plateau
closes the gyre to the west.

At shallower depths, the structure of the South Pacific gyre
is rather different from that on the 50-cl/t surface. On Reid's
(1965) map of the 125-cl/t surface (depth 500-700 m) the gyre en-
compasses a much larger area, and its axis lies to the north along
23°S. The western boundary of the gyre is found along the coast
of Australia (the East Australian Current), and there is only a
slight indication of southerly flow along the east coast of New
Zealand. On Reid's 80-cl/t map the axis of the gyre has shifted
south to about U0°S. At the depth of this isanosteric surface
(1000-1200 m) , New Zealand and the shallow plateau surrounding
it form a major obstacle to the flow, and the subtropical gyre
begins to divide into two gyres, one in the Tasman Sea and a
larger one in the western South Pacific. A poleward boundary
current is now clearly present east of New Zealand. Presumably,
the ridges north of New Zealand serve to separate the two gyres
even more at greater depths , leading to the well-defined closed
gyre observed in the western South Pacific on the 50-cl/t surface.

In the Indian Ocean, only the western tip of the subtropical
anticyclone appears on this map (near 35°S, U0°E) . Associated

32

with the ant i cyclonic gyre is a complex eddy -like feature to the
southwest. This appears to be produced by the shear between the
intense westward Agulhas Current and the eastward Circumpolar
Current. In the Atlantic Ocean too the map does not extend far
enough north to show much of the subtropical gyre. However, the
poleward western boundary current can be seen along the east
coast of South America between 35°S and U2°S.

In addition to the ant i cyclonic gyres normally associated
with low- and middle -latitude wind-driven circulation in the
oceans, there is also a somewhat anomalous gyre south of Australia.
North of the elongated trough centered on roughly ^5°S, the 50-cl/t
surface shoals, indicating a return flow to the west. Although
this feature is evident even in the earliest antarctic sections
{e.g.y Deacon, 193T> PL. XIX ) , there is no mention of it in the
literature .

Systematic hydrographic work in the Australian Bight has
only recently been started, and data are still rather sparse.
Bye (1968) has discussed some of the characteristics of the west-
ward flow, which he called the Flinders Current. According to
Bye, the average westward transport of the Flinders Current is
20 - 30 x 10s m3/s. Most of this water comes around the southern
tip of Tasmania, Bass Strait being too shallow to permit a sig-
nificant flux. The water transported to the west must represent
a mixture of water brought from the north by the East Australian
Current and antarctic water deflected north by the New Zealand
Plateau.

33

The circulation in the Australian Bight is complex. Prom-
inent meanders are present at 132°E and 115°E. In Bye's report,
these areas- are marked by large-scale eddies. More data will be
required to resolve the structure of the circulation in this re-
gion.

II. B. Salinity

Salinity on the 50-cl/t isanosteric surface (Figure h) ranges
from 3^.28 X to 3^.62 %, , a difference of 0.31* % . The range of
potential temperatures which corresponds to the salinity range
for 6Q = 50 cl/t is -1.2°C to 3.1°C At the melting point, water
of 6Q = 50 cl/t has the characteristics 9 = -1.86°C, S = 3^.26 %o .

The salinity distribution, like the flow, is predominantly
zonal, with values increasing from south to north. Perhaps the
most striking aspect of the circumpolar salinity distribution
is its separation into two distinct bands, a southern one in
which the salinity gradient is comparatively steep, and a northern
one in which the gradient is much smaller. Roughly speaking, the
31+.52- %o isohaline marks the boundary between the two zones. In
the Atlantic and western Indian oceans this isohaline follows the
50 °S parallel. Continuing around Antarctica toward the east, the
3^.52-^,, isohaline shifts toward the south, eventually reaching
about 60°S in the eastern South Pacific.

The steep salinity gradient is found south of the 3^.52- %,
isohaline. The gradient is generally confined to a narrow band
approximately 5° of latitude in width. Except in the Ross and

3U

Weddell seas, where large areas of uniformly low-salinity water
occur, the gradient is contiguous to the antarctic coast.

Near the coast, the 50-cl/t surface lies just below the
shallow layer of fresh, warm water which forms as the sea ice
recedes in summer. Vertical mixing with the navifacial layer
provides low-salinity ( ^ 33.9 %> ) water which mixes along the
50-cl/t surface with more saline water from the north, producing
the observed gradient. In winter, the character of the salinity
gradient is probably not significantly altered, for the 50-cl/t
surface intersects the naviface where salinities are less than
3^.0 %, (Gordon and Goldberg, 1970, PL. 3).

Relatively fresh water formed at the naviface in summer
flows out of the Weddell Sea toward the north and east. North-
ward flow occurs east of the South Orkney Islands and along the
western side of the South Sandwich Islands. These two outflows
combine to form the low-salinity tongue surrounding South Georgia.

The cyclonic Weddell Sea gyre is evidenced by the low-salinity
tongue projecting toward the east along roughly 62°S. Return flow
near the coast is indicated by the westward penetration of more
saline water. The anomalously diffuse appearance of the salinity
gradient to the east of the Weddell Sea is probably due to the
gyral circulation.

A second major source of low-salinity water along the antarctic
coast is found in the eastern Ross Sea. This water propagates to-
ward the northwest, splitting into two branches near T0°S, 180° .
One branch continues toward the northwest, extending past the

35

Balleny Islands as far as 57°S, ll+5°E. The other tongue of low-
salinity water flows northeast parallel to the Pacific-Antarctic
Ridge .

The sharp bend in the isohalines at 55°S, lH5°W coincides
with the abrupt change in direction of the circumpolar flow noted
on the depth map. After crossing over the Pacific-Antarctic Ridge,
the Circumpolar Current flows east toward Drake Passage. There is
a tendency for some of the water to recirculate into the Ross Sea,
although the gyre is not as well developed as in the Weddell Sea.

Somewhat higher salinities ( > 3*+. 32 X° , reaching 3^.3^ %» )
than those found in the eastern sector of the Ross Sea are present
in the central and western sectors. This is true throughout the
water column (Countryman and Gsell, 1966, Figure IT) > not just at
50 cl/t . The higher salinities must be the result of local pro-
cesses taking place within the Ross Sea, since the water flowing
into the Sea is of lower salinity.

The low-salinity tongue extending northward to the east of
the Kerguelen Ridge appears to indicate outflow from MacKenzie
Bay and the Davis Sea.

The abrupt transition from the high-gradient to the low-
gradient band is explained by noting that the 3^.52- %> isohaline
generally lies just to the south of the steepest slope of the
50-cl/t surface. According to the geostrophic relation, the
steep slope marks the high-velocity core of the Antarctic Circum-
polar Current. The increased lateral shear and zonal transport
associated with the velocity core quickly smear out the lateral

36

salinity gradient, leading to a uniform salinity distribution
to the north.

Within the uniform circumpolar band, salinities range from
3U.52 %o to about 3^.58 m . The high-salinity water required to
maintain these values comes from two principal sources: the
western South Atlantic and the western Indian oceans. The first
source shows clearly as a tongue of high salinity extending pole-
ward along the east coast of South America. Near U0°S, the high-
salinity water mixes with lower-salinity water which flows north-
ward around the east coast of the Falkland Islands . The resulting
mixture (S 'v* 3^.56 %o ) then flows east in the Circumpolar Current.

The Indian Ocean source is inferred from (l) the saline water
to the south and southeast of Africa, (2) the salinity distribu-
tion on Taft's (1963) 60-cl/t map, and (3) the very high salinity
values found on the 50-cl/t isanostere in the western Indian Ocean
section of Clowes and Deacon (1935). The high-salinity water is
evidently derived from North Indian Deep Water which flows south
in the western Indian Ocean.

Salinity in the Pacific Ocean north of 60°S is exceedingly
uniform. Fortunately, all the data plotted in this region were
taken by electrical salinometer. Therefore, the small variations
that are observed may be used with confidence to trace the circu-
lation. The anticyclonic flow around the subtropical gyre is
marked by a broad northward displacement of the 3^.56- %■ isohaline.
centered on 120°W. Further to the east, salinity increases slight-
ly, suggesting slow poleward flow along the west coast of South

37

America. A similar trend (although less well defined because of
the lower precision of the data) is evident on Taft's (1963)
60-cl/t map.

A distinct tongue of low-salinity water can be seen enter-
ing the eastern Tasman Sea from the south and then curving in an
anticyclonic sense. This pattern is consistent with the deflec-
tion of the Circumpolar Current indicated by the 50-cl/t depth
map.

The salinity distribution in the Australian Bight is confus-
ing. High -salinity water is present at either end of the con-
tinent, but the two regions are separated by a well-defined zone
of slightly lower salinity between 130°E and 135°E. This is the
region where a large cyclonic meander appears on the depth map.
There are insufficient data in the northern half of the Bight to
be conclusive, but one possible interpretation is that the circu-
lation consists of two large eddies, a cyclonic one to the west
of Tasmania and an anticyclonic one near 120°E. These eddies
would transport low-salinity water north into the center of the
Bight.

II. C. Dissolved Oxygen

The dissolved oxygen distribution on the 50-cl/t surface (Figure
5) is remarkably consistent with the salinity distribution. In
particular, the division into two distinct gradient bands observed
in the salinity map also occurs in the oxygen distribution. The
U.5-ml/l oxypleth will be taken as the approximate boundary be-
tween the two bands .

38

South of the U.5-ml/l oxypleth, the dissolved oxygen con-
centration increases rapidly. Values exceeding T«5 ml/1 are found
nearly everywhere along the coast, except to the west of the Ant-
arctic Peninsula, where the dissolved-oxygen concentration does
not reach 6.0 ml/1. Maximum values ( > 8.0 ml/l) are found in
the southwestern corner of the Ross Sea. Typically, the high-
gradient zone is 5° in width, although it is substantially broader
east of the Weddell and Ross seas.

In the Indian and Pacific oceans, the dissolved-oxygen values
decrease monotonically toward the north away from the high-gradient
band. Concentrations of less than k.O ml/1 are observed along the
northern limit of the maps in both oceans. In the Atlantic Ocean,
the concentration of dissolved oxygen reaches a minimum of about
k.O ml/1 along U5°S, then increases toward the north.

Earlier it was pointed out that the 50-cl/t surface coincides
with the vertical minimum of dissolved oxygen over much of the
antarctic. Gordon (1967b) also noted the widespread oxygen min-
imum, which he identified with the upper Circumpolar Deep Water.
Gordon attributed the minimum to depletion by biological activity
in low latitudes of the oxygen-rich North Atlantic Deep Water.
However, it is clear from Figure 5 that the water flowing south
along the east coast of South America is substantially richer in
dissolved oxygen than the water in the lateral minimum. The min-

5

This may be due partly to the scarcity of data in this region,
especially in the Bellingshausen Sea, where heavy ice is always
encountered.

39

imum cannot "be due to the gradual consumption of oxygen in the
North Atlantic Deep Water as suggested by Gordon. The source
of the oxygen minimum is the oxygen-poor water ( < 3.2 ml/l)
found along the west coast of South America (see Barkley, 19&8,
Figure 106) . This water flows south and mixes with the circum-
polar water in Drake Passage. The low-oxygen water flowing
eastward out of Drake Passage stands out as a lateral minimum
across the South Atlantic on the 50-cl/t surface and also in
the Drake Passage and Greenwich Meridian oxygen sections, Figures
1^, l6. Minimum values in the low-oxygen core in the South At-
lantic are: Drake Passage - 3.87 ml/1, U5°W - 3.8U ml/1,
Greenwich Meridian - 3.90 ml/1.

Off the tip of South Africa additional oxygen-poor water
mixes with the eastward-flowing Circumpolar Current. The low-
oxygen water which appears as a tongue along 20° - 25°E origin-
ates in the North Indian Ocean (Clowes and Deacon, 1935). This
water is transported southward along the east coast of Africa,
and it feeds the circumpolar flow via the complex eddy observed
on the depth map. The effect of this water is offset by mixing
with water from the Weddell Sea gyre, which transports high-oxy-
gen water eastward along 60°S. As a result, oxygen values in the
uniform circumpolar belt do not change significantly south of
Africa.

High-oxygen water from the Weddell Sea can be seen flowing
north into the Scotia Sea along the same paths followed by the
low-salinity water. Once again these tongues merge to form a

Uo

prominent tongue which extends past South Georgia.

A high-oxygen tongue along 90°E marks the outflow from Mac-
Kenzie Bay and the Davis Sea. MacKenzie Bay is the third largest
indentation in the antarctic coast, so it is not surprising to
find it acting as a source of oxygen-rich, low-salinity water
similar to that originating in the Ross and Weddell seas.

In the Ross Sea, the dual tongues of low salinity are
matched by a divided tongue of high-oxygen water. High values
extend northwest past the Balleny Islands and northeast along
the Pacific-Antarctic Ridge. The weak cyclonic gyre north of
the Ross Sea is again evident.

The anticyclonic gyre in the Pacific may he discerned in the
northward penetration of relatively high-oxygen water along 120°W,
The southward dip of the oxypleths adjacent to the eastern edge
of the New Zealand Plateau represents poleward transport in the
western boundary current .

The deflection of part of the Circumpolar Current into the
Tasman Sea shows up in the high-oxygen tongue reaching as far as
U2°S. Oxygen values in the Australian Bight are very uniform.
Those in the eastern end off Tasmania tend to be slightly higher
than those in the west. However, the differences are too small
to allow any conclusions to be drawn regarding the flow.

III. THE 30-cl/t SURFACE

In the Antarctic Ocean the 30-cl/t isanosteric surface lies with-
in the deep salinity -maximum layer nearly everywhere (see Figures
13, IT, 18, 20). The principal exceptions are the areas surround-
ing the Ross and Weddell seas, where the salinity maximum is U00-
600 m deeper than the 30-cl/t surface. This is to be expected,
since both regions are sources of dense, low-salinity water 6
which alters to a significant degree the distributions around
them. As the Ross and Weddell sea waters mix with the saline
water from the north, the salinity maximum is eroded and forced
to greater depth.

The deep salinity maximum is a wide-spread feature of the
world ocean. Originating in the North Atlantic, the highly sa-
line deep water can be traced through the South Atlantic, across
the Indian Ocean south of about 30°S, and into the Pacific Ocean
as far as U0°S. The fact that the salinity maximum coincides
with the 30-cl/t surface over such a large area of the ocean makes
this isanosteric surface of particular interest. At the same time
it provides support for the original assumptions upon which this
analysis is founded.

III. A. Depth

The 30-cl/t surface ranges in depth from about 200 m in the Ross
and Weddell seas to more than 3500 m in the central South Pacific

6

The high-salinity Ross Bottom Water is much denser than 30 cl/t
and does not mix with the deep water. See Figure 33.

U2

(Figure 6). In the Atlantic and Indian oceans, the maximum depth
of the 30-cl/t surface is slightly over 2500 m. At none of the
stations used in this study does the 30-cl/t surface intersect
the naviface. It was pointed out earlier that the Deutschtand
Weddell Sea stations show navifacial steric anomalies greater
than 30 cl/t even in mid-winter. It is probable that this
isanosteric surface lies below the naviface throughout the year,
although it may be shallower in winter than in summer.

In addition to the shallow plateaus surrounding New Zealand
and the Falkland Islands, there are several mid-ocean regions
where the entire water column has a potential steric anomaly
greater than 30 cl/t. In such places the 30-cl/t surface inter-
sects the sea floor, which acts as a barrier to flow along the
surface .

In high latitudes there is a close resemblance between the
flow on the 30- and 50-cl/t surfaces. The large-scale deflec-
tions of the zonal circumpolar flow associated with topographic
features are found in the same places on both surfaces. Weak
cyclonic gyres in the Ross and Weddell seas are evident in the
topography of the 50-cl/t surfaces and to an even greater degree
in the 30-cl/t surface .

One high-latitude feature which stands out on the 50-cl/t
surface but can not be seen on the 30-cl/t surface is the ant-
arctic coastal counter cur rent (East Wind Drift). This fact pro-
vides an indication of how shallow the counter current is, since
the 30-cl/t surface is only 300 m below the naviface near the
coast .

U3

A narrow trough in the 30-cl/t surface runs along roughly U3°S
south of Australia. While the topography of the isanosteric sur-
face indicates anticyclonic relative motion, the absolute sense of
flow can not be determined from this evidence alone. This question
will he resolved by examining the distributions of properties.

The small cyclonic eddy south of Africa appears, like its
counterpart on the 50-cl/t surface, to be related to the shear be-
tween the westward-flowing Agulhas Current and the eastward Circum-
polar Current .

In lower latitudes the topographies of the 30- and 50-cl/t
surfaces do not match as well as they do in the circumpolar belt.
The subtropical anticyclone in the South Pacific, which is so
clearly defined on 50 cl/t, is completely absent on 30 cl/t. In
view of the great depth of the 30-cl/t surface in this region, it
is not surprising that the subtropical gyre is not evident.

While the subtropical gyre is not present, there is an indica-
tion of a boundary current along the New Zealand Plateau. Stommel
and Arons (i960) have predicted from a model of the abyssal circu-
lation that a northward-flowing deep boundary current exists in
the western South Pacific Ocean. Recent field work (Reid et at.s
1968) has confirmed the existence of the current and suggests a
transport of 8 - 12 x 106 m3/s. In the present work, the deep
current is reflected not only in the slope of the 30-cl/t surface
but also in the distributions of salt and oxygen on that surface
and in the distributions in the bottom layer.

There is no indication of the subtropical gyre in the Indian
Ocean south of 35°S. However, the data are scarce, and the gyre
may still persist at this depth. In the Atlantic, the gyre does
show up as a narrow trough centered on U0°S.

III.B. Salinity

The total range of salinity on the 30-cl/t isanosteric surface
is from 3^+. 56 %o in the Ross and Weddell seas to 3^.92 % in the
western South Atlantic Ocean (Figure 7) • Over most of the map,
however, the salinity range is much smaller, about 3^.68 -
3^.80 %o , a difference of only 0.12 X • Potential temperature
on this surface decreases from 3.*+°C in the north to -0.8°C near
the Antarctic continent. Over most of the antarctic region, the
meridional change in potential temperature is about 2°C. At the
melting point, water with 6fl = 30 cl/t has the characteristics
9 = -1.8T°C, 5 = 31+.52& .

Three water masses with distinctly different salinities
converge in the western South Atlantic Ocean. Extremely uniform
Circumpolar Deep Water enters through Drake Passage in the west.
The salinity of this water varies from 3^.71 %° along the northern
side of the Passage to 3*+. 73 %> along the Antarctic Peninsula.
After flowing through Drake Passage, the Circumpolar Deep Water
continues east until it impinges on the Scotia Ridge, where it
is forced to divide. Part of the water flows north out of the
Scotia Sea through the passage between the Falkland Islands and

U5

South Georgia, while the remainder flows east around the south-
ern end of South Georgia. Low-salinity water from the former
branch can be seen extending along the continental -shelf of
Argentina and along the northern side of the Scotia Ridge.

Water of salinity lower than that of Circumpolar Deep Water
enters the South Atlantic from the Weddell Sea. The outflow is
marked by a tongue of fresh water (S < 3^.68 %, ) centered on the
South Sandwich Islands. The effects of this water on the salin-
ity distribution can be detected as far north as U7°S.

In sharp contrast to the two water types flowing into the
South Atlantic from the west and from the south is the highly
saline North Atlantic Deep Water. In the tropics, water with
salinity exceeding 3*+. 90 %o is found between 2000 and 3000 m
across the entire South Atlantic (see Fuglister, I960, p. 6l) .
Maximum salinities in the western trough are slightly higher
( *v* 0.02 %o ) than those in the eastern trough. Maximum salinities
on both sides of the South Atlantic decrease toward the south,
but those on the eastern side decrease more than those on the
western side. At 32°S, the maximum in the western high-salinity
core is about 0 .0G %o greater than that in the eastern core (Fug-
lister, I960, p. 57).

Near 35°S the western high-salinity core, which in lower
latitudes is pressed against the east coast of South America,
separates from the boundary. It mixes with the lower-salinity
water from the south and joins the general eastward flow across

1*6

the South Atlantic along roughly UO°S. As it approaches the
African coast, part of the deep water turns north in the sub-
tropical gyre. The northward flow of this slightly diluted deep
water (S = 3^.80 - 3^.8U X ) probably accounts for the greater
poleward decrease in salinity in the eastern South Atlantic rela-
tive to the western South Atlantic.

The deep water which does not turn north in the subtropical
gyre continues east past the tip of Africa. This salty water
shows up clearly in the northern end of the 20°E vertical section,
Figure IT. Near U0°E the 3^.80- %<, is oh aline turns abruptly to
the north, indicating that the high -salinity water is flowing
along the east coast of Africa. The abyssal model of Stommel and
Arons (i960) predicts a northward-flowing deep boundary current
in the western South Indian Ocean as well as in the South Pacific.
At present the hydrographic coverage of this region is regret-
tably sparse, and the prediction of Stommel and Arons remains un-
tested. The present map, however, suggests that such a current
does exist.

Across the Indian sector of the antarctic the 3k-l6-%° iso-
haline marks the boundary between the salty deep water originat-
ing in the North Atlantic and the less saline Circumpolar Deep
Water which flows through Drake Passage. Near 110 °E the salty
deep water shifts several degrees to the south before continuing
eastward in a broad band across the Indian -Ant arctic Basin. In
this region, the vertical salinity maximum associated with the

hi

30-cl/t surface develops into a lateral maximum which can be
traced around the circumpolar belt as far as Drake Passage (see
Figures 18, 20, 13). Maximum values in the high-salinity core
are: 132°E - 3^.755 X , 1T0°W - 2h.lk3 % , Drake Passage -
31+.739 %o , which represents a decrease of less than 0.02 %. in
about 10,000 km.

The fact that a tongue develops in the salinity distribu-
tion south of Australia is due to the presence of relatively low-
salinity water ( < 3^.7^ %» ) in the South Australian Basin north
of 1+Q°S. The Tasman Sea is the only source of such water on this
map. Thus, it appears that the deep flow in the South Australian
Basin is westward, and the large gyre along U3°S is in fact anti-
cyclonic. Transport calculations to be discussed in Section V
indicate that the westward flow is extremely weak- in the deep
layers .

The northward loop in the 3^.1^-Xo isohaline west of New
Zealand indicates deflection of some of the circumpolar flow into
the Tasman Basin. The rest of the Circumpolar Deep Water flows
into the South Pacific and divides into two branches . A portion
(8 - 12 x 106 m3/s, Reid et at. , 1968) turns to the north along
the eastern edge of the New Zealand Plateau. This tongue of rel-
atively high-salinity water marks the beginning of the deep west-
ern boundary current which was mentioned earlier. The deep water
which flows north in the boundary current fills the entire deep
Pacific, there being no sources of deep water in the North Pacific,

kQ

A northward transport of 10 x 10s m3/s in the boundary current
implies a residence time of about 10 years for water in the
layer below 2000 m in the Pacific.

The bulk of the deep water flows eastward across the Pacific
in the Circumpolar Current. This flow is marked by a thin, mean-
dering tongue defined by the 3^.7^- %> isohaline. In the eastern
South Pacific, water of salinity less than 3^.70 % moves south-
ward along the coast of South America. Mixing between this water
and Circumpolar Water produces the uniform deep water which enters
Drake Passage and flows into the western South Atlantic.

Near the antarctic coast, the salinity distribution on 30
cl/t is much like that on 50 cl/t . Low-salinity water originat-
ing in the southwestern part of the Weddell Sea moves north along
the Antarctic Peninsula and then east parallel to the South Ork-
ney Islands. The very distinct tongue along 55° - 60°S shows
that this water continues east as far as 20°E before turning
south in the cyclonic Weddell Sea gyre.

Just to the east of Kerguelen Ridge low-salinity water ex-
tends northward from MacKenzie Bay and Davis Sea. In the Ross
Sea two tongues of low salinity are again evident, one extending
northwestward in the direction of Macquarie Island, the other
eastward along the southern flank of the Pacific-Antarctic Ridge.
The southward extension of the 3^.7^- %> isohaline along 135°E
and the indentation of the 3^.72- %> isohaline along 73°S provide
further evidence of a weak cyclonic gyre in the Ross Sea similar

h9

to that in the Weddell Sea. This feature stands out in the
meridional salinity and dissolved-oxygen sections along 170°W,
Figures 20, 21. The high-salinity, low-oxygen water between
500 and 1000 m near 73°S marks the westward return flow into
the Ross Sea. (The return flow into the Weddell Sea is evident
in the Greenwich Meridian potential-temperature section, Figure
15. Note the relatively warm water near U00 m at 65°S.)

III.C. Dissolved Oxygen

In the Atlantic and Indian sectors of the antarctic the distri-
bution of dissolved oxygen (Figure 8) is distinguished by a
lateral minimum ( < U.5 ml/l) . This feature, like its counter-
part on the 50-cl/t surface, originates in the eastern South
Pacific, where low-oxygen water ( < k.2 ml/l) from the north
mixes with higher-oxygen water ( > U.5 ml/l) in the circumpolar
zone. The resulting mixture (U.3 ml/l < 0 < U.5 ml/l) flows

2

into the western South Atlantic between oxygen-rich North Atlantic
Deep Water and Weddell Sea water, both of which have concentra-
tions well over 5-0 ml/l. The flow of Circumpolar Deep Water north
into the Argentine Basin is indicated by the northward displace-
ment of the oxypleths in this region.

As with salinity, the Circumpolar Deep Water and the North
Atlantic Deep Water can be distinguished by their oxygen concen-
tration across the Atlantic and Indian sectors. The difference
in concentration weakens gradually toward the east. South of

50

Australia, the lateral minimum associated with the Circumpolar
Deep Water and the weak maximum associated with the North Atlantic
Deep Water merge into a broad belt of very uniform oxygen con-
centration.

Further evidence of westward flow in the South Australian
Basin is provided by the oxygen-poor water (0 < U.2 ml/l) north

2

of i+5°S. Low concentrations are also found in the Tasman Basin.
The northward extension of the U.5-ml/l oxypleth along l60°E is
consistent with the deflection of part of the Circumpolar Current
into the Tasman Sea.

In the Pacific Ocean, the oxygen concentration decreases
monotonically from south to north. Northward flow along the west-
ern boundary of the South Pacific is indicated by the prominent
tongue of relatively high-oxygen water along l65°W.

Dissolved-oxygen concentrations exceeding 5-0 ml/1 occur
along the antarctic coast from the Weddell Sea eastward to the
Ross Sea. Maximum concentrations ( > 7.0 ml/l) are found in the.
southwestern corners of the Ross and Weddell seas and in MacKenzie
Bay. High-oxygen tongues extend from the Ross and Weddell seas
along the paths followed by low-salinity water originating in
these seas . However, the water in MacKenzie Bay has a rather
modest influence on the oxygen distribution outside the Bay com-
pared with the strong effect on the salinity distribution noted
in the previous section.

51

IV. THE BOTTOM WATER

Because of the vertical homogeneity of deep antarctic waters, it
is not possible to use isanosteric analysis below the 30-cl/t
surface. However, it is possible to observe the circulation of
abyssal waters in the horizontal distributions of sea-water prop-
erties near the ocean floor. The distributions of three such
properties — potential temperature, salinity, and dissolved oxygen
— have been mapped for this report. Approximately 320 data points
were plotted for each map (see Figure 9 for station positions).
All sample points are from depths greater than 3500 m, and all
but a few are within 300 m of the ocean floor.

IV. A. Potential Temperature

The most commonly measured and mapped bottom-water characteristic
is its temperature, or rather its potential temperature. I have"
already noted that maps of bottom potential -temperature, covering
part or all of the circumpolar zone, have been prepared by Wust
(1933, 1938), Deacon (1937), and Gordon (1966). However, it
seemed worthwhile to prepare a new bottom potential -temperature
map both for the sake of completeness in this report and because
the previous maps either were based on rather poor data grids
(Wust, Deacon) or covered only part of the antarctic region.

Before examining the map of potential temperature, the ef-
fect of geothermal heating on the temperature structure of the
bottom water will be considered. Heat generated in the earth's

52

crust is conducted into the water layer above the sea floor.
The geothermal flux has been measured at a number of locations
in the Atlantic and Pacific oceans. Except over ocean ridges,
where it may be somewhat larger, the flux F is uniform and of
the order 10~6 cal/cm2s (Bullard, 1963). To find out if this
rate of heating will alter to an appreciable degree the thermal
structure of near-bottom waters, a simple model of the circula-
tion near the sea floor will be constructed.

Assume that a thin, vertically homogeneous layer of water
("bottom water") of thickness H flows over the floor of an ocean
basin at a uniform speed U. Assume further that the vertical
uniformity of the water is due to convective overturning caused
by heating from below, and that no mixing occurs between the
bottom water and water above it. What temperature change, AT,
will occur as a water parcel moves along the basin floor 1

The heat Q added to each water column of unit area as it
moves a distance L is

Q = —

Therefore, the downstream temperature gradient due to geo-
thermal heating is

(2) AT/L =

, QJL = JL

pcH pcUH

where p = density of sea water

c = specific heat.

53

The most difficult to determine of the above parameters is
the homogeneous bottom-layer thickness, H (not to be confused
with the Ekman boundary layer at the bottom) . Few hydrographic
stations with closely-spaced bottles near the bottom are avail-
able. Fortunately, some of the stations occupied during Eltanin
cruises in the South Pacific Ocean (Cruise 28) and eastern Indian
Ocean (Cruises 32 and Ul) do have several bottles at 50 - 100 m
intervals near the bottom. These indicate that the bottom few
hundred meters of water are extremely uniform. A choice of 100 m,
which is typical of the sampling level of the data used in this
study, for H would appear to be conservative. The remaining param-
eters will be given the following values:

F = 10"6 cal/cm2s

U = 1 cm/s

c = 1 cal/cm3 °C .

From (2) the horizontal potential -temperature gradient due to
geothermal heating is found to be 0.01°C/1000 km. Observed gra-
dients (from Figure 10) are much larger: Atlantic-Antarctic Basin,
0.08°C/1000 km; Southwestern Pacific Basin, 0.lU°C/l000 km; Pacif-
ic-Antarctic Basin, 0.2T°C/1000 km. It appears that the geothermal
flux does not significantly alter the distribution of potential
temperature in the bottom water.

In the present map of potential temperature (Figure 10) , as
in the earlier ones by Wust and Deacon, the most striking feature

5U

is the extremely cold water ( < -0.8°C) in the Weddell Sea. This
water, which will be called Weddell Bottom Water, originates along
the western shelf of the Sea, then flows east in a narrow tongue
along the southern flank of the Scotia Ridge. Some of the Weddell
Bottom Water enters the Argentine Basin through the deep South
Sandwich Trench. This water continues to flow north along the
western boundary of the Basin. According to Wust (1933) , antarctic
deep water (Weddell Bottom Water) can be detected in the western
trough of the Atlantic as far north as U0°N.

Some cold bottom water from the Weddell Sea also enters the
Scotia Sea, probably through a passage (sill depth 3000 - 3500 m)
near 6l°S, U0°W (Gordon, 1966). A sharp horizontal gradient of
potential temperature is found in the Scotia Sea. Reid and Nowlin
(1971) suggest that the gradient is caused by the shielding effect
of the ridge along 60°W, which allows Weddell Bottom Water to col-
lect in the southwestern Scotia Sea. The strong flow of warm deep
and bottom water in the Circumpolar Current is deflected northward
by the ridge and is confined to the northern half of Drake Passage.
The sharp potential-temperature gradient marks the boundary between
the warm water from the Pacific Ocean and the cold water from the
Weddell Sea.

It is evident from the distribution of potential temperature
that a substantial part of the bottom water produced in the Weddell
Sea continues to flow east along roughly 60°S. Water colder than
-0.U°C fills the entire Atlantic -Ant arctic Basin. The temperature

55

increases steadily from -0.8°C in the Weddell Sea to about -0.5°C
along the western flank of Kerguelen Ridge. Mixing with warmer
deep water accounts for most of the increase in temperature.

Although the flow of bottom water in the Atlantic -Ant arctic
Basin is primarily zonal, there is some northward flow through
gaps in the mid-ocean ridge system. A major break in the ridge
occurs near 50°S, 30°E, connecting the Atlantic -Ant arctic and
Agulhas basins. Wust (1933, 1938) and Deacon (1937) showed water
colder than 0°C penetrating past U0°S, whereas on the present map
the 0°C-isotherm reaches only as far as ^7°S. Generally speaking,
the bottom water in the Agulhas Basin seems to be somewhat warmer
than Wust and Deacon indicated. The distribution of potential
temperature in the Agulhas Basin shown in Figure 10 suggests a
weaker antarctic influence than would be inferred from the earlier
maps .

The Kerguelen Ridge, which separates the Atlantic-Antarctic
Basin from the Indian -Ant arctic Basin, is very shallow. From kj°S
to 62°S the sill depth is less than 2000 m. The only notable pas-
sage through which bottom water from the Weddell Sea can flow is
at 6U°S, where the sill is at 3000 - 3500 m.

East of Kerguelen Ridge, the bottom-water temperature contin-
ues to rise slowly as far as 110°E, where it is slightly less than
-0.U°C. Beyond that point, the potential temperature drops to
nearly -0.6°C. The decrease in potential temperature, along with
changes in the other properties to be examined later, is evidence

56

of a second source of bottom water in this region. It will be-
come evident that the second source is the Ross Sea.

Bottom water from the Indian -Ant arctic Basin flows into the
South Australian Basin through the gap in the Indian -Ant arctic
Ridge at 120°E. As a result, the potential temperature in the
center of the South Australian Basin is slightly lower than it is
to the east and west.

Some bottom water also flows out of the Indian -Ant arctic
Basin through the narrow passage at 59°S, lU6°E. Part of this
water moves north into the Tasman Basin while the remainder flows
into the Southwestern Pacific Basin over the narrow sill at 59°S,
l60°E. Having entered the Southwestern Pacific Basin, the bottom
water flows northeast along the New Zealand Plateau in the deep
western boundary current and spreads into the central South Pacific.

The bottom water south of the Pacific-Antarctic Ridge is sig-
nificantly colder than that north of the Ridge. Part of the water
in the western end of the Pacific-Antarctic Basin originates in
the Ross Sea. The salinity data show that there is also a strong
contribution from the Weddell Bottom Water west of the Macquarie
Rise.

The moderate temperature gradient along the southern flank
of the Pacific- Ant arctic Ridge indicates zonal flow from about
lUO°W to 100°W. The warmer water observed in the northern Pacific-
Antarctic Basin probably comes from the Southwestern Pacific Basin.
According to Gordon (19-66), the warmer bottom water crosses the

57

Pacific-Antarctic Ridge through the complex fracture zone located
at 130 °W.

IV. B. Salinity

The map of bottom-water salinity included in this paper (Figure
ll) is the first to be constructed for the entire circumpolar
region. Previously, the salinity distribution in the Indian and
Pacific sectors was regarded as being too homogeneous to provide
useful information. However, even the small salinity differences
which are observed in this region provide information about the
bottom circulation which cannot be derived from the temperature
and dissolved oxygen distributions.

The bottom salinity has been mapped before in the Atlantic
sector of the Antarctic Ocean. A comparison of the old map ( Wiist,
1933) with the present map reveals one significant difference, a
difference which led Wiist to draw erroneous conclusions regarding
the formation and circulation of bottom water in the Atlantic
Ocean.

On Wust's map and the present one, low-salinity water
( < 3^.65 %o ) is found along the southwestern margin of the Wed-
dell Sea, a region where bottom water is formed. Both maps also
show a tongue of low-salinity water extending along the western
side of the Argentine Basin. However, whereas the present map
shows the salinity increasing steadily from the Weddell Sea
source to the Argentine Basin, Wust's does not. On Wust's map a

58

large region of slightly higher salinity separates the low-
salinity water in the Weddell Sea from that in the Argentine
Basin. This distribution led Wust to conclude that the fresh,
cold water in the Argentine Basin derives from a source other
than the Weddell Sea. He hypothesized that the bottom water
which spreads northward in the western trough of the Atlantic
(which he called antarctic deep water A_) forms near the pack-
ice boundary in autumn and winter, while the bottom water which
forms over the Weddell Sea shelf (which he called antarctic
bottom water, A- ) remains trapped in the western Atlantic-
Antarctic Basin.

I have already stated in Section I.B that the mechanism pro-
posed by Wust for the formation of A_-type water is highly im-
probable. Oceanographers have for some time accepted the notion
that the bottom water which propagates along the western Atlantic
trough originates in the Weddell Sea. It is reassuring to find
that the salinity distribution is in fact consistent with this
view.

The salinity distribution helps define the path taken by
Weddell Bottom Water as it enters the Argentine Basin. It is
clear that the Weddell Bottom Water passes through the South
Sandwich Trench, not along the channel east of the Trench
(Gordon, 1966).

The salinity distribution in the Scotia Sea, like the dis-
tribution of potential temperature, reveals an influx of Weddell
Bottom Water. The salinity increases rapidly toward the west owing

59

to the influx of more saline Pacific water through. Drake Passage.
Examination of several Eltanin stations along the Scotia Ridge
between the Falkland Islands and South Georgia showed no bottom
water of salinity less than 3^.72 %> passing over the Ridge into
the Argentine Basin. The Weddell Bottom Water which enters the
Scotia Sea must be thoroughly mixed with Pacific water before it
flows into the Argentine Basin.

The bottom-water salinity in the Atlantic-Antarctic Basin is
highly uniform. Only in the extreme western and eastern ends of
the Basin does the salinity differ from 3^- .66 %> . (The apparent
uniformity of the bottom water in this basin is partly due to the
fact that all the data were taken by chlorinity titration. Salin-
ometer measurements will undoubtedly reveal some variations in the
salinity distribution.) Low salinities in the southern Agulhas
and Crozet basins are indicative of Weddell Bottom Water flowing
north through the gaps at 30°E and 60°E. The relatively high
salinities found in the northern Agulhas Basin reflect eastward
flow around the tip of Africa of water from the South Atlantic.

Across the Kerguelen Ridge the salinity of the bottom water
increases from 3^.66 %» to 3^.68 %o . Unfortunately, there are no
salinometer measurements in the vicinity of the Ridge, and the
salinity structure over the Ridge cannot be properly investigated.
It is reasonable to assume that in passing over the Ridge the
bottom water mixes with the Circumpolar Deep Water, causing an
increase in salinity near the bottom.

60

The slight salinity minima in the South Australian and Tasman
basins mark the flow of bottom water into these basins. Recall
that weak temperature minima were observed in these basins, also.

In the South Pacific Ocean, the bottom-water salinity distri-
bution is more complex and its interpretation less obvious than
in the South Atlantic and Indian sectors. Low-salinity water
which crosses the sill near 59°S, l60°E spreads north into the
Southwestern Pacific Basin. The tongue of low salinity along the
New Zealand Plateau is another manifestation of the deep western
boundary current .

In the central Southwestern Pacific Basin salinity at first
increases slowly, reaching a maximum of about 3^.715 %o near U0°S.
Beyond this point, salinity decreases slowly, dropping below 3^.70^
at 30°S. The change in sign of the meridional bottom-water salin-
ity gradient is related to changes in the salinity of the deep
water. As deep and bottom water flow north after entering the
South Pacific, the deep salinity maximum is gradually eroded by
mixing. As a result, the salinity of the bottom water increases
slightly. North of about U0°S, where the deep salinity maximum
disappears in the central South Pacific, salinity decreases monot-
onically from the ocean floor to the Intermediate Water. Further
vertical mixing tends to decrease the salinity at the bottom.

The value of the bottom-water salinity map is well demon-
strated in considering the Pacific-Antarctic Basin. The range
of salinity in the Basin is very small, and no doubt some of the

6l

detail shown in the isohalines is due to observation error. How-
ever, I am confident that the pattern shown in the salinity map
does reflect real differences in the bottom water. The data grid
is denser than in any other part of the antarctic, and the major
features have been observed on several different cruises.

The essential features of the salinity distribution in the
bottom water of the Pacific-Antarctic Basin are: saline water
( > 3^.71 %o ) north of 60°S and in the extreme western end of the
Basin x with a band of slightly less saline water ( < 3*+. TO %> )
extending along roughly 65°S.

I have already stated on the basis of the potential -tempera-
ture distribution that the bottom water found in the northern
part of the Pacific-Antarctic Basin probably comes from the South-
western Pacific Basin. The salinity distribution is consistent
with this view. With respect to salinity, the water in the north-
ern Pacific-Antarctic Basin is indistinguishable from that in the
central Southwestern Pacific Basin.

The small region of relatively high salinity in the western
end of the Pacific-Antarctic Basin is the result of outflow from
the Ross Sea. For reasons which are not clear, the bottom water
formed in the western Ross Sea is much more saline than that in
the Weddell Sea. Whereas the cold shelf water of the Weddell Sea
has a maximum salinity of less than 3^-70 %> , that of the Ross
Sea has a maximum salinity of more than 3^.85 & • Typical station
curves for the southwestern reaches of both seas are shown in Fig-
ure 33.

62

Finding "... no stream of cold poorly saline bottom water
sinking from the Ross Sea," Deacon (193T> p. 115) concluded that
the Ross Sea had little or no influence on the bottom water out-
side the Sea. Probably Deacon was looking for water like that
produced in the Weddell Sea, on the assumption that the similar
climatology of the two regions would produce similar water types.
Deacon went on to conclude that any bottom water which is formed
within the Ross Sea must be trapped by the shallow ridge which
runs across the mouth. This thesis has been generally accepted
by oceanographers for many years .

Recent topographic maps of the Ross Sea {e.g.. Countryman
and Gsell, 1966) indicate that the barrier across the outer shelf
is broken by several shallow troughs which extend into the interi-
or of the Sea. These provide paths for Ross Bottom Water to es-
cape. Gordon (1966, 1970) has shown from the distributions of
properties outside the Sea that some bottom water does indeed flow
out of the Ross Sea. It would appear, however, that he has over-
estimated the effect of the Ross Bottom Water, particularly in
the western Pacific-Antarctic Basin. He implied that the cold
bottom water found there is supplied by the Ross Sea alone. It
will be seen that this is clearly not the case. Also, his asser-
tion that the bottom water which flows into the South Australian
Basin is a mixture of Ross Bottom Water and a "locally-produced
water" seems unlikely. If the Ross Sea were contributing large
amounts of bottom water, salinities in the eastern Indian-Ant arc-

63

tic Basin would be much higher than those observed there.

The conclusion that the bottom water found in the western
Pacific-Antarctic Basin is not solely from the Ross Sea is related
to the salinity minimum which extends across the Basin at 65°S.
Figure 20, which is a salinity section along 170°W, shows high-
salinity Ross Bottom Water in the deep basin south of the Pacific-
Antarctic Ridge and less saline water ( < 3^.70 %> ) over the crest
of the Ridge. Figure 22, which is a salinity section along roughly
65°S from lU0°E to 150°W, demonstrates that the low-salinity water
comes from the west. The low-salinity water represents the last
traces of Weddell Bottom Water, which flows up the western flank
of the Macquarie Rise, along the Pacific-Antarctic Ridge, and then
into the western end of the Pacific-Antarctic Basin. Ross Bottom
Water can be observed beneath the Weddell Bottom Water at l60° -
170 °E, suggesting that the outflow takes place along the western
boundary of the Ross Sea. The fact that the salinity minimum per-
sists past 90°W indicates that the flow of Weddell Bottom Water
across the Macquarie Rise must be strong relative to the flux of
Ross Bottom Water into the Pacific-Antarctic Basin.

IV. C. Dissolved Oxygen

The map of dissolved-oxygen concentration for the bottom water
(Figure 12) is strikingly similar to the potential temperature and
salinity maps. The latter two distributions and the circulation
patterns they reveal have been discussed in great detail in the

6k

preceding sections. To avoid unnecessary repetition, only the
most significant aspects of the dissolved-oxygen distribution
will be described.

The highest concentrations of dissolved oxygen ( > 6.0 ml/l)
on this map are found along the western and northern margins of
the Weddell Sea. Eastward flow out of the Sea is marked by a
tongue of high-oxygen water which is pressed against the southern
arc of the Scotia Ridge. Overflow of Weddell Bottom Water into
the Scotia Sea is evident in the high oxygen values east of Drake
Passage. Oxygen-rich water is also observed in the Argentine Basin.
It is clear from the oxygen distribution that the high-oxygen water
in the Argentine Basin is derived from the tongue extending north-
ward along the South Sandwich Trench. No such water was found to
be crossing the ridge separating the Scotia Sea and the Argentine
Basin.

As Weddell Bottom Water moves east in the Atlantic-Antarctic
Basin, the dissolved- oxygen concentration falls from 6.0 to 5.1+ ml/1.
In the western Indian-Antarctic Basin the oxygen concentration re-
mains at about 5 .k ml/1, but in the eastern end of the Basin the
concentration increases to over 5.6 ml/1. This increase, like the
small changes in potential temperature and salinity observed in
this region, is a result of the influx of Ross Bottom Water from
the east.

In the Macquarie Rise sector, the oxygen distribution shows
bottom water flowing out of the Indian Ocean via three paths:
north into the Tasman Basin; across the Macquarie Rise at 59°S and

65

into the Southwestern Pacific Basin; across the Macquarie Rise
along 65°S and into the Pacific-Antarctic Basin. The flow into
the Southwestern Pacific Basin continues east along the Pacific-
Antarctic Ridge and then turns north to feed the deep western
boundary current along l60°W. The eastward flow of bottom water
across the Macquarie Rise and Pacific-Antarctic Ridge shows up
clearly in the oxygen sections along 65°S (Figure 23) and 170°W
(Figure 21). A large proportion of this water is Weddell Bottom
Water. Ross Bottom Water, with a somewhat higher oxygen concen-
tration ( > 5.3 ml/l) , can be seen beneath the Weddell Bottom
Water in both sections.

In the Pacific-Antarctic Basin, the large region with oxygen
concentrations greater than 5-0 ml/l represents both Weddell and
Ross bottom waters and their mixtures. The lower oxygen values
in the northern Pacific-Antarctic Basin are evidence of the in-
flux of bottom water from the Southwestern Pacific Basin and of
mixing with oxygen-poor water along the Chile coast.

66

V. FLUX AND VELOCITY STRUCTURE OF THE ANTARCTIC CIRCUMPOLAR
CURRENT

A shortcoming of the method used in the previous sections is that
it provides only a qualitative view of the circulation. In. this
section the structure of the circumpolar flow, particularly that
of the Antarctic Circumpolar Current, will be examined by more
quantitative means. This will be done principally in terms of
the relative geostrophic velocity field computed for several
meridional hydrographic sections around Antarctica. Deep current-
meter measurements taken along 132°E south of Australia will be
used to compute the "absolute" net flux in the circumpolar zone,
and the result compared with a similar computation of the Drake
Passage transport.

The mass transport of the Antarctic Circumpolar Current has
been estimated by numerous investigators using the dynamic, or
geostrophic, method. These calculations have been carried out
most frequently for the Drake Passage section, which is consider-
ably shorter than any other section across the Circumpolar Current.
Clowes (1933) was the first to compute the zonal flux through
Drake Passage, which he determined to be 110 x 106 m3/s relative
to 3500 db. Later estimates, which range from 90 to 218 x 106 m3/s
(excluding the unrealistically low results of Ostapoff, 196l) have
been reviewed by Gordon ( 1967a) and will not be discussed here.

Sverdrup (19U0), using BANZARE stations along lU5°E, has
estimated the net eastward flux from the naviface to the bottom

67

between Australia and Antarctica at 150 x 106 m3/s. Kort (1963)
computed the eastward flux in the same sector (l65°E) to be
189 x 106 m3/s from data taken by the Ob' in March 1958. Both
Sverdrup and Kort calculated the transport relative to 3000 db
at ikk x 106 m3/s.

Kort (1963) has also computed the net eastward transport be-
tween South Africa and Antarctica. From hydrographic stations
occupied by the Ob ' in February -March 1957 he calculated 150 x
106 m3/s relative to 3000 db and 198 * 106 m3/s relative to the
bottom7. From Discovery data taken along 20°E in April 1938
Kort computed the transport relative to 3000 db to be 205 x 106
m3/s .

While the zonal fluxes computed from the Australian and South
African sections are in reasonable agreement, the flux computed
for Drake Passage is significantly smaller than the other two sec-
tions. Obviously there has been no gain or loss of water in the
Atlantic, Pacific, and Indian oceans, since mean sea level has re-
mained constant for many years. Simple water budget calculations
will show that the mean transports across the Drake Passage, South
African, and Australian sections must in fact be very nearly equal.

In comparison with the zonal water transport of the Antarctic
Circumpolar Current, the average annual difference between evapora-
tion and precipitation -plus-runoff makes a small net contribution

7

The substantial increases in transport between 3000 m and the bot-
tom which Kort reported seem unrealistically large in view of the
very small relative velocities which are observed below 3000 m.

68

to the water budgets of the Atlantic, Indian, and Pacific oceans.
For example, an average difference of 1 m/yr between evaporation
and precipitation over the entire Pacific Ocean would represent
a flux of only 5 x 106 m3/s. The actual average difference, al-
though not well known for the oceanic areas, is probably somewhat
less than 1 m/yr (see Dietrich, 1963, Figure 75).

Aside from the circumpolar zone, the only direct communica-
tion among the oceans is through Bering Strait and the East Indian
Archipelago. Numerous current measurements have been made in
Bering Strait. The mean transport through the Strait is known to
be 1.0 - 1.5 x 10 6 m3/s toward the north (see Coachman and Aagaard,
1966, for a summary of Bering Strait transport calculations). The
net flux through the East Indian Archipelago has not been deter-
mined as precisely as the Bering Strait flux. The only significant
passage through the islands is between Timor and Australia. Wyrtki
(l96l) has estimated the transport of upper water in the Timor Cur-
rent at 1.0 - 1.5 x 106 m3/s toward the west. It is clear from the
deep water in the Timor Trench that the deep flow must be in the op-
posite direction to the surface current (Sverdrup et al.3 19^2, p.
739). It is unlikely that the net transport through the East Indian
Archipelago exceeds 5 x 106 m3/s.

The fact that the net transport figures cited earlier do not
agree either for a given section or among sections might be inter-
preted as evidence of seasonal or longer-term transient motions in
the circumpolar circulation. Kort (1963), Deacon (1963), and
Gordon ( 1967a) have suggested that transient motions are significant

69

in the Antarctic Circumpolar Current. Recent studies indicate,
however, that this is not the case. Boven and Stommel (1968)
analyzed sixteen sections along 20°E taken by the Discovery dur-
ing the period April 1938 to March 1939. No seasonal variations
in the temperature (hence mass) distribution were observed. Reid
and Nowlin (l9Tl) have computed the transport relative to the bot-
tom through Drake Passage using four hydrographic sections made
by different ships between 1930 and 1969 • The range of the trans-
ports was 92 - 117 x 106 m3/s; three of the four were from
113 - 117 x 106 m3/s.

The author has computed the transport relative to the bottom
across two sections south of Australia: 115°E {Discovery , December
1937 - January 1938) and 132°E {Eltanin, December 1969 - January
1970). The net eastward flux through these sections was 1U9 x 10G
m3/s and 156 x 106 m3/s, respectively. The results compare well
with Sverdrup's (19^0) estimate of 150 x 106 m3/s at lU5°E.

It would appear that the net relative transport across any
section is reasonably steady, at least with respect to seasonal
and longer-period fluctuations. A more likely explanation for the
variations in computed transport among the sections is that these
calculations determine only part of the flux. The total transport
consists of the relative component plus the "barotropic" component
(Fofonoff , 1962b) , which corrects for any difference between the
true velocity and the assumed velocity at the reference level.
For a deep reference level the velocity difference is usually small,

TO

resulting in relatively minor changes in the computed velocity
field. However, a small correction to the velocity field may
have an appreciable effect on the transport. A barotropic cor-
rection of 1 cm/s between two stations 250 km apart in k km of
water represents an increment of 10 x 106 m /s in the transport.

In order to determine the absolute transport, current mea-
surements must be taken along with the hydrographic stations from
which the relative transport is to be computed. Over the past two
austral summers two such sections have been made in the antarctic,
the first in Drake Passage (January 1969) and the second along 132°E
between Australia and Antarctica (December 1969 ~ January 1970).
The results of these cruises will be discussed in order to demon-
strate the magnitude of the barotropic correction and to estimate
the total mean transport.

The instruments and techniques used to measure deep currents
in the antarctic have been developed at the Scripps Institution
of Oceanography. (See Isaacs et al.3 1966, for a detailed de-
scription of the current-meter system.) The current meter is
tethered to an anchor on the sea floor for a preset period. At
the end of the period, the meter automatically breaks its mooring
and ascends to the naviface where it is recovered. If the cur-
rent measurements are to be used to compute absolute geostrophic
transports, as in the present case, the instrument is tethered
at or above the level of the deepest bottle on the hydrographic
casts on either side of it. In the antarctic experiments, the

71

meters were usually set 300 - 500 m off the bottom, although, two
were as much as 900 m off the bottom.

Output from the current meter consists of a continuous trace
indicating current direction (magnetic) and a record of the number
of revolutions made by the Savonius rotor per unit time. The
practical threshold of the rotor is 0.5 cm/s . Tests off the Cal-
ifornia coast have shown good reproducibility between adjacent
meters (Isaacs et al.> 1966).

For the present purposes the raw data were reduced to hourly-
mean speeds and directions. Semi-diurnal, diurnal, and daily -mean
velocity components were derived by computer from the reduced data.
In all cases the records were at least one lunar day (25 hours)
long.

The barotropic velocity component was determined by taking
the difference between the measured mean speed normal to the sec-
tion and the geostrophically-computed speed at the same depth.
For those station pairs with no current meter between them, the
barotropic component was determined by linear interpolation be-
tween the values on either side.

Results of the Drake Passage cruise are discussed in detail
by Reid and Nowlin (1971) and will only be summarized here. Five
complete current-meter records of 25 - 99 hours' duration were ob-
tained. At all current-meter stations the daily mean current was
toward the east. Overall mean speeds ranged from about 3 cm/s at
the northern end to nearly 9 cm/s at the southern end of the sec-

72

tion. The geostrophic transport relative to the deepest common
level of each station pair was found to be 113 x 106 m3/s. The
"barotropic transport was 12 U x 106 m3 /s , giving a total eastward
flux of 237 x 106 m3 /s . This estimate does not include any cor-
rection for the small area outside the sampling grid, i.e., be-
tween the continents and the current meters at the ends of the
section, or between the sea floor and the reference level.

The section along 132°E between Australia and the antarctic
continental shelf was made during Ettanin Cruise Ul. Twenty-one
deep hydrographic stations were occupied along the section, and
five complete current-meter records were obtained. The current-
meter results are summarized in Table 2. On the whole, the mean
speeds were somewhat smaller than those measured in Drake Passage,
as might be expected in the wider section.

From the hydrographic data, the geostrophic transport rela-
tive to the deepest common level of each station pair was found
to be 156 x 106 m3/s. The barotropic component, which was deter-
mined in the same manner as for the Drake Passage section, was
77 x 106 m3/s, giving a net transport of 233 x 106 m3/s. Figures
2k and 25 show the relative and absolute velocity and transport
fields .

It is evident from these calculations that the barotropic
component is a significant part of the total transport in the cir-
cumpolar belt. In Drake Passage the barotropic component accounted
for 52$ of the total, while at 132°E it represented 33$ of the to-
tal flux.

73

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The close agreement between the net transports through the
Australian and Drake Passage sections is remarkable in view of
the uncertainties inherent in the method used to compute them.
Although the Pacific Ocean water budget indicates that the two
should agree to within, say, 5 x 10 m3/s, it would be unrealistic
to conclude that the circumpolar flux has been measured to that
precision.

Two major potential errors stand out: (l) The unmeasured
barotropic flux through the side and bottom areas might make a
significant contribution (say, ± 10$) to the total. (2) The
limited number of short-term current measurements upon which
these calculations are based may not be representative of the
long-term average flow. These limitations are particularly evi-
dent in the Australian section, where the meter spacing was much
larger, the gaps in the current-meter coverage {viz, north of U0°S)
greater, and the record lengths shorter than for the Drake Passage
section.

In spite of these shortcomings, the fact that the two trans-
port calculations do agree so well gives some encouragement that
the method provides a reasonable estimate of the mean net circum-
polar flux. Probably the above estimate is low, since extrapola-
tion of the measured currents into the side and bottom boundaries
results in an increase in the net flux. Assuming an increase of
10%, a reasonable estimate of the net circumpolar flux would be
250 x io6 m3/s.

75

The transport of the Antarctic Circumpolar Current alone
varies with longitude. In Drake Passage the eastward Circumpolar
Current occupies the full width of the section and has a trans-
port equal to the net zonal flux. South of Africa and Australia,
on the other hand, westward currents are found along the contin-
ental boundaries, and the transport of the Circumpolar Current
must "be slightly greater ( 'V* 20 - 30 x 106 m3/s) than the net
flux through the section.

Whereas the averaged deep currents measured in Drake Passage
were consistently toward the east, those south of Australia were
not. The record of current meter No. h (record length UT hrs)
gave a strong mean component toward the west. Even with the
oscillatory component included, the current direction was always
between 210° and 260°T. Current meter No. 2, dropped at the same
position two days earlier, provided further evidence of westward
flow. This meter (record length h-5 hrs) gave no speed record,
but the direction trace indicated that the velocity vector was
steady within the arc 220° - 260°T.

Current meters Nos . 2 and h were positioned over the crest
of the Indian -Ant arctic Ridge. Figure 2h shows that over the
Ridge crest the relative geostrophic flow dropped to effectively
zero. As a result the absolute flow over the ridge was toward
the west throughout the water column (Figure 25) .

Without the appropriate current measurements it is not
possible to determine if the current structure observed at 132°E

16

exists elsewhere in the antarctic. However, there are remark-
able similarities between the relative geostrophic velocity field
at 132°E and at a number of other meridional sections across the
Antarctic Circumpolar Current in the Australian sector.

Five additional meridional sections between 115°E and lH6°E
have been worked up. The data were taken over a span of thirty
years. Two of the sections were made by the Discovery (1936,
1938), three by the Eltanin (1968). All the sections cross the
Indian -Ant arctic Ridge, which runs nearly east-west along 50°S
in this region. (See Figures 26 - 30.)

At the northern end of each section, weak westward flow is
observed. This, of course, is the westward Flinders Current, which
was discussed in previous sections. Strong eastward flow with max-
imum navifacial velocities of 15 - 25 cm/s is concentrated in a
narrow band north of the Ridge, over the South Australian Basin.
In the Indian -Ant arctic Basin, the flow is also eastward but slower
(5-10 cm/s) and more uniform than in the north. Just south of
the Ridge crest the relative velocities drop sharply, and the ver-
tical shear practically vanishes. In one instance (11T°E) the max-
imum velocity over the Ridge is 1 cm/s eastward. In all other cases
it is weakly westward .

If the absolute flow field is qualitatively similar to the rel-
ative flow field (as it was at 132°E) , velocity sections south of
Australia indicate that westward flow along the crest of the Indian-
Antarctic Ridge is a common feature of the circulation in this re-

77

gion. Generalizing further, it might be inferred that the current
reversal is in some way caused by the bottom topography. Certain-
ly there is little doubt that major topographic features do influ-
ence the structure of the Circumpolar Current, as shown by the
large-scale meanders noted previously.

The sector south of Africa is topographically similar to the
Australian sector. Deep basins in the north and south are separated
by an east-west ridge along roughly 50°S. A Discovery (1939) sec-
tion along 20°E was worked up in order to examine the structure of
the velocity field with respect to the ridge. Here, too, maximum
eastward navifacial velocities ( ^ 15 cm/s) are present over the
northern flank of the ridge and weak westward velocities over the
southern flank. However, in this section the velocities south of
the ridge are in general very weak, and the current reversal over
the ridge is not as distinct as in the Australian sections. On
the other hand, Kort's (1963) velocity section along 20°E, Figure
31, does resemble closely the situation south of Australia.

The limited observations available indicate that a weak
counter cur rent exists over the mid-ocean ridge in those areas
of the antarctic where both the Circumpolar Current and the mid-
ocean ridge have an east-west orientation. The evidence support-
ing this conclusion is summarized in Table 3, which suggests that
the magnitude of the current reversal increases with the slope of
the southern flank of the ridge. (This table includes only the
six Australian sections; Kort's section did not show the individ-
ual data points necessary to determine the maximum speed over the
ridge. )

78

TABLE 3. Maximum westward relative velocities over the Indian-
Antarctic Ridge.

SECTION MAXIMUM SPEED TOTAL ELEVATION SLOPE OF

OVER RIDGE OF SOUTHERN SOUTHERN

11T°E

+ l.U

132°E

0

115 °E

- 0.3

1U6°E

- l.U

128°E

- 2.0

1U0°E

- 5.0

(cm/s, + east) FLANK (m) FLANK (x 10 3)

900 1.5

1300 1.6

1100 1.7

1100 1.8

1300 2.2

2000 k.2

Although the observations suggest that the countercurrent is
the result of bottom-topography effects, the dynamical explana-
tion for such a relationship is not apparent. Since the flow in
the regions of interest is zonal, fluid elements do not experi-
ence a change in the Coriolis parameter. Therefore, the simple
qualitative arguments based on conservation of potential vortic-
ity, which have been used to explain the meanders of the Circum-
polar Current, are not applicable. If a more complete vorticity
balance is considered, then the production of relative vorticity
by the wind stress must be included. It is perhaps significant
that the maximum westerly winds in the southern hemisphere occur
at roughly 50°S (von Arx, 1957). The sense of the wind-produced
relative vorticity changes from anticyclonic to cyclonic in about
the same latitude in which the countercurrent occurs. It is not
clear how the wind stress distribution and bottom topography might
interact to produce the observed current structure.

79

VI. CONCLUSIONS AND REMARKS

In analyzing the distributions of heat, salt, and dissolved
oxygen in abyssal antarctic waters, I have pointed out certain
characteristics of the antarctic and sub-antarctic circulation.
Some of these features have been noted by other workers, in
which case the present discussion represents an extension or
modification of previously -held ideas. Other aspects of the
circulation which are discussed here are new. All are summar-
ized below. Those items which are new are marked with an asterisk.
(l)* The upper deep water of the antarctic is characterized by
a vertical minimum of dissolved oxygen. The minimum originates
in the eastern South Pacific Ocean, where oxygen-poor water flows
south and mixes into the circumpolar flow. The resulting low-
oxygen mixture is transported eastward in the Circumpolar Cur-
rent, forming a lateral as well as vertical oxygen minimum across
the South Atlantic. Additional low-oxygen water, flowing pole-
ward from the North Indian Ocean, feeds into the Circumpolar Cur-
rent south of Africa. Through lateral mixing, the oxygen-poor
water from these two sources spreads over much of the antarctic
region.

The distributions of salt and dissolved oxygen in the upper
deep water (50 cl/t) are markedly zonal. A narrow belt with sharp
meridional salinity and dissolved-oxygen gradients is found adja-
cent to the Antarctic continent. North of these gradients the
distributions are relatively uniform. The transition from high-

80

to low-gradient zones occurs south of the high -velocity core of
the Circumpolar Current.

(2) Circumpolar Deep Water is also characterized by a vertical
salinity maximum, which is found below the oxygen minimum. North
Atlantic Deep Water, the source of the high-salinity water, flows
south along the western Atlantic trough and joins the circumpolar
flow near U0°S. The saline water is confined to the northern
part of the Antarctic Ocean in the Atlantic and western Indian
sectors. Only in the eastern Indian Ocean south of Australia
does the North Atlantic component become fully integrated into
the Circumpolar Current. In the South Pacific Ocean, Circumpolar
Deep Water is marked by a lateral salinity maximum, which is
slowly eroded as the water flows east, finally disappearing west
of Drake Passage.

In the Pacific and Indian oceans, high-salinity deep water
is observed flowing north along the western boundary. This dis-
tribution is consistent with the theoretical abyssal circulation
model of Stommel and Arons (i960) and with recent field observa-
tions in the South Pacific.

(3)* West of the Macquarie Rise-New Zealand Plateau, deep and
bottom water in the Antarctic Circumpolar Current is deflected
northward into the Tasman Sea. This water feeds a westward
countercurrent which flows south of Tasmania and across the
Australian Bight to at least 115°E. The uniformity of the water
in the Australian Bight makes it impossible to discuss in detail
the flow in this region.

81

(M* The structure of the wind-driven anticyclonic circulation
in the South Pacific Ocean changes dramatically with depth. In
the upper layers the gyre is closed by a western boundary cur-
rent along the east coast of Australia. At greater depths, the
ridge system running north from New Zealand acts as a barrier
to the circulation, and a poleward western boundary current
develops along the east coast of New Zealand. At a depth of
about 1800 m, a well-defined closed gyre is present in the west-
ern South Pacific.

Along the eastern margin of the South Pacific, poleward flow
is evident beneath the Antarctic Intermediate Water. This flow
is less intense than that along the New Zealand Plateau, but
the low-oxygen, low-salinity water it transports to the south
has a much stronger influence on the distributions of properties
in the antarctic.

(5)* A detailed examination of the velocity field in the Aus-
tralian-New Zealand sector has revealed the following:

a. The net eastward geostrophic transport relative to the
bottom is reasonably uniform with longitude and has a magnitude
of about 150 x 10 6 m3/s.

b. The "absolute" net geostrophic transport is approximately
230 x 10s m3/s, which agrees well with the net transport computed
for Drake Passage.

c. The relative geostrophic velocity field consists of weak
westward flow in the Australian Bight (U0° - U5°S) , strong east-
ward flow concentrated in a narrow zone over the northern flank

82

of the Indian -Ant arctic Ridge, weak westward flow just south of
the crest of the Ridge, and moderate eastward flow over the In-
dian-Antarctic Basin. A similar regime exists in the topograph-
ically-similar region south of Africa.

d. Deep current measurements made along 132°E confirm the
structure of the velocity field presented above. In particular,
steady westward flow along the Indian-Antarctic Ridge was detected
"by two meters positioned on the Ridge crest. The westward trans-
port over the Ridge reduced the net eastward flux through the
section "by about 10$.

(6) On these maps, a westward current along the antarctic coast
(East Wind Drift) is observed only at shallow depths ( < 300 m)
and in the sector from the Greenwich Meridian to lU0°E. Large
cyclonic gyres in the Weddell and Ross seas cause westward flow
along the coast in both seas. The Weddell gyre is stronger and
more clearly defined than the Ross gyre.

(T) Ross Bottom Water, which is characterized by its high salin-
ity ( ^ 3^.71 % ), does flow out of the western Ross Sea in small
quantities. Part of the Ross Bottom Water moves west along the
antarctic coast and into the eastern Indian -Ant arctic Basin, while
the remainder flows northeast into the western Pacific-Antarctic
Basin.

The Weddell Sea is a much stronger source of bottom water
than is the Ross Sea. Even in the Pacific-Antarctic Basin, im-
mediately to the north of the Ross Sea, the influx of Ross Bottom

83

Water causes only slight changes in the bottom-water characteris-
tics which were originally established in the Weddell Sea, some
15,000 km distant.

Aside from the Weddell and Ross seas, there are no signifi-
cant sources of bottom water along the coast of Antarctica.
Specifically, there is no evidence of water from the Davis Sea-
MacKenzie Bay region in the bottom layer.

(8)* The Weddell Bottom Water which flows north into the Argentine
Basin (and eventually into the northern hemisphere) does so via the
South Sandwich Trench. Weddell Bottom Water does spill into the
Scotia Basin, but there is no indication that any of this water
crosses the northern arc of the Scotia Ridge and flows into the
Argentine Basin.

I began this paper by stating that understanding the antarctic
circulation is an important part of understanding the general cir-
culation of the world ocean. This study has attempted to refine
our understanding of the antarctic circulation and to quantify
some aspects of that flow. However, at best the conclusions reached
here are only tentative answers to some very significant problems.
I will close by briefly reviewing these problems and discussing
how they might be approached in the future.

(l) Hydrographic coverage of the Antarctic Ocean. The hydrograph-
ic coverage of the Pacific, eastern Indian and western Atlantic
sectors of the antarctic is quite good. The only area where the
coverage is inadequate is the central Indian sector. This region

8U

includes the Kerguelen Ridge, where modern salinity and oxygen
measurements would be extremely useful, especially for investi-
gating the flow of bottom water over the Ridge.

Repeated, systematic hydrographic stations in the Weddell
Sea would be of great interest. Summer stations could be taken
by a specially -equipped icebreaker, for example, the Glacier.
Winter data might be taken by an unmanned station set on the bot-
tom or on the ice. Observations of winter conditions over the
continental shelf of the Weddell Sea would be particularly valuable.
(2) Zonal flux in the Antarctic Ocean. The transport calculations
presented in this report are only a first approximation. However,
the work carried out during Eltanin Cruise kl and the Piquero
Expedition has at least demonstrated the feasibility of measuring
the flux. Future experiments should be carried out to determine:

a. the optimum horizontal spacing of current meters ;

b. the short term ( < 1 month) variability of the Circum-
polar Current ;

c. the transports of the westward boundary currents (Flinders
Current, Agulhas Current, East Wind Drift) associated with
the Circumpolar Current ;

d. whether westward flow is a common feature over the crests
of the Atlantic-Indian and Indian -Ant arctic ridges.

Once the zonal velocity field is known in greater detail than
it is at present, long-term experiments ( ^ 1 year) can be designed
to examine the seasonal variability of the circumpolar flow. The

85

relative flow appears to be steady, but there might be signifi-
cant seasonal fluctuations in the barotropic component. (Note
that the two sets of current measurements reported in this paper
were both made during austral summer.) Also, the magnitude of
the meridional transport of the deep western boundary currents
could be determined by measuring the zonal circumpolar flux up-
stream and downstream of the regions where these boundary currents
originate .

(3) Formation of bottom water. This study did not address the
problem of bottom-water formation, but it is clearly an important
problem in physical oceanography. The most pressing need, in the
context of the general circulation, is to determine the annual
rate of production of Weddell Bottom Water, which is the predom-
inant constituent of antarctic bottom water. From repeated hydro-
graphic stations taken in the Weddell Sea and current measurements
taken at key locations, e.g.3 the Scotia Ridge, South Sandwich
Trench, and western Atlantic-Antarctic Basin, a quantitative assess-
ment of the Weddell Sea source could be made. These data would in-
dicate whether there is a marked winter peak in the rate of bottom-
water production, as Brennecke's model suggests, or if formation
of bottom water continues through the summer, as predicted by the
recent theories of Gill and Turner (1969) and Seabrooke et al.>
(l97l)« Investigations of this sort could eventually lead to an
understanding of the mechanism(s) involved in the production of
bottom water in the antarctic.

86

MAPS (FIGURES 1-12)

Figures 2 and 9 show the distributions of stations for the isan-
osteric and bottom maps, respectively. The key given below identi-
fies the ship symbols. To avoid crowding, only the last two digits
of station numbers are shown on some meridional sections. Those
stations for which there was no originator station-number in the
NODC files are designated by the year the data were taken, enclosed
by parenthesis .

On the isanosteric maps, areas in which the entire water col-
umn is less dense than 50 or 30 cl/t are hatched. On the bottom
maps, areas less than 3500 m deep are hatched.

Key
Eltanin 0

Cruises 28 & 29 (Scorpio Expedition): 0 S

Cruise Ul: 0 E

Discovery #

Ob' A

Meteor +

Operation Deep Freeze □

Atka: AK Northwind: NW

Edisto: ED Staten Island: SI
Glacier: GL Westuind: WE

Glacier (IWSOE-1968 & 1969) ■

Hakuho-Maru +

Thomas Washington A

Umitako Maru >{<

All others O

Anton Bruun: AB
Africana II: AF
Argo: AR
Atlantis: AT
CapCtan Canepa: CC
Diamantina: D
Gascoyne: G
Galathea: GA
Horizon: HO
San Martin: SM
Thorshaven: TH
Vema: V
William Scoresby: WS

87

Figure 1. Major topographic features of the Antarctic Ocean

04»* !<. • ©IBM o *!« j

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-r- —* ANTARCTICA ^

j,. STEREOGRAPHIC POLAR PROJECTION
SCALE I58«I06(AT 60*S)

Figure 2. Stations for the 50- and 30-cl/t maps

89

ANTARCTICA

STEREOGRAPHIC polar projection
SCALE I 58xl06(AT 60*S)

Figure 3. The 50-cl/t surface: depth

90

. * ANTARCTICA *

STEREOGRAPHIC POLAR PROJECTION
SCALE I58»106IAT 60"S)

Figure k. The 50-cl/t surface: salinity

91

} ANTARCTICA ^°

STEREOGRAPHIC POLAR PROJECTION
SCALE I58»I06(AT 60'S)

Figure 5- The 50-cl/t surface: dissolved oxygen

ANTARCTICA *>°

^ STEREOGRAPHIC POLAR PROJECTION
SCALE I58«I06(AT 60*S)

Figure 6. The 30-cl/t surface: depth

93

. * ANTARCTICA *>°

STEREOGRAPHIC POLAR PROJECTION
SCALE I.58«I06(AT 60*S)

Figure f . The 30-cl/t surface: salinity

9h

ANTARCTICA *>°

+ STEREOGRAPHIC POLAR PROJECTION
SCALE I 58«106(AT 60'S)

Figure 8. The 30-cl/t surface: dissolved oxygen

95

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96

ANTARCTICA *

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Figure 10. The bottom water: potential temperature

91

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99

SECTIONS (FIGURES 13 - 31)

The vertical exaggeration for all sections (except Figure 3l)
is 500. In the meridional sections of 9, S, and 0 , the 50-

2

and 30-cl/t isanosteres are shown by heavy dashed lines. At
each station in the meridional salinity sections, the highest
value within the deep salinity maximum is enclosed by parentheses .
At each station in the meridional dissolved-oxygen sections, the
minimum value is enclosed by parentheses. Station numbers are
given along the bottom of each section. If data from more than
one cruise have been plotted in a section, the following abbre-
viations have been used to identify the data source: Atkat
AK; Discovery j DS; Eltanirij ET; Hakuho Maru3 HM.

In the zonal- velocity sections, the heavy dashed line repre-
sents the reference level for computing geostrophic speeds. The
units are cm/s , positive east. The numbers just below the refer-
ence level give the net flux (positive east) for each station
pair, in millions of cubic meters per second.

100

55° S

65° S

5 / %o

DRAKE PASSAGE

34.70

I I I I I 1 I

500

1000

2000

E
\

3000

4000

STAT. 161715 14 13 12 II 10 9

5000

Figure 13. Drake Passage: salinity section. Data from
Piquero III Cruise of the Thomas Washington,
21-27 January 1969 •

101

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60°

65°S

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I I I I l_l L

500

000

2000

\6

3000 £
a

4000

STAT. 16 f7 15 14 13 12 I I 10 9

5000

Figure Ik. Drake Passage: dissolved-oxygen section
Data source same as Figure 13.

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Figure 32. Correction curve for Discovery dissolved-oxygen data.
Data points taken from various cruises over the period
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120

Figure 33 (following page): Station curves for the Ross Sea
(lower) and Weddell Sea (upper). Station positions
are marked on Figure 2. The abscissa of each plot
is potential temperature (°C). The left-hand ordinate
is depth (m); the right-hand ordinate is salinity (%,).
The potential -temperature profile is defined by circles;
the potential temperature, salinity curve is defined by
squares. The slightly -curved lines running diagonally
across the plots are lines of uniform potential steric
anomaly ( cl/t) .

121

122

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VITA

Jeffrey Edwin Callahan was "born in Cambridge, Massachusetts on
2U September 19^3 • He graduated from the United States Naval
Academy in June 1965s receiving a Bachelor of Science degree and
a commission in the United States Navy. After two years' duty
aboard the USS Willis A. Lee (DL-U) in the Atlantic Fleet, he
entered the graduate program in oceanography at The Johns Hopkins
University. He received a Master of Arts degree from the Univer-
sity in 1969. The author is married to the former Evelyn Green
of Bethesda, Maryland.

Thesis

•C1889

1 •>

Thesis

.C1889 Callahan

The structure and
circulation of deep
and bottom waters in
the Antarctic Ocean.

129027

IhesC1889

The structure and circulation of deep an

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