Antarctic polar front zone

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ANTARCTIC
POLAR
FRONT
ZONE
ARNOLD
L.
GORDON
Lamont-Doherty
Geological
Observatory
of Columbia University,
Palisades,
New
York
10964
Abstract. The thermal structure across the antarctic polar front zone in the South Pacific is
inspected. The structure is variable; however, it is often possible to identify some unifying pattern.
A
generalized thermal
section
can be constructed that contains all or most of the structural ele-
ments found by the particular bathythermograph sections. The basic components are as follows:
The
primary polar front zone is characterized by a cold water cell vertically elongated relative to
other ocean features, which is isolated from the main mass of cold water further south. To the
immediate south of the primary front is a warm water zone. The southern component of the polar
front zone is the secondary polar front zone. Here the subsurface temperature minimum
(Train )
that
identifies
the Antarctic Surface
Water
has a very variable structure. Occasional indications
of vertical convection are found. South of the double polar front zone, the Antarctic Surface
Water
is found to lie uniformly over the upper Circumpolar Deep
Water
with
little
sign of
instability.
The
mechanism responsible for the formation of the double polar front zone is not known.
Various
possibilities
are offered. The common occurrence of the double front suggests that the
mean wind may be the cause. However, the mean wind field that could produce the double front
is not consistent with the wind distribution thought to
exist
over antarctic waters. The wind
variations may be more important, and the passing of an isolated storm system may produce the
double front. Another possibility is the influence of bottom topography on the transport of the
Antarctic Circumpolar Current. An additional possibility is the progression of internal waves from
south to north between the Antarctic Surface
Water
and upper Circumpolar Deep
Water.
The
pycnocline gradually weakens between
these
two water masses from the Antarctic Divergence to
the polar front zone. This weakening distorts the internal waves near the northern limit of the
Train
layer, causing a severance into two parts.
To
study the antarctic double polar front zone properly, it would be necessary to conduct a
multiship operation similar to those devoted to the Gulf Stream.
INTRODUCTION
The
zone
separating the antarctic and subantarctic
surface
water masses has
been
subject
to much debate
in
oceanographic
literature. This debate is not limited
to
the finer details of the boundary but
involves
its
fundamental
structure, and
even
the
problem
of the
most
suitable name to
describe
this feature.
Much
of
the
confusion
is
derived
from
the
fact
that
the
zone
shows
a great variability in both
form
and
position
and
that
the available data are too
widely
spaced
for
these variations to be understood properly.
Many
of
the descriptions
given
are true relative to the
specific
data used, but are not
valid
in the general sense.
Some
data
clearly
show
a
convergence
process
with the
Ant-
arctic
Surface Water slipping
below
and
mixing
with
the Subantarctic Surface Water;
hence
the name
Ant-
arctic
Convergence.
Other data indicate
more
compli-
cated
processes
that
imply
divergence,
and so the name
Antarctic
Divergence
has
been
proposed.
Still other
data
show
no
evidence
of either of these
processes,
or
so
complicated
a structure
that
no simple circulation
model
is
obvious.
The term 'polar front' is preferred
by
many oceanographers,
since
it implies a situation
analogous
to an atmospheric polar front. This name
implies
only
that
along
this boundary a
fluid
mass
with
polar characteristics abuts with one with
sub-
polar
properties.
The
varying descriptions of the polar front natu-
rally
lead to varying methods used in its
recognition.
The
most
common
is the presence of a relatively large
drop
in the surface temperature encountered on
pro-
gressing
from
north to south.
Although
this method
is
acceptable in many
crossings,
it
often
leads to am-
biguous
results,
since
occasionally
no sharp
zone
of
temperature change is
found.
This
condition
occurs
in
summer when warming of the upper layers of the
surface
water obliterates the gradient, or when a thin
layer
of Subantarctic Surface Water overrides the
Ant-
arctic
Surface Water. For this reason, a subsurface
feature is
often
preferred in defining the polar front.
205
Antarctic Research Series
Antarctic Oceanology I
Vol. 15
Copyright American Geophysical Union

206
ARNOLD
L.
GORDON
TABLE
1
Surface Subsurface
Reference Expression Expression Name
Meinardus [1923]
Schott [1926]
Deacon [1933]
[1937]
Mackintosh [1946]
Garner [1958]
Wexler
[1959]
Burling [1961]
Ostapoff [19626]
Botnikov [1963]
Houtman [1964]
GWorc
[1967]
Grad
T*
Grad
T
Grad
T
Grad
T
Grad
T
Grad
T followed by a re-
versal in the north-south
surface temperature gradient
Grad
T
Surface position of the 2°C
isotherm (winter only)
Mid-point
of Grad T
Grad
T
Northern limit of Antarctic Bottom
Water
Position
of temperature minimum at
200
meters
Northern limit of the subsurface
temperature inversion
Immediately north of an isolated core
of cold water (> 50 miles wide)
Northern
extent
of the 1°C isotherm
in the
Tmin
layer
The
axis
of the circumpolar
salinity
minimum belt at 200 meters
Northern limit of the 2°C isotherm
within
the
Tmin
layer (summer only)
Position
at which the
jTmin
layer ends
or
shows an abrupt change in depth
Meinardus Line (he
uses
term
'polar front' in 1944 ed.)
Antarctic Convergence
Antarctic Convergence
Antarctic Convergence
Antarctic Divergence
Antarctic Convergence
Polar Front
Antarctic Convergence Zone
Antarctic Convergence
Polar Front Zone
*
Grad T refers to a relatively large north-south surface temperature gradient
with
decreasing temperatures toward the south.
Table
1 lists the methods of identification of various
authors and their
choice
for a name for what is
called
the polar front
zone
in this paper.
In 1946 Mackintosh constructed his much refer-
enced
chart of the mean position of the polar front
based
on crossings by the
Discovery
II and William
Scoresby.
This position must be used with reservation,
since
variations do
occur;
as Mackintosh points out:
Tt
forms
twists and
loops
that
may extend as much
as 100 miles north and south, and it
possibly
even
forms
isolated rings.' Meanders and eddies or rings
appear to be fairly
common,
a
condition
that
prompts
the
often
used
comparison
of the polar front with the
'cold
wall'
of the
Gulf
Stream and its meanders and
eddies.
Some
studies of the seasonal or
longer
time-depend-
ent variations have been made. Botnikov
[1963,
1964]
concludes
that
the seasonal variations are usually 1°
to
2° of latitude
(60-120
miles)
and
that
the
long-
term variations
(from
1901 to 1960) are as much as
4°
of latitude
(180-240
miles).
Gordon
[1967]
shows
a polar front
zone
to be
from
2° to 4° of latitude in
width,
whereas
Ostapoff
[1962a,
19626]
indicates an
even
broader 5°
zone.
Data
from
which
indications
of
the short time variations (days or
weeks)
can be
gained
are not
complete
and
only
vaguely suggest
that
such
fluctuations do
occur.
From
past
work
it can be said with
some
assurance
that
somewhere in a broad circumpolar belt there is a
sharp or
diffuse
boundary separating the Antarctic and
Subantarctic Surface Water masses, along
which
evi-
dence
of either
convergence
or
divergence
may be
found.
It is
believed
[Gordon,
1967]
that
the position
of
this
wide
belt or
zone
may be related to
bottom
topography,
as is the northern limit of the Antarctic
Bottom
Water, but the position of the polar front
within
this
zone
is
controlled
by
wind
and/or thermo-
haline factors.
The
terms 'primary' and 'secondary' were added
to
the polar front
zone
by
Gordon
[1967]
to indicate
the presence of a
double
frontal structure in the west-
ern
section
of the Southeast
Pacific
Basin. The basis
in
performing this separation deals with the detail
structure of the subsurface temperature minimum
(7mm).
A similar structure was noted by
Wexler
[1959]
along the 180° meridian and discussed further
by
Burling
[1961],
Ostapoff
[1962a],
and Houtman
[1967].
In this paper the frontal structure is further
Antarctic Research Series
Antarctic Oceanology I
Vol. 15
Copyright American Geophysical Union

ANTARCTIC
POLAR
FRONT
ZONE
207
investigated using primarily bathythermograph (BT)
and expendable bathythermograph (XBT)
data
gath-
ered by the USNS
Eltanin
cruises 25, 27, 32, and 33.
The
analysis
is
limited
to the
area
east
of the
Mac-
quarie Island. At present the USNS
Eltanin
is
gather-
ing
data
south
of
Australia. When
this
task
is
com-
pleted,
a
study
of
the polar front zone for
that
area
will
be published and related to
this
study.
The
BT and XBT sections to be discussed are shown
in Figure
1.
Discussion
of the
other information
shown
in
this
figure is found
below.
TEMPERATURE
PROFILES ACROSS
THE
POLAR FRONT ZONE
In the course of studying the many bathythermograph
profiles
across
the
antarctic
polar front
zone,
one
is
struck
by the
seemingly unlimited variations
in the
surface and subsurface
temperature
structure.
On pro-
ceeding
northward,
the
water
temperatures
increase;
however,
the
rate
of
warming with
latitude
is
highly
varied with the presence
of
numerous reversals.
The
BT
sections
in
the South Pacific do demonstrate some
similarities, enabling
an
attempt
at a
generalized
ther-
mal section. This was first noticed
in
dealing with the
sections
of the
western Southeast Pacific Basin,
at
which
time
the
term double polar front zone
was
applied.
The
generalized section
is
shown
as
Figure
2,
and
the
following
is a
description of each component.
The
particular
BT sections contain all or most of these ele-
ments. Certainly
the
daily weather conditions influ-
ence
the
surface
temperature;
however, since
the
double
structure
extends
to
depths
as
great
as
800
meters and is found in numerous profiles,
it
is felt
that
it
is
of significance
to
the basic frontal
structure
and
should be considered
in
any
attempt
to
understand
the
polar front dynamics
in the
South Pacific sector
of
antarctic
waters.
Proceeding
from south
to
north,
the
sections
en-
countered are as
follows:
1. The stable zone
is the
area
in
which
the
cold
Antarctic Surface Water mass lies uniformly above the
upper Circumpolar Deep Water. The isotherms tend to
be
horizontal with
a
large gradient
in
temperature
at
the boundary of the surface and deep water. Toward
the
northern
end, the T
m™ layer is variable
in
strength
and depth.
2.
The secondary polar front zone is separated from
the stable zone
by a
drop
in the
depth
of the T
m
\
n
layer
and an
increase
in the
surface
temperature.
Within the secondary zone the
T
min
layer shows great
variability
but
remains well defined. The isotherms
tend to be vertically oriented, with occasional evidence
of
filaments
or
streams
of T
min
penetrating
to
fairly
deep
levels.
3.
The warm water zone
is
the
area
in
which
the
T
min
weakens to
become
discontinuous or absent. This
zone
is relatively warm, with the surface layers demon-
strating
higher stability
than
that
found in the zones to
the immediate north and south.
4.
The primary polar front zone
is
relatively
nar-
row
and shows much vertical stretching
of
the water
properties. This feature
is
vertically elongated only
relative to other ocean features and not in the absolute
sense.
In
this
zone,
the
cold
water
is
cold
and is
marked
by
re-establishment
of the T
m
\
n
layer.
The
patch
of
cold
water
is
generally isolated
in a
north-
south plane from the main
body
of
T
m
\
n
water
to the
south. The influence of the water of
this
cell
occasion-
ally extends to the surface, where
it
lowers the surface
temperature
below
that
of the
surface
of the
more
southerly warm
zone.
At the surface of the
northern
boundary of the primary
zone,
there
is a
fairly rapid
temperature
change.
5.
The Subantarctic Surface Water is the
northern-
most
zone.
This zone
is
warmer
than
the other zones;
it
is
always above 3°C and usually above 5°C.
The
isotherms slope steeply toward the north. Occasional
reversals
of the
slope
of the
isotherm surfaces
are
found.
Each of these components is
a
zone;
that
is, they are
broad regions and not lines on the sea surface.
The
following
is a
display of BT and XBT sections
in an
area
from 115°W westward to
160
°E.
120°W
On
Eltanin
cruise 33 (March
to
May 1968)
a
tem-
perature
profile for the upper 800 meters along
120
°W
from
69
°S
to
55
°S was obtained by the XBT system.
This meridian
traverses
the double frontal zone shown
on
Plate
14 of Gordon
[1967].
With the XBT probes,
the finer details and change with depth of
this
appar-
ently permanent feature
could
be
studied. Figure
3
shows
the
thermal
structure
found along
the
ship's
track.
The
primary and secondary polar front zones were
found
in approximately the positions expected from the
analysis
of
previous BT
data:
the
primary zone
at
57°
30'
to 58
°S
and the secondary zone between
59°
30'
to 63°S (see Gordon
[1967,
pi. 14]). The
T
min
layer,
which
represents
the
Antarctic Surface Water mass,
shows
a
great variation within
the
secondary
zone.
Deep
penetration
of the
cold
surface water
to 600
meters
is
found
at
various positions within
this
zone.
Antarctic Research Series
Antarctic Oceanology I
Vol. 15
Copyright American Geophysical Union

208 ARNOLD L. GORDON
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Antarctic Research Series
Antarctic Oceanology I
Vol. 15
Copyright American Geophysical Union

ANTARCTIC
POLAR
FRONT
ZONE
20 9
100 -
200- 1
300
J
SUBANTARCTI C
|
PRIMARY
POLA R
FRON T
(PPF )
WAR M
WATE R
SECONDAR Y
POLA R
FRON T
(SPF )
STABL E
ANTARCTI C
SURFAC E
WATE R
SURFAC E EXPRESSIO N
OF
PPF
SOUTHER N BOUNDAR Y 0FSPF WAR M
^
UNIFOR M
COLD
Tmin
NOT
FOUND
IN
SURFAC E
LAYE R
WEAK , VARIABL E
/
Tmin
/ '
ISOLATE D WEA K O R
/
'OCCASIONA L
ABSENT / /EVIDENC E
OF
Tmin
SEGMEN T
SINKIN G
RELATIV E
SURFAC E
TEMPERATUR E
•0
HO O
1-200
-300
Fig. 2.
Schematic
diagram
of the
antarctic
double
polar
front
zone.
between
which
occur
columns of warmer deep water.
At
62°S
(XBT 76) water of
over
2°C ascends to
within 180 meters of the surface. Within the secondary
zone,
the vertical trend of the isotherms suggests
that
the
r
m
i
n
forms many filaments or streamers, perhaps
similar to the cellular
convection
discussed by Turner
[1967]
and
Foster
[1968],
However, in the above
case,
salinity is the stabilizing influence
rather
than
temperature, as discussed by Turner. The arrows
shown
on Figure 2 indicate the possible vertical mo-
tion of the water with the secondary polar front
zone.
South of the secondary polar front zone is the zone
with a more stable water column. Here the isotherms
tend to be horizontal, whereas in the secondary zone
and to its north the isotherms are more vertical.
Th e
primary polar front zone is characterized in a
manner described by
Wexler
[1959].
Along
the sea
surface
from
north to south, the temperature first
drops
sharply, then reverses gradient, resulting in a
temperature minimum immediately south of the high
temperature gradient.
Below
the minimum is a ver-
tically
elongated
cell
of
cold
water isolated in a north-
south plane
from
the main
body
of
cold
surface water
further south.
Between
the two polar front zones is a region of
warmer water. In Gordon
[1967],
it is suggested
that
this
warm water,
occupying
the region
from
58° to
59°30'S
for the section shown in Figure 3,
results
from
upwelling,
that
is, a zone of divergence. In
this
case,
the salinity of
this
zone should be above
that
of
Antarctic Surface Water. However, the salinity of
this
zone
for the upper 300 meters is shown in Figure 4
to
be no higher
than
the Antarctic Surface Water mass,
that
is, less
than
34.2%
c,
which is more characteristic
of
Subantarctic Surface Water and certainly less
than
that
of the upper Circumpolar Deep Water
(34.6%c).
Th e
higher temperatures, together with similar salini-
ties, indicate
that
this
water may be warmed Antarctic
Surface
Water.
115° W
Figure 5 shows the temperature profile for the upper
80 0
meters along 115°W constructed
from
cruise 11
data.
The upper 275 meters are taken
from
the BT
observations
(see Gordon
[1967,
pi. 13]).
Below
this
depth the hydrographic station
data
are used;
since
these observations are spaced much
farther
apart
than
the BT
data,
the section represents
only
general
structure. The temperature distribution shown in
Figure 5 is similar to
that
along
120°W
(Fig. 3) in
that
the double polar front is present.
Th e
r
m
in
layer continues
below
BT depth, although
it is not as extreme as it is at shallower levels. The
level
character of
this
layer is more probably the
product
of too widely spaced
data
than
a reflection of
reality. The deep penetration of the 2°C isotherm at
60°30'S
and 61°30'S suggests a
convective
process
similar to
that
found along the
12 0 ° W
section. The
elongated
cold
cell
of the primary zone exists, but the
northern extent of the
cell
is uncertain
owing
to a
larger spacing of the hydrographic stations at these
latitudes.
12 8 ° W
During cruise 25 another BT section was obtained in
the region of the double frontal
structure
shown in
Plate 14. The BT section is shown as Figure 6. In
this
section
the primary polar front
zone,
although present,
is not very well developed. The surface temperature
drops
suddenly
from
6° to 5°C at
55°40'S,
and a
slight minimum of about 4°C occurs at
56 ° 15'S ,
be-
lo w
which is the upper
part
of a
cell
of
colder
than
3.5°C.
Although these values are a few degrees above
those in the primary zone previously encountered, the
Antarctic Research Series
Antarctic Oceanology I
Vol. 15
Copyright American Geophysical Union

210 ARNOLD L. GORDON
(SH313W)
Hid3d
Antarctic Research Series
Antarctic Oceanology I
Vol. 15
Copyright American Geophysical Union

ANTARCTIC
POLAR
FRONT
ZONE
211
34. 0
34.2 34.4 34.6 34.8
SALINIT Y (°/oo )
Fig. 4. Temperature /
salinity
diagram of hydrographic
sta-
tions
824-828
along
120 ° W .
The numbers
next
to symbols
are observation
depths
in meters.
general form persists. The surface water salinity
(found
from hydrographic stations) south of the pri-
mary front is approximately
34.0%c
to
34.1%o,
iden-
tifying
the water as Antarctic Surface Water and the
primary front as a genuine boundary of water masses.
Th e
secondary zone is found between 59° to 60°
30'S .
The
structure
is fairly typical in
that
the
r
min
layer undergoes a depth increase on entering the zone
from
the south accompanied by a relatively large
sur-
face
temperature
gradient. Within the secondary
zone,
the
r
min
is variable. Owing to the lack of
closely
spaced
deeper
data,
it is not known if convection exists
as found along 115° and
12 0 °W .
Th e
warm zone is fairly broad and is divided into
two
packets of warmer water: at 56°30
/S and at
58°S.
Th e
two hydrographic stations in
this
zone (STD
sta-
tions 603 and 604; see Jacobs and Amos
[1967])
indi-
cate no shallow T
m
in layer, but a weak T
m
in zone at
700
meters of approximately
0.5°C
amplitude;
this
situation is similar to the warm zone along
12 0 °W .
155°W
(AVERAGE)
Th e
return
track of cruise 25 is the first section dis-
cussed
which is west of the previously defined extent
of
the double frontal
structure.
The BT section is
shown
as Figure 7. It indicates a double zone similar
to
Figure 6 in
that
the form is present but the temper-
atures
of the primary zone are higher
than
expected.
Th e
well-developed characteristics of a rapid change of
surface
temperature
followed
by a minimum above a
cold
cell
occur
between 55° to
55°30'S.
A well-de-
veloped
warm zone is found between
55 ° 30 '
and 56°
30'S .
The secondary zone between
56 ° 30 '
and 59°S
cannot be investigated in detail because of the lack
of
data
below
BT depth; however, two regions of sink-
ing appear to be centered at 56°50'S and
58°S.
The
southern boundary of the secondary zone is again
found
by the change in the depth and continuity of
the
T
min
layer and a high surface
temperature
gradient.
170°W
(AVERAGE)
Th e
next profile to the west was also obtained on
cruise 33 with XBT and supplementary BT observa-
tions. This
temperature
section is shown as Figure 8.
PRIMAR Y
POLA R
FRON T
ZON E SECONDAR Y POLA R FRON T ZON E
23 0
231 232
23 3 23 4
235 236 237 238 239 240 241 242
LATITUD E
IS. ) ELTANI N
CRM
DE C
1963
Fig. 5. Temperature profile
along
115°W;
upper 275 meters
taken
from Gordon [1967, pi. 13]; below
these
depths
hydrographic
stations
230 to 242 are used,
Eltanin
cruise
11.
Antarctic Research Series
Antarctic Oceanology I
Vol. 15
Copyright American Geophysical Union

212
ARNOLD
L.
GORDON
(UJ )
No
developed double frontal
structure
is
found,
al-
though the
T
min
layer between 60° and
61
°S does in-
dicate vertical elongation similar
to a
primary
zone;
however,
the isotherms are not isolated from the main
body
of
r
m
in
to the south, nor are
there
any large re-
versals of the north-south surface
temperature
gradient.
Therefore,
only
a
singular polar front zone
is
shown,
as defined in Table
1.
178°E
The
BT section obtained
on
cruise
27 is
shown
in
Figure
9. No
clear division into
a
double zone
is
found.
However,
the
Tmm does change
in
both tem-
perature
and depth immediately south
of
63
°S,
and
an increase
in the
surface
temperature
gradient
is
found.
Therefore,
the
region between 62°
and
63°
is labeled
as a
secondary polar front
zone,
although
the lack of
a
warm zone
to
the north does not permit
definition
of the
northern
boundary.
The
region
at
60°30
/
is
called
the
primary polar
front
zone,
although again the lack of the warm zone
and no observed isolation of
a
cold
cell
suggest
a
single
polar front. Therefore,
this
section can be divided into
a double zone only
by the
variation
at
63
°S.
The
other factors used
in
definition of
this
phenomena are
absent.
176°E
During cruise 32 of the Eltanin,
a
series of BT's and
XBT's
(the probes used for
this
section extend to 500
meters) were
taken
along
a
track
that
recrossed ap-
proximately the same region
three
times
in
four days.
During
this
time, variations were found
in
the frontal
structure.
These sections (A to C) are shown as Figure
10.
Both sections B and C display strong double struc-
ture.
Section
A
indicates only
a
poorly defined pri-
mary polar front
zone,
although
it is
possible
that
the
large spacing between BT observations 223 to 227 may
have missed the center of the
cold
cell.
The
northern
limit of the
T
min
layer in the secondary
zone
changes
its
form somewhat, although
not
dra-
matically. Most of the variation
is
found
in
the pri-
mary
zone.
Comparing
the
center
of the
cold
cell
shown
in
sections
B
and C,
it is
found
that
it
moves
toward the north
at
17 cm/sec and elongates slightly.
The
1.5°
C
isotherm moves upward from 100
to 60
meters with
a
vertical velocity
of 4 X
10~
2
cm/sec.
The
coldest water within
the
primary front
is
below
—0.5
°C.
The
northern
limit of
this
isotherm with the
Tmin
layer
to
the south
is at
64°
30'S
or
over 200 km
to the south of the center
of
the primary front.
The
amount
of
water with
a
temperature
below
0.0
°C
is
Antarctic Research Series
Antarctic Oceanology I
Vol. 15
Copyright American Geophysical Union

ANTARCTIC
POLAR
FRONT
ZONE
213
(w) Hid
greater
in section C. The variations in the primary
front between sections A and B are in the same direc-
tion as are the variations between sections B and C,
but they are more intensive.
The
cold
cell
appears
to be growing and becoming
more
intense from sections A to C. The decreasing
temperature
is also
apparent
at the surface, where the
temperature
drops from 4.5° to 3.5°C from sections
A
to C. The intensification is a product of
transport
perpendicular to the BT sections (zonal
transport)
rather
than
motion parallel (meridional transport),
since
the isolation and intensity of the
cold
cell indi-
cate no way in which
cold
water can be added to the
primary front except by zonal
flow.
The
three
BT sections are positioned so
that
section
B
crosses the primary polar front zone 9 km
east
of
the point
that
section A crosses
this
feature, and
sec-
tion C crosses it 55 km to the
northeast
of B. Since
the
satellite
navigation system was not operative
dur-
ing these
three
BT sections, the accuracy of these dis-
tances is only fair. It is possible to visualize a number
of
circulation
patterns
that
would be consistent with
the observation and would be in steady
state
(no time
variation). These are: (1) a westward
flow
along the
core
of the
cold
cell with diffusion; (2) no motion at
all; (3) an eastward
flow
in the
cold
cell with re-
newal of
cold
water from the south at some point at
the western end of the
cold
cell (between sections A
and B). None of these conditions is realistic, since an
eastward
current
is most probable for the Antarctic
Circumpolar
Current,
and
there
is no evidence of a
renewal of
cold
water from the main
body
of
!Fmin
water to the south, especially from the section colder
than
—
0.5°C.
Therefore, it is fairly certain
that
a time
dependent component is important, although it is
diffi-
cult to
separate
completely the space from the time
variations without a number of simultaneous measure-
ments of
temperature
at a
great
many points.
Assuming an eastward flowing
current,
its magnitude
may be such
that:
(1) the ship would cross the pri-
mary front at the same relative point (this requires a
current
of 14 cm/sec between sections A and B and
48 cm/sec between sections B and C) ; (2) the ship
may gradually approach (or 'overtake') the core of
the
cold
cell from the west, i.e., the
current
is less
than
14
and 48
cm/sec,
respectively, between the two
sec-
tion pairs; or (3) the core of the
cold
cell is 'catching
up'
with the ship, i.e., a
current
of
greater
than
14 and
48 cm/see, respectively. Certainly the first
choice
is
not realistic, since the significant changes in the ex-
tent
and
temperature
of the
cold
cell indicate zonal
Antarctic Research Series
Antarctic Oceanology I
Vol. 15
Copyright American Geophysical Union

214 ARNOLD L. GORDON
Antarctic Research Series
Antarctic Oceanology I
Vol. 15
Copyright American Geophysical Union

ANTARCTIC
POLAR
FRONT
ZONE
215
flow
relative to the ship. However, the values 14 and
48
cm/sec
are current magnitudes which are probably
in the range of the actual
velocity
in
this
area;
there-
fore,
in addition to the motion of the
cold
cell
relative
to
the ship, it is expected
that
the
cold
cell
is under-
going
deformation.
The
vertical motion of the 1.5°
C
isotherm and the
northward migration of the center of the
cold
cell
referred to earlier are most likely both time and space
dependent. From the wind
data
collected
aboard the
ship, it is found
that
strong Ekman divergence exists
above
the
cold
cell
region. The increase in the vertical
extent of the
cold
cell
may be explained by
this
diver-
gence.
The Ekman meridional
transport
as a function
of
latitude
is shown in Figure 11 for sections A, B,
and C. These values were calculated from the equation
based on the Ekman theory
Te = r\/p f
where Te is the Ekman
transport
along a meridian;
T\
is the zonal component of the wind stress, / is the
Coriolis
parameter, and p is the water density. In the
regions
where Te is increasing toward the north, diver-
gence
results, and the continuity equation requires
either upwelling or variations in zonal
transport,
or
both.
The steeper the gradient of Te with latitude, the
greater must be the upwelling and/or zonal
transport
variation. The maximum divergence is found directly
above
the primary polar front
zone.
Therefore, the
divergence
theory put forward by
Wexler
[1959]
is
supported by these
data,
However, problems still exist,
since
upwelling alone cannot explain the primary polar
front
zone.
The remaining questions are: Where does
the
cold
water within the primary front
come
from,
since
water
cold
enough to act as a source is found far
to
the south, and why does the double frontal struc-
ture
extend to depths greater
than
the surface Ekman
layer, which is expected to be 100-200 meters deep?
Although
the
three
sections for cruise 32 do not
lead to conclusive results, they do indicate
that
the
primary polar front zone varies zonally and
could
easily be missed by hydrographic profiles because of
its limited
latitudinal
extent as well as its
apparent
dis-
continuous zonal
structure.
174°E
The
BT profile obtained on cruise 32 is shown as
Figure 12. Unfortunately, the depth penetration was
less
than
the
usual
275 meters, and it is unknown if
the
r
min
layer continues north of
62
°S in the
near
surface layers. The double frontal zone is
poorly
de-
fined, although the characteristic dip in the T
m
in layer
Antarctic Research Series
Antarctic Oceanology I
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Copyright American Geophysical Union

216
PRIMAR Y
POLA R SECONDAR Y
FRON T POLA R FRON T ZON E
LATITUD E (SOUTH )
PRIMAR Y
SECONDAR Y
POLA R
POLA R FRONT ZON E
FRON T
PRIMAR Y SECONDAR Y
POLA R POLA R FRON T ZON E
LATITUD E (SOUTH )
Fig.
10. Bathythermograph and expendable bathythermograph sections along
176°E;
section C is partially com-
posed of T4 XBT probes; BT observations 203 to 269; XBT observations 1 to 13, Eltanin cruise 32.
Antarctic Research Series
Antarctic Oceanology I
Vol. 15
Copyright American Geophysical Union

ANTARCTIC
POLAR
FRONT
ZONE
50
40
I
i 1 • i
T
k ! \
-
/ \
./ c
\.
x ^
1
N
•
i • i . i.N
63 °
LATITUD E
(SOUTH )
Fig. 11. Meridional transport
within
the Ekman
layer
for
sections
A, B, and C
shown
in Figure 10.
to
the immediate south of the secondary polar front
zone
is present, as is the
cold
intrusion at
6 0 ° S
similar
to
the
cold
cell
of the primary front. The temperature
structure
is similar to
that
found on the two crossings
of
cruise 25, in
that
the primary front is present but
is
composed
of water warmer
than
that
found in other
sections.
OTHER
BT
SECTIONS
SOUTH
OF
NE W
ZEALAND
In addition to the Eltanin BT sections, a number of
sections
obtained by other ships south of New Zealand
also
show
a double frontal structure. The BT and
hydrographic
station
data
of the
HMNZS
Kaniere
[Houtman,
1967],
whose track is shown in Figure 1,
show
a clear double frontal structure, as do the BT
data
discussed by
Wexler
(1959—two
sections of USS
Glacier
and one of
HMNZS
Pukaki). Since these
last
three
sections all represent the same region,
only
the
position
of the
second
Glacier
BT section is shown in
Figure 1.
Tw o
interesting BT sections are discussed by Burl-
ing
[1961].
These are sections taken by
HMNZS
Pukaki and
HMNZS
Hawea
only
five
miles
apart
at
the same time. They are shown as one line on Figure
1. Burling's figures 46 and 4c are reproduced in
Figure 13. Both
show
a strongly
developed
primary
and secondary polar front
zone,
with
only
minor vari-
ation between the two sections. The BT sections taken
during the northerly track of the two ships are far
apart.
The Hawea
followed
the 180° meridian and the
Pukaki the 169° E meridian. The BT observation spac-
in g
on Hawea was too large to detect the narrow pri-
mary front. The Pukaki section shows a weak pri-
mary front (neither of these section positions are
shown
on Figure 1).
(w)Hid30
Antarctic Research Series
Antarctic Oceanology I
Vol. 15
Copyright American Geophysical Union

218
ARNOLD
L.
GORDON
PRIMAR Y
POLA R
FRON T
ZONE
SECONDAR Y
POLA R
FRON T
ZONE
60° 62°
LATITUDE
(SOUTH)
Fig. 13a.
Bathythermograph
section
obtained
by HMNZS
Pukaki
(modified
from
Burling
[1961]).
DISCUSSION
The
positions of the primary and secondary polar front
zones
of each of the BT and XBT sections discussed
are shown in Figure 1, in addition to the mean posi-
tion of the Antarctic Convergence determined by
Mackintosh
[1946].
The primary front is often in
close
proximity to the Mackintosh position, whereas
the secondary front is 1 or 2 degrees of
latitude
fur-
ther
south. Between
170
°E to 180° the primary front
is found to be consistently south of the Mackintosh
mean position. It is probable
that
the southernmost
penetration of the Subantarctic Surface Water around
Antarctica occurs between 170°E to 180°.
SECONDAR Y
POLA R
FRON T
ZONE
LATITUDE
(SOUTH)
56° 58°
Q.
200
3001
HAWE A
<<<:>>{
DECEMBE R
1956 X- 1
68°
I73°40'EI74°44'E
I75°53'E I76°55'E I78°05'E I79°32'E
|79°57W
LONGITUDE
Fig. 136.
Bathythermograph
section
obtained
by HMNZS
Hawea
(modified
from
Burling
[1961])
Antarctic Research Series
Antarctic Oceanology I
Vol. 15
Copyright American Geophysical Union

ANTARCTIC
POLAR
FRONT
ZONE
219
For
cruise 32 sections A to C it was shown above
that
Ekman divergence of the surface water is found
above
the primary polar front zone and
that
the en-
suing upwelling is consistent with the
structure
of
this
zone.
The wind
data
collected simultaneously with
the other
Eltanin
BT and XBT sections were also in-
spected
for similar
structure.
It is found
that
diver-
gent conditions
occur
over the primary zone for
sec-
tions 5 and 7, but not for the other sections. There-
fore,
the double
structure
cannot be explained entirely
by
the local wind for
this
reason and for the
follow-
ing additional reasons: (1) the
structure
penetrates
to
depths far
below
the Ekman layer; and (2) the
com-
mon
occurrence of the double
structure
also suggests
at least a semipermanent presence, since it would be
fortuitous
that
most BT sections are obtained during
the period
that
the double
structure
exists, unless
this
period
is long. If it is the wind which is responsible
for
this
phenomenon, it is more likely the mean wind
or
the mean
effect
of the passage of storm systems.
It is possible to envision a mean zonal wind field
that
would create an Ekman circulation within a me-
ridional plane consistent with the suspected
pattern
in
the double polar front
zone.
The
latitudinal
variation
of
the proposed mean wind would show: (1) a west
wind
maximum over the southern boundary of the
secondary
polar front
zone;
(2) a minimum or slightly
negative value (east wind) over the warm
zone;
(3) a
very
rapid increase in the west wind over the primary
polar front
zone.
A circumpolar belt of low pressure
caused by the passage of storm systems over the pri-
mary front can produce such a wind
pattern.
How-
ever,
storm
tracks
most likely pass south of
this
front
[Weyant,
1967] with an average position in the re-
gion
of the Antarctic Divergence, which divides the
easterly directed Antarctic Circumpolar
Current
from
the westerly directed coastal
current
[Koopman,
1953].
In
this
case, the zonal wind is expected to increase
toward the north from the Antarctic Divergence and
would
not create the above circulation
pattern.
Per-
haps the mean zonal wind and average storm
tracks
are not as important as the short period fluctuations
in the wind field. Wind fluctuations may contribute
to the mean ocean circulation
[Veronis,
1966].
The
occasional
passage of a storm north of the average
storm track may divide the
r
m
i
n
layer into the two
components
of the double front. The double
structure
is produced by the single storm and
once
produced is
not destroyed by the
return
of the
usual
west wind
profile.
The
cold
cell
flows
eastward and
slowly
dis-
sipates by diffusion, perhaps months after its creation
by
a storm.
It is possible to differentiate the relative 'age' of
the double polar front
zone.
Cruise 25 sections with
a weak primary frontal zone may represent the final
stages, i.e., the
cold
cell
is warmer
than
usual
and al-
most
dissipated. A young or active stage is represented
by
cruise 33, section 5, and cruise 32, section 7.
In both cases, the local wind
pattern
would produce a
strong Ekman divergence over the primary front.
Sec-
tion 5 displays a continuous
T
min
layer, but the dis-
tortion of
this
layer between 60° to
62
°S suggests the
beginning of a vertically oriented
cold
cell
and subse-
quent primary front. The cruise 32 section
taken
along
173
°E a few weeks before the
176°
E sections
shows
dissipated conditions, indicating
that
during the
intervening month a new double polar front was
formed.
The
sections along
115°
W and
120
°W
represent
'middle-age'
conditions, since the double frontal struc-
ture
is well developed, but Ekman divergence does not
occur
over the primary front in either case. The dis-
sipated condition is indicated by the cruise 27 section,
in which the
r
m
in
of the secondary front moves north-
ward to extend to the previous position of the primary
front.
The
speculation of
Wexler
[1959]
on the formation
of
the
cold
cell
by the wind leads to his suggestion
that
the
entire
polar front zone is a region of diver-
gence.
The
cold
cell
results
from strong winter winds
north of the
northern
limit of the pack ice. The short
summers do not allow sufficient time to destroy the
cold
cell.
Wexler, by analogy with other upwelling
areas,
shows
that
the maximum upwelling is expected
to
occur
100 miles or so north of the maximum pack
ice
extent. His theory, while explaining the proposed
upwelling
within the primary polar frontal
zone,
does
not explain the secondary front nor the problem of
supplying
cold
water to the isolated
cold
cell.
Although
a wind based theory is
attractive,
since the
double
polar front includes surface layer phenomena,
the problems listed above are difficult to
overcome.
As
pointed out by Gordon
[1967],
the primary and
sec-
ondary polar frontal zones in the western
part
of the
Southeast Pacific Basin converge to a point directly
above
the Usarp
Fracture
Zone.
This zone contains a
deep
canyon cutting across the Mid-Oceanic Ridge.
Therefore,
it was suggested in
this
earlier study
that
a
'wake'
phenomenon may
occur
and be in a quasi steady
state.
The
3000-meter contour shown in Figure 1 indicates
that
both north and south of Macquarie Island
(55°S,
160°E)
are deep passages similar to the Usarp Frac-
ture
Zone.
From inspection of the dynamic topog-
Antarctic Research Series
Antarctic Oceanology I
Vol. 15
Copyright American Geophysical Union

220
ARNOLD
L.
GORDON
raphy of the sea surface relative
to
2500-db level,
it is
found
that
a
steep gradient occurs over
the
northern
passage. Such
a
constriction
of
the streamlines
at the
sea surface
is
similar
to the
pattern
above
the
Usarp
Fracture
Zone
[Gordon,
1967,
pi.
11].
The
north-
ern passage was crossed on Eltanin cruise 16.
A
5500-
meter channel was found
at
52
°S,
161°30'E.
In
addi-
tion,
the
mean position
of the
Antarctic Convergence
passes directly over
the
Usarp
Fracture
Zone
and
probably
north
of
Macquarie Island,
rather
than
in-
tersecting
the
island
as
shown
by
Mackintosh.
It ap-
pears
that
the
structure
of the water column has some
similarities,
and
that
propagation
of a
double polar
frontal zone may
occur
over both regions. Perhaps
its formation may
be
related
to the
theory
of
Burling
[1961]
:
The
northern
extremity
of a
meander
in the
polar front breaks off, forming
a
cold
eddy which,
in
conserving
its vertical component of potential vorticity
as
the
latitude
decreases, must elongate into
the
ob-
served
cold
cell.
The deep channels may cause produc-
tion
of
meanders
that
culminate
in the
form
of
cold
eddies.
A
third
possible mechanism
that
may produce
the
double
polar front zone
is
based
on
internal
waves.
The
main pycnocline
in
antarctic waters occurs
be-
tween the Antarctic Surface Water and upper Circum-
polar Deep Water. On
this
pycnocline
internal
waves
may
occur.
The wave
patterns
found
in
the isotherms
below
the T
min
layer
on all
BT sections south
of the
secondary
polar front zone are most likely due
to
such
internal
waves. Calculation of wave length
is
dubious,
since
the section is obtained over
a
period of days.
The
density variation across
the
pycnocline
de-
creases toward the north. The upper circumpolar deep
water decreases from
a
<J
t
of
27.8
near
the
Antarctic
Divergence
to
27.6
near
the polar front
zone.
The
cr
t
of
the T
min
layer decreases from 27.4
to
27.3 across
the same distance.
The
separation between
the two
layers
is
slightly greater
near
the polar front zone
than
in
the
Antarctic Divergence vicinity. Therefore,
the
strength
of the
pycnocline decreases slightly from
south
to
north.
The
internal
waves
are
initiated
at
the Antarctic Di-
vergence
owing
to the
passage
of
storm systems.
The
waves
travel northward.
As the
pycnocline weakens,
the waves progress more
slowly
and the
wave length
decreases. Accompanying
this
shortening of the wave,
the amplitude would increase. The increase
in
ampli-
tude and decrease
in
wave length
is
due
to the
reduc-
tion
of
the restoring
force
as the
pycnocline weakens.
Smaller amounts of energy are needed to produce simi-
lar amplitude waves
for a
weak pycnocline
than
for
a strong pycnocline. Therefore,
the
internal
waves be-
come
large amplitude waves
or
relatively short-crested
waves
near
the
northern
limit
of
the
T
min
layer. Such
a situation
is
analogous
to
the flapping of
a
flag
in
the
wind,
where the tension on the cloth
is
less
at
the free
end,
so
that
it
oscillates more violently
than
does
the
flag
near
the
flagpole.
The
internal
waves
in the
vicinity
of the
northern
boundary
of the T
min
layer
may
become
unstable with turbulence destroying
the
crests and troughs leaving only
the
sides of the wave.
Such
a
process would produce
the
isolated
cold
water
cell
of the
primary polar front zone
and the
cells
of
sinking within the secondary polar front
zone.
CONCLUSION
The
present paper deals only with
the
area
east
of
Macquarie Island. The double polar front zone may
exist
further
west. The
future
Eltanin cruises will
in-
vestigate
this
possibility.
If it is
found west
of
Mac-
quarie Island, either
the
bottom topography theory
would
be
proven wrong
or
another deep channel fur-
ther
west may exist.
None
of the above theories
is
conclusive. However,
with these
in
mind field work can
be
planned
to
best
obtain
an
understanding
of the
double polar front
zone.
Certainly time series
data
are
needed,
as is a
rapid survey
of an
area
of
limited extent over
the
primary polar front
zone.
Acknowledgment.
Grateful
acknowledgment
is due to R.
Tsgonis
for
preparing
the
figures,
to L.
Child
for
drafting
them,
and to J.
Stolz
for
secretarial
assistance.
The
manu-
script
was
critically
read
by
Stanley
S.
Jacobs,
Kenneth
Hunkins,
and
Neil
Opdyke.
The
work
is
financially
sup-
ported
by the
National
Science
Foundation
grants
GA-1309,
GA-10794,
and
GA-19032. Lamont-Doherty
Geological
Observa-
tory
contribution
1499.
REFERENCES
Botnikov,
V. N.,
Geographical
position
of the
Antarctic
Con-
vergence
Zone
in the
Antarctic
Ocean,
Soviet
Antarctic
Exped. Inform.
Bull.,
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