JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. C5, 3162, doi:10.1029/2001JC001227, 2003
Southern ACC Front to the northeast of South Georgia: Pathways,
characteristics, and fluxes
Michael P. Meredith,1 Jon L. Watkins, Eugene J. Murphy, Peter Ward, Douglas G. Bone,
Sally E. Thorpe, Sharon A. Grant, and Russell S. Ladkin
British Antarctic Survey, Cambridge, UK
Received 16 November 2001; revised 16 September 2002; accepted 19 December 2002; published 27 May 2003.
[1] In December 2000 we conducted high-resolution hydrographic and towed undulator
transects across the South Georgia shelf and into the deep waters of the Georgia Basin.
The Southern Antarctic Circumpolar Current Front (SACCF) was observed flowing
northwestward close to the base of the slope at 53.7S, with a second manifestation around
53.4S having flow in the reverse direction. Both crossings had a significant barotropic
component aligned with the flow. The region of the SACCF was characterized by largescale undulations of properties on horizontal scales of 5–10 km, similar to the local
internal Rossby radius. We observe a distinct surface temperature gradient associated with
the SACCF in the vicinity of South Georgia and demonstrate the usefulness of this by
tracing the course of the front around the island with advanced very high resolution
radiometer data. A distinct northward deflection of the front to the north of South Georgia
is probably due to topographic steering by the North Georgia Rise. The signature of the
SACCF is pronounced in the Circumpolar Deep Water, with strong isopycnal interleaving
indicative of cross-frontal mixing. The salinity of the Weddell Sea Deep Water is
marginally higher between the two manifestations of the SACCF, reflecting the different
pathways from its source regions in the Weddell Sea. Remarkably strong vertical
gradients of potential temperature and salinity exist in the bottom 100 m of the water
column (at depths of around 3500 m) because of the vertical juxtaposition of waters that
INDEX TERMS: 4207
have taken different routes around the Northeast Georgia Rise.
Oceanography: General: Arctic and Antarctic oceanography; 4512 Oceanography: Physical: Currents; 4536
Oceanography: Physical: Hydrography; 4528 Oceanography: Physical: Fronts and jets; 4275 Oceanography:
General: Remote sensing and electromagnetic processes (0689); KEYWORDS: South Georgia, SACCF, fluxes,
water masses, SST
Citation: Meredith, M. P., J. L. Watkins, E. J. Murphy, P. Ward, D. G. Bone, S. E. Thorpe, S. A. Grant, and R. S. Ladkin, Southern
ACC Front to the northeast of South Georgia: Pathways, characteristics, and fluxes, J. Geophys. Res., 108(C5), 3162,
doi:10.1029/2001JC001227, 2003.
1. Introduction
[2] South Georgia lies on the North Scotia Ridge toward
the eastern end of the Scotia Sea (approximately 54S,
37W; Figure 1). It is separated from the deep ocean by a
shelf region of around 50 – 150 km width; this shelf is
typically shallower than 300 m, but is frequently dissected
by submarine canyons. Waters over the South Georgia shelf
often have different characteristics from the water masses
beyond the shelf break [Brandon et al., 1999, 2000; M. P.
Meredith et al., Variability in oceanographic conditions to
the east and northwest of South Georgia, 1996 – 2001,
submitted to Progress in Oceanography, 2002, hereinafter
1
Now at Proudman Oceanographic Laboratory, Bidston Observatory,
Prenton, Wirral, UK.
Copyright 2003 by the American Geophysical Union.
0148-0227/03/2001JC001227$09.00
referred to as Meredith et al., submitted manuscript, 2002],
a consequence of retention processes combined with significant fluxes of heat and freshwater. Occasionally, the
boundary between these different waters takes the form of a
distinct shelf-break front [Brandon et al., 1999]; at other
times, a much more gradual horizontal transition is apparent
(Meredith et al., submitted manuscript, 2002). Significant
interannual variability has been observed both over the
South Georgia shelf and beyond the shelf break (Meredith
et al., submitted manuscript, 2002), with both small-scale
local processes and large- (basin-) scale processes contributing. At the very large scale, climate anomalies have been
observed precessing to South Georgia, resulting in interannual variability in temperature around the island’s shelf
[Trathan and Murphy, 2002]. These anomalies can transfer
from over the deep ocean environment onto the South
Georgia shelf, and significantly perturb local conditions
there, with consequences for the local ecosystem (Meredith
et al., submitted manuscript, 2002). The mechanisms by
31 - 1
31 - 2
MEREDITH ET AL.: WATER MASSES AND CIRCULATION NEAR SOUTH GEORGIA
Figure 1. Schematic of the locations of the ACC fronts across the Scotia Sea. Frontal locations were
adapted from Orsi et al. [1995] by Meredith et al. [2001] following work by Thorpe [2001] and Naveira
Garabato et al. [2002]. Fronts are (north to south) the Subantarctic Front (SAF), the Polar Front (PF), the
Southern ACC Front (SACCF), and the Southern Boundary of the ACC (SB). Some important
topographic features are marked: North Georgia Rise (NGR), Northeast Georgia Rise (NEGR), Georgia
Passage (GP), and Orkney Passage (OP). The 1000 and 3000 m isobaths are marked; regions shallower
than 3000 m are shaded.
which these climate anomalies are transferred from the
Pacific into the South Atlantic sector of the Southern Ocean
and toward South Georgia involve advection by the Antarctic Circumpolar Current (ACC).
[3] The ACC is a banded structure consisting of a number
of bottom reaching fronts with high current speeds separated
by broader, more quiescent zones of water. Much of our
present-day understanding of the ACC derives from work
undertaken during the International Southern Ocean Studies
(ISOS) program in the 1970s and 1980s [e.g., Sievers and
Nowlin, 1984]; this concentrated on Drake Passage, and
the fronts observed here were termed (north to south) the
Subantarctic Front (SAF), the Polar Front (PF), and the
Continental Water Boundary (CWB). More recently, Orsi
et al. [1995] used a comprehensive set of historical hydrographic measurements to trace these features around Antarctica. They noted that the term CWB was somewhat
illogical, since it was actually a circumpolar feature, and in
some other areas was situated at a great distance from
the Antarctic continent. The name Southern ACC Front
(SACCF) was suggested as an alternative. A Southern
Boundary to the ACC was also identified; south of this in
the Scotia Sea lie the waters of the Weddell-Scotia Confluence (WSC), characterized by reduced vertical gradients
of water mass properties because of the injection of dense
shelf waters near the tip of the Antarctic Peninsula [Whitworth et al., 1994].
[4] Over the deep ocean environment around South
Georgia, waters in the upper 200 m are generally termed
Antarctic surface waters. Typical surface temperatures during the austral summer (when most in situ measurements are
made) are around 2 – 3C over the South Georgia shelf, the
slope, and the adjacent deeper ocean (Meredith et al.,
submitted manuscript, 2002), although values approaching
4.5C have been observed. Salinities are generally around
33.6– 33.8. It is extremely rare, but not completely unknown, for the ocean to freeze around South Georgia in
winter. In the austral summer, the Antarctic surface waters
feature a pronounced temperature minimum at around 100
m depth. This is the remnant of the previous winter’s mixed
layer, and is formed seasonally by deep convection. It is
commonly termed Winter Water (WW) [Mosby, 1934].
Typical values for the temperature of the WW close to
South Georgia are around 0C although they can be as low
as 1C (Meredith et al., submitted manuscript, 2002).
Salinities in the WW layer are generally around 33.9.
[5] The most voluminous water mass in the region is
Circumpolar Deep Water (CDW), derived from the North
Atlantic Deep Water (NADW) that becomes incorporated
into the ACC [Reid et al., 1977; Whitworth and Nowlin,
1987]. This is divided into two components, specifically
Upper CDW (UCDW) and Lower CDW (LCDW). UCDW
is characterized by a maximum in potential temperature
(and a minimum in dissolved oxygen) at around 500 m
MEREDITH ET AL.: WATER MASSES AND CIRCULATION NEAR SOUTH GEORGIA
depth, while LCDW is characterized by a maximum in
salinity around 1000– 1500 m depth. The densest water in
the Scotia Sea and Georgia Basin is Weddell Sea Deep
Water (WSDW), which forms through mixing of mid-depth
Warm Deep Water (WDW) from the Antarctic subpolar
gyres with cold, saline shelf water [Gill, 1973; Carmack
and Foster, 1975; Foster and Carmack, 1976] and is part of
the continuum that comprises Antarctic Bottom Water.
[6] Orsi et al. [1995] found the properties of some of
these water masses to be useful in determining the location
of the SACCF. Specifically, they located stations north of
the SACCF as having: potential temperature greater than
1.8C along the potential temperature maximum at depths
greater than 500 m; salinity greater than 34.73 along the
salinity maximum at depths greater than 800 m; and
dissolved oxygen concentrations less than 4.2 mL l 1 along
the oxygen minimum at depths greater than 500 m. Stations
south of the SACCF were identified as having potential
temperature less than 0C along the potential temperature
minimum at depths shallower than 150 m. They stated that
each of these generalized criteria might not apply in a given
region. An example of this is given by Thorpe [2001], who
noted that on the World Ocean Circulation Experiment
section A23 close to South Georgia, the progression of
temperature at the subsurface temperature minimum was
actually reversed, a consequence of local mixed layer
processes.
[7] Approximate mean positions of the ACC fronts across
the Scotia Sea (derived largely from historical hydrographic
data, though supplemented with some more recent measurements) are marked in Figure 1. The SACCF is believed to
be especially important in dictating oceanographic conditions at South Georgia since it loops anticyclonically around
the island from the south before retroflecting to the east, and
thus remains in close proximity to an extended region of the
slope. Trathan et al. [1997, 2000] examined hydrographic
data from surveys conducted to the northeast of South
Georgia, and observed some apparent variability in the
location of the SACCF. They argued that a more easterly
location of the SACCF was associated with warmer waters
close to South Georgia. Thorpe et al. [2002] used a highresolution map of historical geopotential anomaly to identify the mean position of the SACCF retroflection north of
South Georgia: this was found to be close to 36W,
approximately 400 km further east than it was depicted in
the earlier climatological map of Orsi et al. [1995]. Thorpe
et al. [2002] also presented evidence for temporal variability
associated with the SACCF in the South Georgia region,
and used drifter data to demonstrate variability in the
western extent of the SACCF retroflection north of South
Georgia and for the presence of eddies in the region.
[8] There have been rather few calculations of the
SACCF flux in the Scotia Sea region. Orsi et al. [1995]
derived a value of 15 Sv from a historical section crossing
the central Scotia Sea, but this was solely for the baroclinic
component relative to 3000 m or the deepest common level.
The WOCE section A23 crossed the SACCF three times in
the vicinity of South Georgia, and baroclinic transports of
16.5 Sv, 13 Sv and 14 Sv were obtained for these crossings
[Thorpe et al., 2002].
[9] The pathways and fluxes of the SACCF have implications for the South Georgia ecosystem, since the local
31 - 3
population of Antarctic krill (Euphausia superba) is not
self-sustaining [Everson, 1984], and thus requires input
from other regions. Krill is the key food species for the
vast colonies of higher predators at South Georgia; variability in the reproductive success of these predators is
connected with fluctuations in the local abundance of krill
[Croxall et al., 1988, 1999]. It has been hypothesized that
krill are advected by the SACCF across the Scotia Sea from
their source regions near the Antarctic Peninsula and in the
southern Scotia Sea [e.g., Hofmann et al., 1998; Murphy et
al., 1998; Fach et al., 2002]. Direct observations of krill
flux have been significantly lacking, however; this is discussed in detail by Murphy et al. [2003].
[10] As with the other ACC fronts, the SACCF can have a
strong effect on the deep water masses in the Scotia Sea and
the Georgia Basin farther north [Naveira Garabato et al.,
2002; Meredith et al., 2001]. Being a full depth feature, it
can act as a hydrodynamic barrier for the northward spreading of water masses from southern sources, and may also act
to increase abyssal mixing rates. Interaction of the front
with the convoluted topography of the region produces a
number of different pathways for water masses entering the
region from Drake Passage to the west or the Weddell Sea to
the south [Arhan et al., 1999; Naveira Garabato et al.,
2002], with the different routes being discernible through
the different mixing histories of the water.
[11] In this paper, we present results from a fine-scale
hydrographic survey of the SACCF in the southern Georgia
Basin close to South Georgia. This consists of a stationbased section that profiles the full depth features of the
waters encountered, and a towed undulator section that
provides a characterization of the upper layers of the
SACCF at spatial scales unattainable using conventional
stations. Physical characteristics of the front and its impact
on the various water masses encountered are discussed, with
special emphasis on the role of the front (and its interaction
with topography) in dictating the spreading and mixing of
the waters. The potential for infrared remote sensing of the
SACCF close to South Georgia is demonstrated, and we use
this potential to trace the course of the front as it loops
anticyclonically around the island, interacts with the bathymetry, and retroflects to the east. Physical fluxes in both the
baroclinic and barotropic modes associated with the front
are presented.
2. Methods
[12] In situ data used in this paper were collected on RRS
James Clark Ross cruise 57, during which a transect
oriented northeast from South Georgia (Figure 2) was
conducted. An initial outward leg was performed during
daylight on 26 December 2000 using towed and underway
instrumentation (described below), followed by a return
occupation of the transect during which hydrographic
stations were conducted. The locations of these stations
are shown in Figure 2. At each station, data were collected
with a SeaBird 911plus conductivity-temperature-depth
(CTD) instrument. Station separation was approximately
10 km, close to the Rossby radius of deformation in this
region [Houry et al., 1987]. The deviation from a straight
line transect at station 81 was a diversion to avoid an
iceberg. Discrete water samples were obtained from a Sea-
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MEREDITH ET AL.: WATER MASSES AND CIRCULATION NEAR SOUTH GEORGIA
Figure 2. Locations of hydrographic stations used in this study (RRS James Clark Ross cruise 57, 26–
31 December 2000). The section runs from just in front of Cumberland Bay on the South Georgia shelf
out to the deep Georgia Basin. Station spacing is approximately 10 km. The small diversion from a
straight line transect at station 81 was to avoid an iceberg. Bottom topography is from Smith and
Sandwell [1997].
Bird 12-position carousel water sampler carrying 10 L
Niskin bottles. These were analyzed for salinity on a
Guildline 8400B salinometer using ampoules of standard
seawater (batches P133 and P137) prepared by Ocean
Scientific International Ltd. CTD salinities were calibrated
using these discrete measurements, and CTD downcast
profiles were averaged on 2 dbar intervals for analysis.
On the initial outward transect, a Chelsea Instruments
NvShuttle undulating oceanographic recorder (UOR) was
employed. This cycled between around 5 and 140 m,
completing a full undulation approximately every 2 km;
temperature, pressure and conductivity were among the
suite of parameters logged. The UOR data was calibrated
using collocated CTD casts. Since these were not conducted
simultaneously (the UOR transect was conducted prior to
the CTD section), there is a potential source of error here:
the estimated accuracy of the UOR salinity is 0.007. The
UOR transect does not cover as much of the South Georgia
shelf region as the CTD section because of initial problems
with the undulator data logging.
[13] Continuous measurements of water velocity in the
upper part of the water column were made using a 153.6
kHz RD Instruments vessel-mounted Acoustic Doppler
Current Profiler (VM-ADCP), sited in a sea chest to afford
protection from ice. The ADCP was configured to record
data in 64 8 m bins, and data were captured in ensembles
of 2 minute duration. The ‘‘blank beyond transmit’’ was
set to 4 m; coupled to the transducer depth (6 m), this
gives the centre of the first bin depth at 14 m. Absolute
velocities were obtained by using data from the Global
Positioning System obtained with an Ashtech ADU-2 to
correct for drift and error in the ship’s gyrocompass, with
bottom track data being used to correct for misalignment
of the Ashtech antenna array relative to the ADCP transducers and for the inherent scaling factor associated with
ADCP velocities.
[14] Satellite imagery was obtained from the AVHRR
flown on the National Oceanic and Atmospheric Administration series of weather satellites. Images were captured at
the British Antarctic Survey (BAS) receiving station at
Rothera, with sea surface temperature (SST) being obtained
using the split window retrieval algorithm on channel 4 (11
mm) and channel 5 (12 mm) following Llewellyn-Jones et al.
[1984]. Images were contrast stretched to improve the
apparent thermal resolution of the ocean surface for the
temperature range of interest, and false colored for clarity.
3. Results
3.1. Identification of the SACCF
[15] Full depth vertical fields of potential temperature (q),
salinity (S), neutral density (gn) [Jackett and McDougall,
1997] and baroclinic velocity are shown in Figure 3. These
sections show a doming of isopleths centered near station
88. Clearly there is a recirculation, with perpendicular flow
oriented to the northwest of the section between stations 91
MEREDITH ET AL.: WATER MASSES AND CIRCULATION NEAR SOUTH GEORGIA
31 - 5
Figure 3. CTD-derived vertical fields of (a) potential temperature, (b) salinity, (c) neutral density
[Jackett and McDougall, 1997], and (d) baroclinic velocity referenced to the deepest common level
between adjacent stations. CTD station positions are marked in Figure 2 and indicated along the upper
horizontal axis in this figure. The upper 500 m are plotted on an exaggerated vertical scale.
and 101, and perpendicular flow to the southeast of the
section between stations 72 and 85. This is the signature of
a loop in the SACCF, and is particularly pronounced in the
bidirectional structure of the baroclinic velocity field refer-
enced to the deepest common level (Figure 3d). For the
purposes of this paper, we term these manifestations of the
SACCF on the section crossing 1 (closest to South Georgia)
and crossing 2 (farthest from South Georgia). Crossing 1 is
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MEREDITH ET AL.: WATER MASSES AND CIRCULATION NEAR SOUTH GEORGIA
Figure 3. (continued)
a narrow feature, being around 30 km total width. Crossing
2 is somewhat broader, with lower geostrophic velocities. It
is important to note that the station pair farthest to the
northeast on the section (stations 69 and 72) is consistent
with flow in the reverse direction to that associated with the
adjacent crossing 2. This seems to identify station 72 as
being the centre of a second, less pronounced recirculation,
though with only one station farther out from South Georgia
than this, it is difficult to be more expansive from the
hydrographic data alone.
[16] On traversing crossing 1, the temperature at the core
of the WW changes from 0.32 to 0.41C, the salinity at the
core of the LCDW changes from 34.705 to 34.708, and the
potential temperature at the core of the UCDW changes from
1.85 to 2.08C. For traversing crossing 2, the temperature at
the core of the WW changes from 0.04 to 0.44C, the salinity
MEREDITH ET AL.: WATER MASSES AND CIRCULATION NEAR SOUTH GEORGIA
31 - 7
Figure 4. (a) Potential temperature versus salinity for stations marked in Figure 2. Stations are grouped
according to their near-surface properties (69 – 75 dotted; 78– 97 solid; 101– 119 dashed). Contours of
neutral density (gn) [Jackett and McDougall, 1997] are superposed. (b). Expanded scale version.
at the core of the LCDW changes from 34.704 to 34.722, and
the potential temperature at the core of the UCDW changes
from 1.91 to 1.98C. It is apparent that, in the locality under
study, the large-scale criteria presented by Orsi et al. [1995]
do not provide reliable indicators for the position of the
SACCF. Orsi et al. [1995] state specifically that their criteria
are intended solely as useful indices, and that regional
variability in the circumpolar property fields [e.g., Gordon
and Molinelli, 1982] implies that one or more of them need
not apply at any given location. Indeed, it has been shown at
other locations that the fronts change along-stream as they
split and merge and are subject to seasonal cycles in heat and
freshwater flux [e.g., Belkin and Gordon, 1996]. In addition
to this, we note that strong isopycnal mixing is observed on
our section (see below); this will change the property values
at their core extrema, and thus further reduce the local
applicability of large-scale criteria based on them.
3.2. Impact of the SACCF on Water Mass Properties
3.2.1. Surface Waters
[17] Figure 4 shows the characteristics of the water masses
encountered on the section in potential temperature/salinity
space. On our section, the WW properties clearly fall into two
categories. Stations 78– 97 (between the two crossings of the
SACCF) exhibit water at the temperature minimum that is
colder than 0.3C, with a neutral density higher than 27.55
(s0 > 27.35). All other stations on the section have much
warmer potential temperature minima (q > 0.4C) that are
less dense (gn < 27.55; s0 < 27.35). The potential temperature minima of stations 78 –97 are, in general, also margin-
31 - 8
MEREDITH ET AL.: WATER MASSES AND CIRCULATION NEAR SOUTH GEORGIA
ally more saline. This is clearly the influence of the SACCF,
with the WW properties between crossing 1 and crossing 2
being characteristic of waters of more southerly origin.
[18] The coldest WW on the section (q < 0.5C) is
present at station 88, the centre of the SACCF loop. Cold
WW is also found at the adjacent station 91 (q < 0.2C),
and also at station 78 (q < 0C). This latter station also
features a lens of cold water at around 300 m intruding into
the deeper water below. The temperature minimum layer
extends onto the South Georgia shelf (Figure 4); here it is
capped by waters that are even warmer than northeast of
crossing 2, and also become progressively fresher with
decreasing distance from land. These are the effects of
water mass retention on the shelf coupled to freshwater
discharge and warming through insolation [Brandon et al.,
1999; Meredith et al., submitted manuscript, 2002].
[19] The fine-scale (minimum order of 2 km) spatial
characteristics of the two crossings of the SACCF can be
seen in the vertical sections derived from the UOR covering
the upper 150 m of the water column (Figure 5). In addition
to the same broader-scale (minimum order of 10 km)
structure of the SACCF observed in the CTD section, there
is evidence for large-amplitude (approximately 50 m) undulations in properties beneath the seasonal halocline, the base
of which is around 75 m depth between the two SACCF
crossings. These undulations typically occur over horizontal
scales of 5 – 10 km. The undulations of salinity and density
are strongly correlated, as is expected for waters at low
temperature.
[20] Initial concepts of the SACCF were that it did not
separate surface water masses, and hence should not have a
surface signature [Orsi et al., 1995]. In some regions,
evidence has been presented to support this [e.g., Heywood
et al., 1999], although some fine-scale surveys do seem to
show a surface expression of the SACCF [e.g., Holliday
and Read, 1998]. Figure 6 shows the CTD-derived temperature at the 1 dbar level across our transect; clearly there are
large horizontal gradients between stations 91 and 107
(approximately 0.03C/km) and stations 72 and 78
(approximately 0.05C/km). These coincide spatially with
the locations of crossing 1 and crossing 2 of the SACCF,
and the sense of the temperature gradient (negative closest
to South Georgia, positive farthest) is in accord with
the different directions of flow in these two crossings. The
presence of a surface temperature gradient across the
SACCF close to South Georgia is more than just a curiosity,
since it will have consequences concerning (for example)
the solubility of biologically and climatically important
gases, and the behavior of temperature-dependent ecosystems functions. It also offers the potential for tracing the
course of the SACCF using satellite-borne infrared radiometers, as will be demonstrated below.
3.2.2. Deep and Bottom Waters
[21] UCDW typically inhabits the approximate ranges 1.6
< q < 3.2C, 34.00 < S< 34.71, 27.55 < gn < 28.00 [Naveira
Garabato et al., 2002]. Previous definitions of this water
mass based on potential density placed it in the range s0 >
27.35, s2 < 37.00 [Reid et al., 1977; Sievers and Nowlin,
1984]. LCDW generally occupies the neutral density range
28.00 < gn < 28.26 [Naveira Garabato et al., 2002],
equivalent to s2 > 37.00, s4 < 46.04 [Sievers and Nowlin,
1984]. In addition to these, WDW also has an influence on
the properties of the waters observed. WDW is derived from
the LCDW of the ACC that becomes incorporated into the
eastern Weddell Gyre through a broad discontinuity in the
southwest Indian Ridge [Orsi et al., 1993], and circulates
cyclonically within it before exiting over or around the
South Scotia Ridge. Whilst having similar density characteristics to LCDW, WDW can be distinguished through the
effects of its different mixing history: it has lower potential
temperature (0.2 < q < 0.6C) and salinity (less than 34.69 at
the maximum).
[22] On our transect, the potential temperature maximum
of the UCDW was strongest between station 97 and the
slope, where it exceeded 2C (Figure 3), and northeast of
station 78, where it again exceeded 2C. Between stations
81 and 94, the potential temperature maximum of the
UCDW was much lower, at just above 1.8C (again, the
influence of the SACCF). Very strong isoneutral interleaving is apparent for the density range of UCDW (Figure 4a).
This is dominant for stations 78– 97 (the same stations that
featured the cooler type of WW), and much weaker for the
other stations. The potential temperature/salinity curves of
stations 78– 97 are consistent with waters from equatorward
of the SACCF in the ACC zonation mixing with cooler,
fresher waters of a more southerly origin that spread north
toward South Georgia within and south of the SACCF.
[23] None of the deep waters sampled along our transect
in the density range 28.00 < gn < 28.26 fully meet the
criteria for WDW, and thus are more properly termed
LCDW. However, as with the UCDW, there is evidence
of profound isoneutral interleaving of the LCDW sampled
with cooler, fresher waters (Figure 4b); that is, the waters
observed are the product of significant mixing between
LCDW and WDW. This is the signature of mixing upstream
with WDW. That the interleaving is so intense is probably
related to the SACCF and its retroflection: the large property gradients, coupled with the strong vertical and horizontal shears, will act to enhance the mixing.
[24] WSDW typically has characteristics around 0.7 < q
< 0.2C, S < 34.703, and 28.26 < gn < 28.40 or 46.04 < s4 <
46.16 [Arhan et al., 1999; Naveira Garabato et al., 2002]. It
circulates cyclonically in the Weddell Gyre, and can escape
through gaps in the South Scotia Ridge to flow through the
Scotia Sea prior to exiting through Georgia Passage [Locarnini et al., 1993]. WSDW can also spread northward on the
eastern side of the South Sandwich Arc. Once having
traversed the Scotia Sea or circulated around the South
Sandwich Arc, two routes are possible for WSDW to reach
the sampling location, namely anticyclonically around the
Northeast Georgia Rise (NEGR; Figure 1), or through the
gap between the NEGR and the South Georgia shelf break.
Naveira Garabato et al. [2002] showed that water denser
than around gn = 28.29 kg/m3 lies too deep in the water
column to flow along this latter route.
[25] Striking in the potential temperature, salinity and
density fields (Figure 3) is a remarkably intense abyssal
layer, around 100 m thick, with very large gradients in
properties (stations 78 – 94). This layer exists in waters
around 3500 m deep, and possesses vertical gradients
similar to those found in the near-surface thermocline.
The most likely explanation is that vertical stacking of
different types of WSDW is occurring in the Georgia Basin,
with different layers having taken different routes from the
MEREDITH ET AL.: WATER MASSES AND CIRCULATION NEAR SOUTH GEORGIA
31 - 9
Figure 5. Upper ocean vertical fields of (a) temperature, (b) salinity, and (c) density anomaly from the
undulating oceanographic recorder. The line surveyed is the same as in Figure 2 but does not extend as far
onto the South Georgia shelf as did the CTD section because of instrumental difficulties.
Weddell Sea. Densities within the very bottom layer are
greater than gn = 28.29 kg/m3, thus this abyssal layer will
have reached the sampling location by circulating anticyclonically around the NEGR [Naveira Garabato et al.,
2002]. Conversely, the layer immediately above this can
enter the basin through the gap between the NEGR and the
South Georgia slope. The vertical juxtaposition of these
different types of WSDW within the Georgia Basin locally
creates the strong abyssal gradients in properties.
[26] Within this dense abyssal layer, two separate types of
WSDW exist: stations 85 and 88 show WSDW that is more
saline (by around 0.002 – 0.003) than WSDW elsewhere
along the transect (Figure 4b). These two stations are
situated between the two crossings of the SACCF. This
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MEREDITH ET AL.: WATER MASSES AND CIRCULATION NEAR SOUTH GEORGIA
Figure 6. CTD temperature at 1 dbar across the transect
(solid line) and corresponding SST from the cloud-free
section of the AVHRR image shown in Figure 7a (dashed
line). The temperature traces agree to within the radiometric
resolution of AVHRR (0.6C). Station locations are
marked above the upper axis, also in Figure 2. Note the
distinct temperature gradients associated with the two
crossings of the SACCF.
small difference in salinity is indicative of different pathways followed by the waters from their source regions in the
south. One branch of WSDW crosses the South Scotia
Ridge from the Weddell Sea to enter the Scotia Sea; this
water subsequently exits the Scotia Sea through Georgia
Passage [Locarnini et al., 1993]. A second branch of
WSDW flows northward on the eastern side of the South
Sandwich Arc, around 26W (Figure 1). The former type of
WSDW becomes freshened by interaction with dense
waters that spill off the shelf near the tip of the Antarctic
Peninsula [Whitworth et al., 1994], while the latter branch is
not subject to this freshening. Arhan et al. [1999] and
Meredith et al. [2001] used the resulting small difference
in salinity to identify different pathways from the Weddell
Sea into the Georgia Basin and Scotia Sea respectively. For
the present section, the WSDW in the dense abyssal layer at
stations other than 85 and 88 has clearly crossed the South
Scotia Ridge, flowed through the Scotia Sea, exited via
Georgia Passage and reached the location of the transect by
circulating anticyclonically around the NEGR. Conversely,
WSDW at stations 85 and 88 has flowed around the South
Sandwich Arc and anticyclonically around the NEGR to
reach the location of the transect. Thus the difference in
WSDW salinity witnessed along the section is evidence of
the SACCF interacting with the complex bathymetry of the
region to control the spreading pathways of the abyssal
waters.
3.3. Spatial Characteristics of the SACCF Close
to South Georgia
[27] We have seen that the SACCF exhibits a temperature
gradient close to South Georgia such that surface temperatures change by around 1.5C over spatial scales around 50
km (Figure 6). These values compare to typical radiometric
and spatial resolution of surface temperature imagery from
AVHRR of around 0.6C and just over 1 km respectively.
This clearly offers a capability for tracing the course of the
SACCF using satellite data, subject to the availability of
cloud-free satellite passes.
[28] One such image (partially cloud contaminated) is
shown in Figure 7a. The data were recorded on 26 December 2000 at 20:06 GMT, thus it is an image from the
evening before our hydrographic transect commenced.
The surface structure in the southern and western areas of
the image is obscured by high cloud (white). The image is
somewhat confused by the presence of low cloud, which
has temperatures comparable to those of the sea surface.
(This low cloud was identified using different contrast
stretching and coloring to those used for Figure 8). The
northwest limit of the low cloud has been approximately
marked on the image with a black line; red pixels to the
south and east of this line should not be interpreted as
representing areas of warm sea surface. The formation of
this low cloud is clearly associated with the mountainous
topography of South Georgia, since it starts at the island as
cooler, higher cloud (blue pixels), and warms as it sinks
during its spreading to the east. If such cloud formation and
eastward spreading were a common occurrence at South
Georgia it could have significant consequences for the local
ecosystem, since levels of incident solar radiation at the
ocean surface would tend to be higher on the western side of
the island than the east.
[29] Despite the partial cloud contamination, broad areas
of ocean surface are visible. These show the frequent
mesoscale meanders and eddies characteristic of the baroclinically unstable ACC fronts. The PF can be distinguished
beneath the patchy cloud cover at the northwest side of the
image, looping around the southern side of the Maurice
Ewing Bank [Moore et al., 1997; Arhan et al., 2002].
Marked on the image is the location of the hydrographic
transect (red line), and the positions and directions of the
SACCF crossings derived from the hydrographic data.
Crossing 1 is partially obscured by low cloud, so it is
difficult to make assertions concerning the spatial structure
of the SACCF here. Crossing 2 is cloud free, and it is
immediately obvious that it represents the southwestern
edge of a detached warm-core ring of SACCF water situated
at approximately 34.5W, 53.2S. This is consistent with the
observation from the hydrographic section of reverse direction flow for the station pair at the very northeastern end of
the transect (stations 69 and 72); clearly this station pair was
situated to the northeast of the centre of this warm-core ring.
We note that the surface waters in Figure 8 just north of the
SACCF (around 52S, 35W) are similar in temperature to
the waters at the PF close to the Maurice Ewing Bank. This
would appear to be an indication that the PF and SACCF
come very close at the point, and raises the possibility that
they occasionally merge.
[30] The warm-core ring with which crossing 2 is associated can be traced back in the AVHRR image to the
eastward flowing retroflected manifestation of the SACCF.
Modelling studies tracing particles in the output of Ocean
General Circulation Models (OGCMs) have indicated that
while flow in the northwest flowing SACCF and its retroflection is relatively rapid, water is often transferred into the
MEREDITH ET AL.: WATER MASSES AND CIRCULATION NEAR SOUTH GEORGIA
31 - 11
Figure 7. (a) SST image for 26 December 2000 (20:06 GMT) from AVHRR. Much of the ocean surface
to the south and west of the image is obscured by cloud (white). Low (warm) cloud also obscures the
ocean surface immediately to the north of South Georgia and east of this region; the northwestern limit of
this cloud is marked approximately (black line). The hydrographic transect discussed in this paper is
marked (red line), as are the two crossings of the SACCF (black arrows). Note that the crossing of the
SACCF farthest from South Georgia is associated with an isolated ring that has spun off the SACCF
retroflection. (b) Progression of the positions of the SACCF as predicted by the OCCAM general
circulation model. Note the detached warm-core ring in the same location as observed in Figure 7a. [from
Thorpe et al., 2002].
region between them from the retroflected SACCF in the
form of rings (Figure 7b) [adapted from Thorpe et al., 2002].
Our observation of a warm-core ring shed to the south of the
eastward flowing SACCF retroflection seems to verify this
OGCM-predicted process. The modeled rings typically
feature high velocities, but tend to remain roughly stationary
for extended periods, and are thus relatively ineffective at
transferring material into or out of the vicinity. The implication is that material contained in the warm-core ring may
have been present in the region significantly longer than
material present in crossing 1. This has profound conse-
quences for ecosystem operations and the interpretation of
biological measurements [Ward et al., 2002].
[31] A second example of the use of SST imagery in
tracing the course of the SACCF close to South Georgia is
shown in Figure 8. This is an AVHRR image from 12 July
2000, and thus is not coincident in time with the occupation
of the hydrographic transect. However, it is notable for
showing the course of the SACCF around South Georgia
with comparatively little interference from cloud cover.
(Such cloud contamination as exists around the edges of
the image is depicted with white pixels.) It is clear from the
31 - 12
MEREDITH ET AL.: WATER MASSES AND CIRCULATION NEAR SOUTH GEORGIA
Figure 8. SST image for 12 July 2000 (05:27 GMT) from AVHRR. Much of the ocean surface around
the edges of the image is obscured by cloud (white). The anticyclonic looping of the SACCF around
South Georgia is clear, as are mesoscale meanders in its path. Rings appear to be forming at the
retroflection region immediately north of South Georgia.
image that the SST signature of the SACCF broadly follows
the predicted course of the front, looping anticylonically
around the island from the south, and subsequently retroflecting to the east on the northern side of the island.
Noteworthy however is that at the time of image capture
the SACCF appears to first come close to the South Georgia
slope at around 40W, 55S, i.e., close to the southwestern
edge of the island’s shelf. This is contrary to the general
depiction in climatological maps of the SACCF, which tend
to have the front first approaching the slope much farther
east (typically around 34W, i.e., the southeastern edge of
the shelf; see Figure 1). There is a large approximately
meridional bathymetric ridge (around 2200 m depth) running along 39W in the Scotia Sea between around 55.5 and
58S (Figure 1); our SST image from July 2000 suggests
that (at the time of image capture) the SACCF was lying
north of this feature rather than south. The region of this
bathymetric feature is obscured by cloud in the AVHRR
image; further data are needed to gain more insight into this,
and into possible variability of the front’s position. In either
case, the proximity of the SACCF to the southern side of the
South Georgia shelf is potentially important for the local
ecosystem. It has been argued that the SACCF is a major
advective instrument in the transfer of krill across the Scotia
Sea from upstream near the Antarctic Peninsula [Hofmann
et al., 1998; Fach et al., 2002]. Regions where the front is
closest to the shelf may be areas where krill transport into
the shelf waters is facilitated. Although clearly influenced
by the topography of the South Georgia slope, the SACCF
also shows some large mesoscale meanders during its loop
around the island. Particularly prominent is the large south-
ward excursion of warm water at around 36W to the south
of South Georgia, and an eddy spinning off this excursion at
around 57S, 35W.
[32] To the north of the island, the SACCF appears to be
deflected away from South Georgia at around 37W. A very
similar deflection was observed previously at this location in
the track of an iceberg that had been apparently following the
path of the SACCF close to the bottom of the slope adjacent
to South Georgia [Trathan et al., 1997, Figure 12]. The
deflection of the iceberg was clearly associated with the
presence of the North Georgia Rise (NGR; an approximately
1500-m-high topographic feature centered on 37W, 52.5S).
This iceberg subsequently moved closer to South Georgia
having rounded the NGR, and followed a cyclonic course
around the periphery of the basin and north to the vicinity of
the PF. The presence of the deflection in the satellite image of
the SACCF again suggests the role of the NGR in steering the
flow at the north side of South Georgia. After its deflection to
the north, the SACCF appears to adopt a hammerhead eddy
configuration, and sheds a number of rings to the west. It then
retroflects to the east, and can be seen to meander again
greatly as it flows north.
3.4. Fluxes Associated With the SACCF
[33] Baroclinic volume transports associated with crossings 1 and 2 of the SACCF can be obtained by spatially
integrating the velocity field shown in Figure 3d. To account
for bottom triangles, we extrapolate the baroclinic velocity
profiles downward from the deepest common layer of each
CTD pair. The resultant fluxes perpendicular to the section
are shown in Figure 9a. The two crossings of the SACCF are
MEREDITH ET AL.: WATER MASSES AND CIRCULATION NEAR SOUTH GEORGIA
Figure 9. Vector plots. (a) Baroclinic flux (full depth) perpendicular to the transect. (b) Two-minute
ensemble VM-ADCP vectors from 22 m along the transect. The two crossings of the SACCF are clear.
The northeasternmost crossing is a part of an isolated ring; its circulation is apparent in the veering of the
vectors close to the northeast end of the transect. (c) Barotropic flux (full depth) perpendicular to the
transect. The vectors appear less spatially variable than the baroclinic flux vectors (Figure 9a) since more
than two adjacent stations were averaged in places to increase the accuracy of the barotropic velocity
derivation. (d) As for Figure 9a, but for the upper 250 m. (e) As for Figure 9b, but for the upper 250 m. (f)
Surface and near-surface ageostrophic flux perpendicular to the transect, with station numbers.
31 - 13
31 - 14
MEREDITH ET AL.: WATER MASSES AND CIRCULATION NEAR SOUTH GEORGIA
clearly visible, with the highest magnitudes of flux between
individual station pairs being 4.5 Sv (stations 94 and 97; 1
Sv = 106 m3/s), and 3.2 Sv (stations 75 and 78). At the
northeastern end of the section (stations 69 and 72), flow in
the reverse direction to the adjacent station pair is observed;
this is the manifestation in the velocity field of the SACCF
ring observed previously (e.g., Figure 7).
[34] In addition to the baroclinic component, we are able
to quantify the barotropic (depth-independent) component
of velocity across the section. To do this, we use direct
velocity measurements from the VM-ADCP. 2-minute
ensemble ADCP vectors from 22 m depth are shown in
Figure 9b, with the most obvious features again being the
two crossings of the SACCF. Note that the vectors associated with crossing 2 are at a more obtuse angle to the
section than those of crossing 1, and veer across to reverse
direction at the northeastern end of the section (the ring).
The angle of the vectors associated with crossing 2 indicates
that the section cut across the ring to the south of its core.
[35] To derive the barotropic component, we average the
ADCP current vectors between station pairs, resolving
perpendicular to the ship’s track to enable comparison with
baroclinic velocity. The tidal components of the VM-ADCP
velocities were subtracted using version 3.1 of the Oregon
State University global tidal model [Egbert et al., 1994].
Tidal velocities removed were always less than 2 cm/s for the
meridional component, and always less than 0.5 cm/s for the
zonal component. Below the ageostrophic layer (typically
around 40– 50 m depth), the baroclinic component (from the
CTD data; e.g., Figure 3d) and the total velocity (from the
averaged ADCP data) become close to parallel. The offset
between the different velocity profiles represents the barotropic component of velocity. For some of the station pairs
used here, the baroclinic and total velocity profiles were
closer to parallel when adjacent stations were also considered, i.e., using two or more sets of stations pairs in one
averaging procedure. In such circumstances, the barotropic
component is more reliably derived than by considering each
station pair separately, hence we adopt the averaged velocities and make the assumption that the average velocity
applies to each separate station pair. The uncertainty in the
offset (barotropic) velocity is estimated as ±2 cm/s.
[36] Figure 9c shows the full depth fluxes associated with
the barotropic component of the velocity field. The vectors
appear less spatially variable than the baroclinic vectors in
Figure 9a because of the averaging of more than two
adjacent stations. The barotropic flux vectors are generally
aligned in the same direction as the baroclinic flux vectors,
i.e., northwestward in crossing 1 and southeastward in
crossing 2. We interpret this as indicating that the SACCF
has a barotropic flux component that enhances the magnitude of the baroclinic component. The one location where
barotropic and baroclinic fluxes are notably oriented in
opposing directions is directly in between the two crossings
(stations 85 to 91), for which we have no clear explanation.
Barotropic fluxes at the slope are smaller than the fluxes
near the centre of the two SACCF crossings (Figure 9c).
This is a reflection of the shallower water depth rather than
lower barotropic velocity however, since velocities are
actually substantially higher at the slope (see below).
[37] Total geostrophic fluxes are obtained by summing
the barotropic and baroclinic components. For crossing 1,
the total geostrophic flux is 14.1 ± 2.4 Sv. Approximately
9.8 Sv of this are in the baroclinic mode, and 4.3 Sv in
the barotropic mode. For crossing 2, the total geostrophic
flux is 10.2 ± 3.6 Sv, with approximately 5.7 Sv in the
baroclinic mode and 4.5 Sv in the barotropic mode.
[38] Thorpe et al. [2002] deduced baroclinic fluxes of
13– 16.5 Sv for the SACCF near South Georgia from an
approximately meridional section undertaken in 1995. Our
baroclinic fluxes are close to this, with that of crossing 2
being marginally smaller. At the western end of the Scotia
Sea, Cunningham et al. [2001] used combined hydrography
and lowered ADCP measurements to deduce a transport for
the SACCF at Drake Passage of 9.3 ± 2.4 Sv, again broadly
consistent with our measurements. Whilst our measurements of transport across the section appear generally
consistent with most previous estimates of SACCF transport, it is interesting to note that the maximum velocities
associated with the SACCF in our measurements seem very
high. Maximum baroclinic velocities at crossing 1 were
above 35 cm/s; addition of the barotropic component raises
this velocity still further. By comparison, Thorpe et al.
[2002] describe maximum baroclinic velocities associated
with the SACCF close to South Georgia between 7.5 and 10
cm/s. Heywood et al. [1999] show no values higher than 15
cm/s for the region of the SACCF and SB near 85E in the
Southern Indian Ocean. Typical values for the surface
velocity of the SACCF at Drake Passage lie in the range
10– 20 cm/s [e.g., Whitworth et al., 1982; Roether et al.,
1993]. The consistency of our transport measurements with
previous estimates, combined with the extremely high
velocities we measured, is a consequence of crossing 1
being a very narrow, intense jet, even compared to other
crossings of the SACCF. It is possible that local topography
acts to constrict and accelerate the flow of the SACCF as it
loops anticyclonically around South Georgia. Given that the
SACCF is generally found in waters with depths approximately 2500 – 3000 m, it could well be that the gap between
the South Georgia shelf and the NEGR (Figure 2) is
responsible for this, with the SACCF effectively being
funneled through.
[39] Figures 9d and 9e show the baroclinic and barotropic
fluxes associated with the upper 250 m of the water column,
derived as above. The same general directions of flow are
present, with the barotropic and baroclinic parts being
generally coaligned for both crossing 1 and crossing 2. In
addition to these geostrophic components, we can quantify
also the surface ageostrophic flux caused predominantly by
direct wind forcing and inertial effects. For this, we compared the geostrophic (baroclinic plus barotropic) velocity
profiles averaged between stations with the directly measured total velocity from the VM-ADCP. For each comparison, the part of the water column closest to the surface
(generally shallower than 40 to 50 m depth) showed
significant deviation of total velocity from geostrophic
velocity. In each case, this deviation was oriented to the
northwest of the transect. We subtracted the geostrophic
component from the total velocity in these layers, and
integrated the residual velocity to obtain ageostrophic
volume fluxes perpendicular to the transect (Figure 9f).
The fluxes are very much smaller than the geostrophic
fluxes, thus any error in them will have minimal impact
on the total flux calculated. The ageostrophic flux being
MEREDITH ET AL.: WATER MASSES AND CIRCULATION NEAR SOUTH GEORGIA
oriented to the northwest along the transect is consistent
with the general wind forcing of the region; predominantly
westerly winds will cause a northward Ekman transport.
[40] For the upper 250 m, the total volume transport
associated with crossing 1 is 2.9 ± 0.3 Sv; around 2.0
Sv of this is in the baroclinic mode, 0.8 Sv in the
barotropic mode, and 0.1 Sv due to ageostrophic components. For crossing 2, the total volume transport is 1.2 ± 0.3
Sv; around 1.1 Sv of this is in the baroclinic mode, 0.3 Sv in
the barotropic mode, and 0.2 Sv is due to ageostrophic
components. Of particular note is the large barotropic flux
between stations 101 and 104 (over the South Georgia
slope) that is closer to South Georgia than the highest
baroclinic velocities (Figure 3d). This is suggestive (but
by no means proof) of a topographically steered barotropic
mode at the South Georgia slope; further measurements are
needed to better examine this possibility. The impact of
these upper layer fluxes on the horizontal advection of krill
is examined by Murphy et al. [2003].
4. Summary
[41] We have observed several important effects associated with the SACCF in the southern Georgia Basin. The
impact of the front on the deep waters is profound: it
interacts with topography to control the northward spreading of abyssal waters, with WSDW between the two crossings of the SACCF having been taken along a different
transport path from WSDW outside the SACCF. The
WSDW in the Georgia Basin flows north to become a
component of the Antarctic Bottom Water of the global
thermohaline circulation, thus restriction and steering of the
flow in the Georgia Basin and farther upstream has potential
consequences on a much broader scale than simply local
circulation. The WSDW in the Georgia Basin also exhibits
extremely large vertical gradients of properties in the
abyssal layer, most likely because of different routes taken
by waters of different densities around the North East
Georgia Rise. The SACCF seems to be a region of enhanced
deep mixing, with very large isoneutral intrusions at the
density ranges of CDW.
[42] In the upper ocean beneath the seasonal halocline,
we have observed fine-scale (5 – 10 km) undulations of
properties across the SACCF. Such ‘‘streakiness’’ has been
observed previously across ACC fronts [e.g., Pollard et al.,
1995], and has been attributed to differential advection due
to the large velocities of the front. Water mass properties are
transferred along the axis of the front more rapidly than
either side; this can act to sharpen cross-frontal gradients.
Pollard et al. [1995] argued that such differential advection
can lead to phytoplankton patchiness, and it is well-known
that mesoscale upwelling at fronts can also lead to chlorophyll patchiness on the scale of the local internal Rossby
radius [Strass, 1992].
[43] We have shown that, at the time of our section, a
surface temperature signal was associated with the SACCF
close to South Georgia. We have used this surface temperature signal in satellite imagery to identify an isolated ring of
SACCF water at the end of the transect (half of which
coincided with crossing 2). Such rings were predicted by
OGCMs to form in this region; this is good observational
verification of this process. We also used satellite imagery to
31 - 15
trace the course of the SACCF right around South Georgia.
We note that a surface temperature signal associated with the
SACCF close to South Georgia is also apparently present in
the monthly composite images produced during the ongoing
global survey of SST fronts conducted at the University of
Rhode Island (I. M. Belkin, et al., Global survey of ocean
fronts from Pathfinder SST data: 1. Atlantic Ocean, manuscript in preparation, 2003). This suggests that the presence
of the SST gradient is not specific solely to the times of the
data presented in this paper, but a more persistent feature,
and offers scope for future remote sensing investigations of
the SACCF close to South Georgia.
[44] We were able to detect some important features of
the SACCF in the region, such as its close proximity to the
southern slope of South Georgia, and its apparent northward
deflection by the North Georgia Rise. Whether these features are persistent or relevant solely to the time of image
capture requires further investigation. One of the images
presented shows the SACCF appearing to shed rings to the
west at the point of its retroflection. These rings seem to
move south from the point of their generation, and hence
they may be important in transferring biological material to
the northwestern end of the island. Such circulations often
feature enhanced local upwelling; this would tend to raise
nutrient levels in the near-surface layers, thereby promoting
primary production. The region around Bird Island, at the
western tip of South Georgia, is one of the most biologically
active areas of the entire Southern Ocean, with large
colonies of predators that depend on krill as a food source.
It has been argued that upwellings induced by mesoscale
circulations may be critical in determining the characteristics of Southern Ocean ecosystems [e.g., Barth et al.,
2001], thus the presumed persistence of the SACCF retroflection, and the possible frequency of the ring shedding,
may be crucial. If so, the western extent of the retroflection,
and the ring generation and translation processes, deserve
further attention. Satellite infrared remote sensing offers the
capability to study this, subject to the availability of cloudfree imagery.
[45] Acknowledgments. We thank the captain, crew and scientific
party of RRS James Clark Ross cruise 57 for their support of this work. We
are grateful to Steve Colwell, Jose Xavier and Steve Hart for assistance with
the satellite data. We are especially grateful to Igor Belkin for valuable
advice concerning the SACCF, and specifically its surface temperature
signal close to South Georgia. Our thanks to the two reviewers whose
comments greatly improved the original manuscript.
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M. P. Meredith, Proudman Oceanographic Laboratory, Bidston Observatory, Prenton, Wirral CH43 7RA, UK. ([email protected])