GEOPHYSICAL RESEARCH LETTERS, VOL. 40, 6186–6190, doi:10.1002/2013GL058125, 2013
Modified shelf water on the continental slope north
of Mac Robertson Land, East Antarctica
Annie P. S. Wong1 and Stephen C. Riser1
Received 25 September 2013; revised 25 November 2013; accepted 27 November 2013; published 9 December 2013.
[1] We report on under-ice profiling float observations of
cold, dense, and oxygenated bottom layers on the continental
slope of Mac Robertson Land (60°–72°E) in East Antarctica.
This bottom layer water mass, with potential temperature in
the range 1.8°C < θ < 0.4°C, is identified as modified
shelf water. It is a downslope variety of dense water formed
on the Antarctic continental shelf in winter and plays an
important role in ventilating the deep Southern Ocean. The
seasonal evolution of its thickness and density follows the
sea ice cycle of growth and decay, reaching a maximum in
October–November. The characteristics and location of this
modified shelf water are similar to Cape Darnley Bottom
Water, thus suggesting the same primary source in the Cape
Darnley polynya region. These float data support recent
results that the continental shelf along Mac Robertson Land
is a significant source of dense waters in East Antarctica.
Citation: Wong, A. P. S., and S. C. Riser (2013), Modified shelf water
on the continental slope north of Mac Robertson Land, East Antarctica,
Geophys. Res. Lett., 40, 6186–6190, doi:10.1002/2013GL058125.
1. Introduction
[2] Dense waters on the continental shelf around Antarctica
are formed in winter as a result of brine rejection and deep convection during active sea ice production. Their production is
enhanced in areas where the presence of coastal polynyas
leads to increased sea ice formation. These dense waters have
near-freezing temperatures and higher oxygen contents than
surrounding waters and persist year-round as a bottom layer
in major depressions on the Antarctic continental shelf. With
suitable bathymetry and current transport, they can flow off
the shelf, mix with the surrounding modified Circumpolar
Deep Water (mCDW), and sink along the continental slope
as modified shelf water (MSW). Thus, dense shelf waters
around Antarctica are an important component of the thermohaline circulation in the Southern Ocean, as their export and
sinking along the continental slope contribute to the formation
of Antarctic Bottom Water (AABW).
[3] AABW is the water mass that occupies the bottom layer
of the Antarctic Circumpolar Current and plays an important
1
School of Oceanography, University of Washington, Seattle, Washington,
USA.
Corresponding author: A. P. S. Wong, School of Oceanography, University
of Washington, Campus Box 355351, Seattle, WA 98195, USA.
([email protected])
©2013 The Authors. Geophysical Research Letters published by Wiley on
behalf of the American Geophysical Union.
This is an open access article under the terms of the Creative Commons
Attribution-NonCommercial-NoDerivs License, which permits use and
distribution in any medium, provided the original work is properly cited,
the use is non-commercial and no modifications or adaptations are made.
0094-8276/13/10.1002/2013GL058125
role in ventilating the abyssal depths of the world ocean.
Three primary source regions of AABW have been established
around Antarctica: the Weddell Sea [Foster and Carmack, 1976;
Fahrbach et al., 2001], the Ross Sea [Jacobs et al., 1970;
Whitworth and Orsi, 2006], and the Adélie Land coast [Rintoul,
1998; Williams et al., 2010]. More recently, Ohshima et al.
[2013] identified the Cape Darnley polynya region as a fourth
source of AABW. Cape Darnley is situated at the eastern end
of Mac Robertson Land, which is a portion of East Antarctica
between approximately 60° and 72°E. The Cape Darnley
polynya is located at approximately 65°–69°E and covers an
offshore area > 104 km2. It was estimated that this newly identified bottom water accounted for 6–13% of the circumpolar
total of AABW.
[4] Year-round observations of dense waters on the continental shelf and slope around Antarctica are sparse because
the oceans at those high latitudes are covered by sea ice for
most of the year. Ohshima et al. [2013] documented winter
measurements collected by seal-mount instruments that showed
high-salinity waters over the continental shelf of Cape Darnley
and overflowing dense waters on the continental slope. In this
paper we report on observations made by under-ice profiling
floats during 2010 and 2011 that show the presence and characteristics of MSW on the continental slope north of Mac Robertson
Land (Figure 1). These float-based observations show that local
production of dense shelf waters is a perennial event and that
their export onto the continental slope north of Mac Robertson
Land extends across a wide region. More importantly, this
independent confirmation of MSW supports the results from
Ohshima et al. [2013] that the Cape Darnley area is a significant
dense water production region in East Antarctica.
2. Data
[5] Observations of MSW along the Mac Robertson Land
coast were made by four profiling floats equipped with the
ice-avoidance algorithm discussed in Wong and Riser
[2011]. These four profiling floats were part of a larger group
of floats deployed in the East Antarctic sea ice zone around
117–128°E and 64–65°S during September–October 2007
[Williams et al., 2011]. Most of the floats from this deployment moved westward with the Antarctic Slope Current
(ASC) but became inactive before reaching 70°E. Only four
floats remained active after they migrated west of 70°E and
subsequently recorded the presence of MSW north of Mac
Robertson Land in 2010 and 2011 (Table 1).
[6] The combination of the ice-avoidance algorithm and
the use of the Iridium communication system for data telemetry allowed these floats to collect and store conductivitytemperature-depth (CTD) data beneath sea ice in winter and
transmit the accumulated data when the sea surface is free
of ice in summer. CTD data collected by this group of floats
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Figure 1. Mac Robertson Land coast. The surrounding ocean is under an ice cover for 8 months of the year. Colored dots mark
the known positions of the four floats during the ice-free months. Dashed lines with same colors mark the estimated trajectories
when the floats were sequestered from the sea surface. The boxed region is the approximate area where MSW was observed by
the floats. M1, M2, M3, and M4 mark the mooring locations from Ohshima et al. [2013]. Schematic flow of the ASC is also shown.
were 2-dbar bin-averaged and were collected during ascent
from 2000 dbar every 7 days. Between each CTD profile the
floats parked and moved with the currents at 1000 dbar. The
four floats described in Table 1 remained active for more
than 4 years, which included five austral winters. Their CTD
data showed no sensor drift over time and were estimated to
have an accuracy of 0.01 for salinity, 0.002°C for temperature,
and 2.4 dbar for pressure. These float data accuracies are
consistent with results reported in Riser et al. [2008].
[7] Two of the four floats (World Meteorological Organization
buoy identification number (WMO ID) 2900123 and 2900126)
were equipped with Aanderaa 3830 Optode sensors that
collected dissolved oxygen samples at discrete levels during
ascent at the same time as the CTD profiles. The oxygen
sampling levels were every 50 dbar from 2000 to 400 dbar,
every 20 dbar from 400 to 360 dbar, then every 10 dbar from
360 dbar to the surface. The dissolved oxygen data were
postprocessed by using a method similar to Takeshita et al.
[2013], giving an accuracy of 10 μmol kg 1 for the adjusted
values, which is sufficient for illustrating the high-oxygen characteristic of MSW.
3. Observations
[8] MSW over the Antarctic continental slope is identifiable as a sharp turn toward cold, fresh values at the bottom
of vertical CTD profiles. This bottom feature is ubiquitous
in float data from the vicinity of the Mac Robertson Land
coast. We focus our description on the most prominent variety
of MSW from this area: the bottom layers that show potential
temperature values lower than 0.4°C. Note that this is not
used as a temperature boundary to identify the top of the
bottom layers, but rather, it is used to identify CTD profiles
whose bottom values are significantly colder than surrounding
waters. As such, the choice of 0.4°C was determined by
examining all float data from the East Antarctic region.
[9] MSW in the range 1.8°C < θ < 0.4°C was found in
data from 41 float profiles between April 2010 and
November 2011 (Table 1). Almost all of these profiles were
collected when the ocean was under an ice cover and the
floats were unable to surface to obtain position fixes from
satellites. Positions for these under-ice profiles are not known
accurately but can be estimated by interpolating between
float positions that have satellite fixes from the ice-free
months. Linearly interpolating between known positions is
a simple way to estimate the mean direction of flow, but it
does not account for deviation of the float trajectories from
the mean caused by eddy variability and topographic
steering. A previous study by Wong and Riser [2011] indicated that along the East Antarctic coast, linearly interpolated float positions could differ from reported positions
by an average of 20 km. Thus, by using interpolated positions, we estimated that MSW was observed in the region
between 62–72°E and 66–67°S, with an estimated error of
0.5° in longitude and 0.2° in latitude. The mean direction
of flow according to interpolated positions is westward.
This agrees with the sea surface height field from Meijers
et al. [2010], which shows that the regional flow field is
dominated by westward jets associated with the ASC and
follows the contours of the shelf break. Maximum CTD
pressures indicate that most of these MSW profiles were
collected at locations with water depths between 1300 and
2000 m (Figure 1), thus confirming their positions near the
continental slope.
Table 1. The Four Profiling Floats That Observed MSW North of Mac Robertson Landa
WMO ID
Date of MSW Observations
Estimated Longitude Range
Cycle #
Optode O2
2900116
2900117
2900123
2900126
14 Jun to 5 Sep 2011
9 Sep to 24 Nov 2011
5 Apr to 7 Jun 2010
28 Feb to 20 Jun 2011
65.7–71.2°E
62.5–66.8°E
65.5–69.6°E
62.4–69.0°E
193 to 205
209 to 220
131 to 136, 138 to 140
178, 180, 184, 190, 191, 193, 194
No
No
Yes
Yes
a
Data are available from the Argo Global Data Centers (http://www.coriolis.eu.org; http://www.usgodae.org) by using WMO ID of the floats. Numerical
designations of profiles within individual float data record that sampled MSW are listed under Cycle #.
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Figure 2. Vertical profiles of (a) potential temperature, (b) salinity, (c) neutral density, and (d) adjusted dissolved oxygen,
from the float data listed in Table 1. (top row) Data below 1200 dbar from May to November. (bottom row) Data below 1400
dbar from February to April. MSW is identifiable as a cold, fresh, and oxygenated bottom layer. MSW with γn > 28.27 kg m 3
is highlighted in red (top row) and blue (bottom row).
[10] The cold, fresh, bottom signature of MSW is accompanied by elevated dissolved oxygen values in excess of
220 μmol kg 1 (Figure 2). Potential temperature within the
observed MSW reached values as low as 1.77°C, with corresponding salinity at 34.48, recorded at 1280 dbar during
June 2011. The near-freezing bottom temperature indicates
a shelf origin, and the elevated dissolved oxygen values
provide further evidence that the water mass has recently
been near the surface. Salinity as high as 34.67 was observed
within this variety of MSW, with θ = 0.75°C at 1930 dbar
during October 2011.
[11] θ-S curves from float data show that the observed
MSW exists as a dense water mass underriding the salinity
maximum of regional mCDW (Figure 3). Whitworth et al.
[1998] defined this as AABW on the continental slope and
used the neutral density criterion of γn > 28.27 kg m 3 to
delineate mCDW from AABW (or MSW). Using this definition, we estimated that the thickness of the observed MSW
varied between 50 m and 970 m. This wide range of MSW
thickness is due to seasonal variations in MSW properties.
During the period from May to November (Figure 2, top
row), the MSW layers are thicker and show more interleaving
than the period from February to April (Figure 2, bottom row).
[12] The annual cycle of formation and modification of
dense shelf water is related to the cycle of sea ice growth
and decay. The months from May to November are the active
winter sea ice growth period when brine rejection and
enhanced vertical convection lead to the formation of thick
layers of dense water on the continental shelf. December to
March are the spring melt and summer ice-free period with
no vertical convection and hence no production of new shelf
water. The remaining shelf water from previous winter gradually loses its near-freezing characteristic through mixing
and becomes warmer and thus lighter in density. In a case
study of the Adélie Depression, Williams et al. [2008] show
that shelf water densities reach a peak during September–
October and then gradually decrease from November. Since
MSW is the downstream variation of dense waters from the
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WONG AND RISER: MODIFIED SHELF WATER AT EAST ANTARCTICA
Figure 3. Potential temperature versus salinity (θ-S) diagram
from the float data listed in Table 1. Labeled lines are γn contours (in kg m 3). MSW with γn > 28.27 kg m 3 is highlighted
in red (May to November) and blue (February to April), as in
Figure 2. The thick black line marks the θ = 1.9°C isotherm,
which is the freezing point of sea water in the 34.5–34.7 salinity range, referenced to 0 dbar.
continental shelf, the seasonal evolution of MSW properties
on the continental slope naturally follows the processes on
the shelf. Our MSW observations on the continental slope
show that both the thickness and the density of MSW increase
steadily from February to November, with density reaching a
maximum in October and thickness reaching a maximum
in November (Figure 4a). The increase in density from
February to June is mainly due to the decrease in temperature,
while from June to October, density increase is mainly due
to the increase in salinity (Figure 4b).
[13] No MSW was observed during December and January.
A single January profile showed a sharp hook at the bottom
toward cold, fresh, oxygenated values, but its minimum potential temperature only reached 0.2°C. This is a curious fact
as MSW of the θ < 0.4°C variety was observed during
February and March, much later in the shelf water modification period. We suspect that this sampling irregularity is less
a result of seasonal variability and more due to spatial variability of the water mass. Of the four floats that were profiling
along the Mac Robertson Land coast, only two were in the
estimated MSW region during the austral summer (WMO ID
2900123 and 2900126). Their known positions from those
ice-free months indicate that they remained east of ~69°E
and north of Prydz Bay during December and January
(Figure 1). The lack of MSW observations during December
and January could be an indication that the export of cold,
dense shelf water from Prydz Bay is sporadic.
4. Discussion and Conclusions
[14] Our observations of MSW from profiling floats are
consistent with the results reported by Ohshima et al.
[2013]. They documented MSW on the continental slope
between 67.5° and 71°E based on seal-mount CTD data, with
θ-S characteristics similar to our observations and with a
shift to denser bottom values west of ~68.5°E (see their
Figure 3c). The key result from Ohshima et al. [2013] comes
from their moored time series data (M3 at water depth
2608 m), which show plumes of newly ventilated AABW
cascading down the slope to the bottom of the canyons northwest of Cape Darnley from May to January. This Cape
Darnley Bottom Water (CDBW) has 1°C < θ < 0.5°C
and 34.6 < S < 34.67 (see their Figure 2) and is estimated
to have a thickness of > 300 m. It is believed to originate primarily from the Cape Darnley polynya region, an area noted
to have the second highest sea ice production of all regions
around Antarctica. High sea ice production leads to formation of a relatively large volume of dense shelf water, which
flows out of the shelf, mixes with the ambient mCDW, and
ultimately gets channeled downslope via canyons to abyssal
depths as new CDBW. Our observed MSW shares the same
θ-S range and thickness as CDBW, with an estimated position
in the same location as CDBW, albeit at shallower depths.
These similarities with CDBW suggest that the MSW reported
in this study has the same primary source as CDBW.
[15] Not all modified shelf waters on the Antarctic continental slope descend to abyssal depths as dense plumes via
submarine canyons. Some of these waters flow westward
along the slope as they descend in approximate geostrophic
balance. These broad sheets of dense waters serve to ventilate
the interior of the Southern Ocean at their adjusted density
range [Baines and Condie, 1998]. Since there is some uncertainty in the positions of the float-based MSW profiles, we
Figure 4. Seasonal evolution of MSW properties. (a) Thickness
in meter (black line) and neutral density γn in kg m 3 (blue
line). (b) Salinity (blue line) and potential temperature °C
(red line). Values shown are the median for the range of
MSW observed in each month from February to November.
No MSW was observed during December and January.
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WONG AND RISER: MODIFIED SHELF WATER AT EAST ANTARCTICA
cannot determine the exact extent of the MSW westward
spreading from its source. However, we note that the four
floats listed in Table 1 sampled MSW over an average period
of 81 days. During that time they moved in a general
westward direction at an average speed of 0.03 ms 1. This
equates to a longitudinal distance of about 210 km, or about
5° of longitude at 67°S, over which the bottom characteristics
of MSW remain distinguishable. Such spatial coverage is
comparable to the dense shelf water overflows on the continental slope between 144°E and 148°E, northwest of the
Mertz Depression [Williams et al., 2010].
[16] Williams et al. [2010] pointed out that dense shelf
water sources along the East Antarctic coast were found in
discrete coastal polynya regions, with the most conspicuous
location being the Adélie and George V Land at 140–149°E.
Despite having collected nearly 3000 CTD profiles from
autonomous floats along the East Antarctic coast between
50°E and 128°E, the Mac Robertson Land coast was the only
area where MSW with θ < 0.4°C was observed. It is inconclusive whether this absence of MSW from other areas in the
float data record implies a lack of other AABW sources in
the polynya regions of East Antarctica. There could be other
dense shelf waters that were not sampled by floats, due to
the float trajectories being too far from the Antarctic continental margin. Nevertheless, our MSW observations north of Mac
Robertson Land from 2010 to 2011, along with those of
Ohshima et al. [2013] from 2008 to 2009, highlight the
Cape Darnley polynya region as a significant source of dense
shelf water for the Weddell-Enderby Basin. This deep basin
between the Weddell Sea and the Kerguelen Plateau is a climatically sensitive area, as its bottom waters serve to ventilate
the Atlantic sector of the Southern Ocean. We therefore echo
the conclusions from these previous authors that this region
of East Antarctica should be studied in more detail in order
to assess the contribution of its shelf circulation to deep and
bottom water formation in the Weddell-Enderby Basin.
[17] Acknowledgments. The authors wish to thank Robert Drucker for
providing the adjusted dissolved oxygen data from the two floats that carried
the Aanderaa Optode sensors. Comments from an anonymous reviewer
helped improve the manuscript. The profiling floats used in this study were
fabricated and programmed at the University of Washington by Dana
Swift, Dale Ripley, and Rick Rupan. The float data were made freely available
by the International Argo Program (http://www.argo.net). This work was
funded by National Oceanic and Atmospheric Administration (NOAA) grant
NA17RJ1232 task 2 to the University of Washington in support of the Argo
float project.
[18] The Editor thanks an anonymous reviewer for his assistance evaluating
this manuscript.
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