Journal of Oceanography, Vol. 59, pp. 629 to 635, 2003
Short Contribution
Seasonal and Spatial Variations of Iceberg Drift off
Dronning Maud Land, Antarctica, Detected by Satellite
Scatterometers
S HIGERU AOKI*
The Graduate University for Advanced Studies (SOKENDAI), Center for Antarctic Environment
Monitoring, National Institute of Polar Research, Kaga, Itabashi-ku, Tokyo 173-8515, Japan
(Received 8 September 2002; in revised form 17 December 2002; accepted 29 January 2003)
Seasonal and spatial variations of iceberg drift were studied using continuous satellite scatterometer images off Dronning Maud Land, East Antarctica. Generally, iceberg drift speed showed a westward increase to the Greenwich Meridian. Seasonal
variations of the drift speed were high in autumn–early winter and low in spring, and
their magnitudes also increased westward. Seasonal variations of the drift speed were
significantly correlated with variations of sea levels at Syowa and Mawson Stations,
and hence qualitatively consistent with geostrophic current variations. Thus, the
scatterometer data are demonstrated to be useful in monitoring iceberg trajectory
and oceanic current variations.
Keywords:
⋅ Satellite
scatterometers,
⋅ iceberg drift,
⋅ sea level,
⋅ seasonal variation,
⋅ spatial variation,
⋅ Antarctica.
tween the oceanic current and wind stress variations. This
suggests a possible link of coastal currents over the continental slope off Dronning Maud Land. However, this
link has not been thoroughly established and it is crucial
to investigate the seasonal and spatial variations in this
region.
Iceberg drifts have been used to investigate and validate the coastal current and its variation (e.g., Tchernia
and Jeannin, 1980; Ohshima et al., 1996; Gladstone et
al., 2001). Off East Dronning Maud Land, the land-fast
ice margin is usually located along the shelf break. Off
the land-fast ice edge there are coastal polynyas, and further offshore there is a pack ice region (Yamanouchi and
Seko, 1992; Ishikawa et al., 1996). This sea ice distribution suggests that the iceberg drift along the continental
slope is largely controlled by oceanic current (e.g.,
Yamonouchi and Seko, 1992). Ohshima et al. (1996)
showed that seasonal variation of the drift speed of icebergs off Dronning Maud Land is in phase with
geostrophic current variation derived from sea level variation at Syowa Station. However, the iceberg drift data
they used, the Antarctic iceberg archive of the National
Ice Center (NIC), U.S., were tracked only intermittently.
The number of available icebergs is small in 0°–90°E
compared to the other regions, and this area has not been
surveyed extensively so that the average time interval was
about one month for six icebergs (B01, C02, C07, D11,
D12, and D13, numbered with the NIC code). Thus the
1. Introduction
The continental margin of the Weddell Sea is one of
the important regions of deep and bottom water formation. The circulation pattern in the Weddell Sea is characterized as a cyclonic gyre with intense westward flow
along the continental margin (Deacon, 1937; Gill, 1973).
While estimates of the volume transport of the gyre differ by several factors (e.g., Carmack and Foster, 1975;
Gordon et al., 1981; Whitworth and Nowlin, 1987;
Fahrbach et al., 1994; Heywood et al., 1998; Schröder
and Fahrbach, 1999), Fahrbach et al. (1994), with their
intensive current mooring transect over the gyre, showed
that over 90% of the total gyre transport is concentrated
in boundary currents on the continental slope at about
10°W. This indicates the importance of the coastal current input from the Indian sector along the continental
slope. Moreover, they showed that the coastal current has
a significant seasonal variation with a high in autumn–
winter and a low in spring–summer. In Lützow-Holm Bay
at about 40°E, off Dronning Maud Land, Ohshima et al.
(1996) observed a seasonal current variation similar to
the one found at 10°W, and suggested a relationship be-
* E-mail address: [email protected]
Current affiliation: Institute of Low Temperature Science, Hokkaido
University, Sapporo 060-0819, Japan.
Copyright © The Oceanographic Society of Japan.
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B9A
C05
D11
D12
Enderby
Land
Weddell Sea
Mawson
Syowa
Lutzow-Holm Bay
Neumayer
Dronning Maud Land
Fig. 1. Drifts of the four icebergs (B9A, C05, D11, and D12, numbered with the National Ice Center code) off Dronning Maud
Land. The locations of coastal stations with tide gauge and wind observations are indicated with dots. The GEBCO bathymetric
contours are drawn for 1000 m, 2000 m, and 3000 m.
Fig. 2. Sample images used for deriving the position of B9A on May 1 and June 30, 1999 (from ERS-2) and September 1 and
November 1 (from QuikSCAT).
data coverage was not sufficient either in time or space
to reveal their detailed structures off Dronning Maud
Land.
Recently, application of satellite scatterometers to
polar regions has made it possible to monitor snow and
ice conditions continuously at short intervals (at least a
few days) and homogeneously around Antarctica (e.g.,
Long and Drinkwater, 1994, 1999; Remund and Long,
1999). This enables us to discuss detailed iceberg drift
properties, uncontaminated with spatially or temporally
irregular samplings. In this study, the trajectories of icebergs were derived from the scatterometer images to examine their spatial and seasonal variations quantitatively
in the continental margin off Dronning Maud Land (Fig.
1). Tide gauge data at Syowa and Mawson Stations were
also used to constrain estimates of surface geostrophic
current, and were compared with the variations of iceberg drift speed. This study uses the austral definition of
seasons; summer (December–February), autumn (March–
May), winter (June–August), and spring (September–
November).
2. Scatterometer Images and Tide Gauge Data
The data used in this study are the satellite
630
S. Aoki
scatterometer images and tide gauge time series.
Scatterometers measure microwave backscatter from
the earth’s surface. The backscatter property is significantly different between over ice/snow-covered areas and
open water surfaces, and the difference can be used as a
means to identify individual features such as drifting icebergs. Trajectories of icebergs were derived from the satellite scatterometer images. The images from ERS-1/2 and
QuikSCAT (Long, 2000a) were used from 1994 to 2000;
the ERS-1/2 AMI images are provided at intervals of three
days for 1994–1999 and the images from SeaWinds on
QuikSCAT are provided daily for 1999–2000 (the latter
was re-sampled at intervals of three days). Four iceberg
(C05, D12, D11, and B9A, numbered with the NIC code;
Table 1) were tracked. Their positions were read by eye
through graphic software (Fig. 2); each snapshot was
zoomed by several factors to resolve each pixel and the
central position of the iceberg was read as horizontal and
vertical coordinates of the image. This central position
does not necessarily coincide with the center of each pixel
grid. The central position was converted to x/y pixel numbers and then converted to the longitude/latitude using
Scatterometer Image Reconstruction (SIR) software developed by the Microwave Earth Remote Sensing research
Table 1. Drift information for the four icebergs: time when
each iceberg passed at 70°E, 10°E and 30°W and its size at
70°E.
Code
70°E
10°E
30°W
Size (km)*
C05
D12
D11
B9A
1994/10/04
1996/11/12
1997/03/26
1999/04/25
1996/06/30
1998/03/08
1998/07/24
2000/07/05
1996/11/15
—
1998/12/24
2000/11/15
33 × 54
41 × 52
17 × 89
28 × 43
*From the National Ice Center archive at http://
www.natice.noaa.gov/southberg.htm
Fig. 3. Movement of the longitudinal positions of the four icebergs.
group, the Brigham Young University (BYU). Generally
there was significant brightness difference between the
icebergs and sea ice and it was easy to distinguish icebergs from the surrounding sea ice, except relatively small
difference in summer. The icebergs C05, D11, and B9A
were tracked from 70°E to 30°W, while the iceberg D12
was traced westward only to about 10°E because of a difficulty in distinguishing from the surrounding sea ice. The
C05 trajectory has a large gap in 1996, corresponding to
the period of transition from ERS-1 to ERS-2. Drift speed
in that interval was derived from successive two positions. Since the pixel size of the images is about 12 km
for ERS-1/2 images and 31 km for QuikSCAT images at
65°S, monthly drift speeds are expected to have a formal
error of 2 (31 km/30 days) for QuikSCAT data, assuming that the error estimate in position is equal to the pixel
size. Comparison with six data for B9A in the NIC archive gave an average difference of 22 km, and this value
is slightly better than the pixel-size estimate. The drift
data with the ocean depth shallower than 300 m were
excluded in subsequent analysis to avoid possible contaminations caused by grounding. The bottom topography data of Smith and Sandwell (1994) were used.
Tide gauge data at two coastal stations (Fig. 1) were
studied to constrain estimates of surface geostrophic current variations. Hourly data at Syowa Station (39.6°E,
69.0°S) for nearly the same period of 6 years from 1994
to 1999 were used. Differential GPS observations on the
land-fast ice indicated that the accuracy of the tide gauge
data was 0.01–0.02 m on time scale of about a year (Aoki
et al., 2002). The data were provided through JARE Data
Reports (e.g., Yoritaka and Masuyama, 2001). Hourly sea
level data are also available at Mawson Station (62.9°E,
67.6°S) for the same 6 years from 1994 to 1999. The diurnal and semi-diurnal tidal components were estimated
and removed using a harmonic analysis method called
the Bayesian Tidal Analysis Program—Grouping method
(Ishiguro et al., 1981; Tamura et al., 1991). To remove
direct signals of variations in surface atmospheric pressure, sea level data were adjusted for the inverse barom-
eter response with the coefficient of 0.01 m sea level to 1
hPa atmospheric pressure. For Syowa Station, we have
hourly sea surface pressure data (e.g., Japan Meteorological Agency, 1999). For Mawson Station, 3-hourly pressure data are available from the British Antarctic Survey
and they were interpolated into hourly intervals with cubic splines.
3. Results
The characteristics of the spatial pattern and seasonal
variations of the iceberg drift were investigated, and then
their relationship with sea level variations at the coastal
stations were studied.
Generally, the four icebergs drifted westward along
the continental slope. Longitudinal positions of the icebergs are shown in time sequence (Fig. 3). It took about
500 days to move from 70°E to 10°E. However, there were
some differences in their behaviors, such as a large meridional excursion in the eastern region from 60°E to 65°E
(Fig. 4). Iceberg C05 moved northward at around 65°E
and then drifted westward again further offshore. Iceberg
D12 experienced a cyclonic motion and fluctuated before its subsequent westward drift.
The drift speeds of the icebergs were derived from
the variations in their positions. To derive their alongshore
component, the large meridional drifts of C05 and D12
in 61°–64°E were excluded. The longitudinal distribution
of the drift speed is shown in Fig. 5. The drift speeds
generally revealed a westward increase to around the
Greenwich Meridian. The drift speeds were spatially averaged to derive the temporal mean structure; the length
of the averaging bin was set to 20° for the eastern regions
of 50°–70°E (the Mawson region) and 30°–50°E (the
Syowa region), 30° for 5°–35°E (the middle region), and
40° for 20°W–20°E (the Greenwich Meridian region).
Average drift speed showed an increase from 0.06 m/s in
the Mawson region to 0.14 m/s in the Greenwich MeridAntarctic Iceberg Drift by Scatterometers
631
ian region.
The temporal distribution of the drift data provided
sufficient coverage to resolve its seasonal variation for
the region east of the Greenwich Meridian; the icebergs
C05, D11 and B9A drifted in almost the same seasons
west of around 10°E (see Table 1) and there were little
data in autumn. Seasonal variations of iceberg drift were
clearly detected with a high in autumn-early winter and
low in spring (Fig. 6). The drift speed was 0.18 m/s in
June–July and 0.02 m/s in October in the middle region,
and it was 0.14 m/s in May and 0.003 m/s in November
in the Syowa region. In the Mawson region the speed was
0.11 m/s in May and 0.03 m/s in October. The magnitude
of the seasonal variation also increased westward from
0.09 m/s in the Mawson region and 0.14 m/s in the Syowa
region to 0.16 m/s in the middle region.
Seasonal variations of coastal sea level were derived
at the two tide gauge stations (Fig. 7). At Syowa Station,
the data revealed a seasonal cycle of about 0.15 m range
with a high in April–June and a low in October. At
Mawson Station, sea level showed a variation of about
0.13 m range with a high in March–June and a low in
October. The correlation coefficient of the seasonal variations between the iceberg drift and sea level was 0.84
(this is significant with 99.5% confidence level) in the
Syowa region and 0.58 (significant with 95% confidence
level) in the Mawson region. The seasonal sea level variation was also larger at Syowa Station. Coastal sea level
variation can constitute a significant portion of cross-shelf
sea level difference, and thus it is suggested that the seasonal iceberg drift variations are significantly in phase
with the surface geostrophic current variations.
4. Discussion and Conclusions
The iceberg drift speed increased westward off
Dronning Maud Land, and both the iceberg drift speed
and sea level were high in autumn–early winter and low
in spring. The ranges of the seasonal variation were also
3000
Fig. 4. Drifts of the four icebergs in the region around 60°–
65°E. Symbols are as in Fig. 1.
Fig. 5. Spatial variation of iceberg drift speeds for 30°W–70°E
(positive westward). Yellow, red, green, and blue dots are
derived drift speeds for B9A, C05, D11, and D12, respectively. The solid line denotes the average drift speed in the
longitude bins (20°W–20°E, 5°–35°E, 30°–50°E, and 50°–
70°E).
Fig. 6. Seasonal variation of iceberg drift speeds in (a) 5°–35°E, (b) 30°–50°E, and (c) 50°–70°E (positive westward). Symbols
are as in Fig. 5.
632
S. Aoki
Fig. 7. Seasonal variations of sea level (thick line) and iceberg
drift speed (thin line) in the Syowa and Mawson regions.
increased westward to around the Greenwich Meridian.
This drift pattern can be explained by the effect of
geostrophic ocean current. The geostrophic current at the
surface can be estimated by g∆ η /f∆y where ∆η is the
cross-shelf sea level difference, ∆y is the cross-shelf distance that the sea level difference occurred, g acceleration due to gravity, and f the Coriolis parameter. The crossshelf current profile at 15°W (Heywood et al., 1998)
showed that the strong current region was confined within
50 km or less. The cross-shelf sea surface dynamic topography also shows a change in about 40 km distance
(see Appendix). Although the sea level difference can be
used to estimate the current at the surface, the current
affecting the iceberg drift is that of the subsurface due to
its large draft. The draft of each iceberg was not observed,
but is assumed to be typically around 200–250 m (e.g.,
Gladstone et al., 2001; Lichey and Hellmer, 2001). One
of the expendable current profiler (XCP) observations
revealed a surface-intensified current structure on the
continental slope region at 24°E (figure 9 in Nakamura
and Noguchi, 1993); the current speed at 100 (200) m
depth was about 40 (30)% of that at 25 m depth. Heywood
et al. (1998) observed that the absolute current speed at
100 (200) m depth was about 75 (40)% of that at the surface. Using the above estimates of 50 km cross-shelf distance and 50% reduction caused by the drafts, the range
of the seasonal variation was 0.11 m/s for the Syowa region and 0.10 m/s for the Mawson region, with f of
–1.37 × 10–4 s–1 at 70°S and an assumption of negligible
variation in the offshore sea level. These rough estimates
suggest that the effect of the geostrophic current variation is consistent with the drift speed variation, and the
oceanic factor is of primary importance.
The possible direct effect of the wind is not straightforward to describe. Climatological wind speed (1954–
1990) at Mawson Station (Russell-Head and Simmonds,
1993), wind speed in 1992–1994 at Neumayer Station,
and wind speed in 1960–1990 at Syowa Station were compared with the observed iceberg drift speed (Fig. 8). The
dominant wind direction was nearly along-shore: SE, NE,
Fig. 8. Seasonal variations of wind speed at Neumayer (triangles), Syowa (crosses) and Mawson (circles). Thick lines
denote the climatological estimates, thin lines the average
for 1994–1999, and the broken lines the averages for 1992–
1994.
and E at Mawson, Syowa, and Neumayer Station, respectively. Although the wind speed variation at Syowa Station is significantly correlated with the drift speed variation, the wind speed variation at Mawson Station is poorly
correlated with the drift speed variation. Moreover, the
mean wind speed does not show a simple westward increase. These facts may indicate that the direct effect of
wind is not the dominant factor in the iceberg drift patterns in this region.
Effect of the sea ice as a “collector” of the wind
momentum (Lichey and Hellmer, 2001) is not clear in
this region. While the sea ice concentration is dense in
the western Weddell Sea, it is relatively low in this eastern region. The correlation between the variations of wind
field and sea ice drift is poor in this region (Kimura, 2002),
and this may indicate a relative importance as a collector
of the ocean current momentum. The fact that the drift
speed was low in spring and summer suggests that a locking in land-fast ice did not play the main role in this
seasonality. These facts suggest that the change in the sea
ice condition is not the primary factor of the seasonal
variation of the drift speed in this region, either.
As a mechanism of the seasonal variations, temporal variation in Ekman transport along the coast of
Enderby Land was studied by Ohshima et al. (1996). The
along-shore sea level difference is proportional to the distance along the coast. The observed westward increase
of the range and the averaged drift speed can be qualitatively explained by the cumulative effect of the alongshore wind stress on the longer distance. This scenario
explains the observed iceberg drift pattern, connects the
current observations at 10°W and 40°E (Fahrbach et al.,
1994; Ohshima et al., 1996), and extends eastward to
70°E.
It was demonstrated that the satellite scatterometers
are useful in monitoring the iceberg drifts. Continuous
Antarctic Iceberg Drift by Scatterometers
633
monitoring is required to increase the data coverage both
in time and space. However, the present normal resolution of the scatterometers limits the size of the icebergs
that can be detected (typically 20 km or more). Enhanced
resolution analysis (Long, 2000b) will be useful in augmenting drift data by detecting the smaller icebergs.
Moreover, combined usage with other techniques such as
Synthetic Aperture Radar will be helpful in revealing
detailed structure of the oceanic current and its variation.
Acknowledgements
A. Ibaraki processed the scatterometer image data
and read many of the positions of the icebergs. Y. Nogi
provided useful information on bottom topography data
around Antarctica. K. Ichikawa provided helpful comments on sea level variation at Syowa Station. Thanks
are extended to two anonymous reviewers for their invaluable comments, which improved the manuscript considerably. The ERS-1/2 data were obtained from the
NASA sponsored Scatterometer Climate Record Pathfinder at the BYU through the courtesy of David G. Long.
ERS-1(2) Antarctic images are available at: http://
scp.byu.edu/data/ERS/SIR/ers1(2)/Ant.html The
QuikSCAT data were obtained from the NASA Physical
Oceanography Distributed Active Archive Center at the
Jet Propulsion Laboratory, California Institute of Technology. QuikSCAT Antarctic images are available at: ftp:/
/podaac.jpl.nasa.gov/pub/oceanwind/quikscat/
sigma0browse/data/ The SIR software is courtesy of D.
G. Long at the BYU. The tide gauge data at Mawson Station were provided by the National Tidal Facility, Australia. Atmospheric pressure data at Mawson Station were
provided by British Antarctic Survey. The wind speed data
at Neumayer Station was available through http://enet.awi-bremerhaven.de/MET/Neumayer/Tabellen.9294.html The General Bathymetric Chart of the Oceans
(GEBCO) 97 was used for the bathymetry in Fig. 1.
Appendix: Mean Sea Surface Dynamic Topography
Temporal mean field of sea surface dynamic topography (MSSDT) can be a measure of the surface
geostrophic current. The MSSDT can be derived by subtracting the geoid from mean sea surface height; it was
derived from the OSUMSS95 mean sea surface height
and the EGM96-based geoid model (Lemoine et al.,
1998). Although the cumulative geoid error is estimated
as 0.075 m to the spherical harmonic degree 36, simple
subtraction of the EGM96 geoid was not sufficient to
derive the detailed distribution of MSSDT. The f/H averaged MSSDT increases shoreward by about 0.3 m over a
width of about 40 km, centered at about 700 m depth on
the continental slope. However, the actual error is not
clear, and future satellite gravity missions will reduce the
uncertainty on this spatial scale.
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S. Aoki
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