Blue Water Zone Project
A View of the Circulation from Altimetry and Drifters
Shane Elipot
March 23, 2004
1 Introduction: What is in this report?
This report describes the work carried out by Shane Elipot as part of the Blue Water Zone
(BWZ) cruise to evaluate ocean circulation in the cruise domain by using altimeter data, drifter
data, and wind observations.
2 Altimetry data
Altimeter data provided by AVISO (Archiving, Validation and Interpretation of Satellites
Oceanographic data) at CLS (Centre Localisation Satellite), Toulouse, France, were used.
AVISO delivers near-real time data, named Ssalto/Duacs NRT data. Upon approved request,
global maps of sea level anomalies and derived geostrophic velocities are available twice a week
on Wednesday and Saturday (every 3.5 days) for the data from a week before [AVISO, 1996].
For this project we used a gridded data set that merges observations from four different
satellite altimeters (JASON-1, TOPEX/POSEIDON, ERS-2 and ENVISAT). The merged data
product has been shown to give more realistic sea level and geostrophic velocity statistic than a
single altimeter alone and also very good agreement with drifters in the North Atlantic [Ducel et
al., 2000]. Elipot and Gille (2003) also showed a statistically significant increase in the
agreement between drifter and altimeter-derived geostrophic velocities in the Southern Ocean.
During the cruise, we used 15 maps from January 21 to March 10. The data are gridded
with a Mercator projection, with 1/3 degree resolution in longitude, and latitudinal resolution
adjusted to match the spatial separations in longitude [AVISO, 1996]. This grid takes into
account the convergence of the satellites' tracks toward the poles which increases the spatial
resolution. The resulting average spatial resolution for our study region is about 18 km.
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Drifter data
A total of 36 drifters were deployed for the purpose of the project. Sixteen were deployed
during the January 2004 AMLR cruise and 20 were deployed during the BWZ cruise. Roughly
every two days, we received by email from Mayra C. Pazos (pazos@aoml.noaa.gov) raw
ARGOS drifters data from the AOML in Miami, Florida. These data consisted of drifter position
whenever they were in sight of the ARGOS satellite array. The time interval between two
consecutive drifter positions was 78 minutes on average but with a more typical (modal) value of
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100 minutes. Drifter velocities were estimated from their displacements in order to compare
them with geostrophic velocities from altimetry (see section 5).
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Geostrophic currents from hydrography
CTD profiles of temperature and salinity versus pressure were used to compute
geopotential anomalies at the ocean surface following the method described in Gill (1982). In the
preliminary computation, 77 downcast profiles from stations 03 to 79 were used. These casts
were done over a time interval of 20 days from February 20 to March 11. Regular CTD profiles
were used and, when not available, Trace Metal Clean CTD profiles were used. The depth of the
casts was 1000 m.
Since the station grid formed a regular mesh, the geopotential anomalies were mapped on
a regular grid of approximately 14 km in the meridional and zonal directions (15 degrees by 7.5
degrees), using a tension curvature surface gridding algorithm describe by [Smith and Wessel,
1990]. From the gradient of the geopotential anomaly at the surface, surface geostrophic currents
were calculated referenced to the 1000 db pressure level (Figure 18).
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Preliminary Analysis of the Altimeter Data
The BWZ study region is a region of low eddy kinetic energy EKE 1 compared to the eddyintense Drake Passage, with the Subantarctic Front to the north, the Polar Front in the center, and
the Southern ACC Front to the south [Lenn et al.,]. Despite the lower EKE levels in the BWZ
region, altimeter data provide useful information about the circulation. The dynamic height
difference across Drake Passage, computed from hydrography is about 1 meter [Gouretski and
Janke, 1998]. Across our study region, the difference can be of about 0.10 meters leading to a
typical 10 cm s-1 north eastward flowing geostrophic currents. However, within the study region,
altimeter data show energetic signals, with typical sea level anomalies of about 0.40 m and
geostrophic currents of up to 60 cm s-1.
5.1
Eddy field general features
The variability of the surface geostrophic flow can be represented, as in Figure 1, using
variance ellipses. Figure 1 represents variance during the time period of our study, from January
21 to March 10 2004. Each individual map of sea level anomalies with their corresponding
geostrophic velocities are plotted in Figures 6 through 16. Contemporaneous drifters' trajectories
are plotted in green with red diamonds indicating the beginning of the trajectories. The Laurence
M. Gould's track is also plotted when she was in the region.
One of the main feature of the flow variability is the orientation of the currents along the
Shackleton Fracture Zone on its west and east sides. The ridge found there, which rises suddenly
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EKE = {(u'2+v'2)/2)}0.5 where u' and v' are zonal and meridional velocity anomalies
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from about 6000m to 1000m, acts as a barotropic barrier for the flow.
The currents' orientation on the continental slope is along the topography southwest of the
Fracture Zone. More specifically, the flow is oriented along the 1000 to 4000m isobaths. This
orientation pertains north of Elephant Island. There, the ellipses' axes rotate from the southwestnortheast direction to the south-north direction, following the orientation of the 3000m isobath.
Along the isobaths, the geostrophic currents are flowing toward the southwest from January 21 to
February 4 and then to the northeast until March 10.
As can be seen from the variance ellipses or from each individual maps, an eddy feature either
rotating clockwise or anticlockwise was located east of the Fracture Zone for the major part of
the survey. When compared to sea color MODIS images, it coincides to the bluest surface
waters southeast of the region.
The details about the circulation are complex and highly variable. In general, the
presence of several eddies caused narrow jets oriented along the Fracture Zone's direction to flow
in alternate direction across the region.
5.2
Correlation with the drifter data
Drifter trajectories showed good qualitative agreement with the geostrophic anomaly
flow suggesting once again that the mean current is of not much impact in the region.
The drifter trajectories showed typical inertial oscillations uncorrelated with the
geostrophic velocities. The drifters' positions and velocities were consequently first daily
averaged (equivalent to a low-pass filtering) in order to quantitatively compare them with
geostrophic velocities. Preliminary results are presented in Figure 17. The top panel shows the
correlation between the drifters and geostrophic velocities for the zonal and meridional
components. In general, the correlation and its statistical significance increases as more daily
velocities are available to compute these parameters: the correlation is generally greater than 0.5
for both velocity components for the drifters released in January. The correlation drops and even
becomes negatives for data from the drifters released last.
Since velocity is a vector quantity, a complex correlation can be computed between the
drifters velocities and the geostrophic velocities. The absolute value of the complex correlation
shows approximately the same thing than the correlation for the individual components: higher
correlation for the drifters released in January. The angle of the complex correlation is generally
low for corresponding high absolute values showing a good agreement between the drifters' and
geostrophic velocities directions.
As for now, it is hard to say if there is a relationship between one drifter's geographical
location and its correlation with the geostrophic flow. It seems that the agreement is better in
the``blue'' waters where the flow speed is greater than in the green waters close to the shelf
where the geostrophic speeds are smaller.
It can be hypothesized that the lack of correlation for some parts of the data can be due to winddriven currents. The drifters' drogues are usually located in the mixed layer where both the
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influence of geostrophic currents and Ekman-type currents are of importance. Analysis of
scatterometer wind fields should be helpful to analyze the influence of the Ekman component of
the surface flow.
5.3
Correlation with the geostrophic shear
As shown by the surface geostrophic currents maps from altimetry, our survey region has a lot of
variability. The good agreement found between the drifters and the altimeter data is an indication
that the geostrophic approximation for computing surface currents from maps of sea level
anomalies is sensible. Bearing this in mind, one should also notice that length scales of
variability smaller than our region's geographical extend can be found in these maps. Moreover,
the theoritical first baroclinic Rossby radius of deformation is of about 3km. Consequently, given
our average stations grid spacing and sampling time, one cannot hope to capture the vertical
geostrophic shear at the time scale of the transient eddies. As a consequence, surface
geostrophic currents mapped in Figure18 are certainly biased by the sampling schedule.
The mapped currents certainly exhibit the strong jet flowing between the southern tip if
the Shackleton Fracture Zone and Elephant Island where the sea floor is deeper, turning then
along the Fracture and flowing toward the north. This water path is often seen in the drifters'
trajectories. A second branch of the current past Elephant Island emerges from this map, flowing
towards the northeast across the survey region. This water path has not been followed yet by the
drifters and the statistical significance of this water path is dubious. I hypothesis that this feature
is the resulting average flow from the alternating jets flowing along the northwest-southeast
direction across our survey region and carried there by transient eddies.
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Figures
Figure 1: Variance ellipses for geostrophic surface velocities derived from maps of sea level
anomalies. A reference variance ellipse of (10cm.s-1)2by (5cm.s-1)2, oriented at 45º is given as a
reference to the lower left corner of the figure.
Figure 2: Sea level anomalies and geostrophic current anomalies for January 21 2004.
Figure 3: Sea level anomalies and geostrophic current anomalies for January 25 2004.
Figure 4: Sea level anomalies and geostrophic current anomalies for January 28 2004.
Figure 5: Sea level anomalies and geostrophic current anomalies for January 31 2004.
Figure 6: Sea level anomalies and geostrophic current anomalies for February 4 2004.
Figure 7: Sea level anomalies and geostrophic current anomalies for February 7 2004.
Figure 8: Sea level anomalies and geostrophic current anomalies for February 11 2004.
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Figure 9: Sea level anomalies and geostrophic current anomalies for February 14 2004.
Figure 10: Sea level anomalies and geostrophic current anomalies for February 18 2004.
Figure 11: Sea level anomalies and geostrophic current anomalies for February 21 2004.
Figure 12: Sea level anomalies and geostrophic current anomalies for February 25 2004.
Figure 13: Sea level anomalies and geostrophic current anomalies for February 28 2004.
Figure 14: Sea level anomalies and geostrophic current anomalies for March 3 2004.
Figure 15: Sea level anomalies and geostrophic current anomalies for March 6 2004.
Figure 16: Sea level anomalies and geostrophic current anomalies for March 10 2004.
Figure 17: Top panel: correlation between drifters velocities and geostrophic velocities.
Correlation for the zonal and meridional velocity components for each single drifter are given.
Drifter numbering is arbitrary. 95\% statistical significance levels are given based on the number
of degrees of freedom for each drifter. Middle panel: absolute value of complex correlation.
Lower panel: angle of complex correlation.
Figure 18: Surface geostrophic currents referenced to the 1000db pressure level. CTD profiles
used for this computation are located by green arrows. Seafloor topography contours at 1000m
intervals are plotted in the background as a reference.
References
AVISO (1996). AVISO User Handbook for Merged TOPEX/POSEIDON products.
AVISO/Altimetry, edition 3.0 edition.
Ducet, N., Le~Traon, P.~Y., and Reverdin, G. (2000). Global high-resolution mapping of ocean
circulation from TOPEX/Poseidon and ERS-1 and-2. J. Geophys. Res.-Oceans,
105:19477-19498.
Elipot, S. and Gille, S. (2003). Comparing drifter and altimeter derived velocities in the
Southern Ocean. In from TOPEX-POSEIDON to JASON - SWT meeting 2003 - Book of
abstracts, page 112. NASA-CNES.
Gill, A. E. (1982). Atmosphere-Ocean Dynamics. Academic Press.
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Gouretski, V. V. and Jancke, K. (1998). A new climatology for the world ocean. WHP SAC
Tech. Rep. no.3, woce report no. 162/98, WOCE Special Analysis Centre, Max-Planck
Institute, Hamburg.
Lenn, Y.-D., Chereskin, T., and Sprintall, J. Momentum eddy balance in the Drake Passage. In
AGU Ocean sciences meeting 2004.
Smith, W. H.~F. and Wessel, P. (1990). Gridding with continuous curvature splines in tension.
Geophysics.
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