New estimates of the greater Agulhas
Surface Current dynamics from satellites
IAF
International Aeronautical Federation
!
GMES networking between ESA Member States, Russia and Ukraine
J.A. Johannessen (NERSC), B. Chapron (IFREMER), F. Collard (CLS),
Coordinators:
Stein
Sandven,
NERSC (CSIR)
, and Vladimir
Kudryavtsev,
NIERSC
A. Mouche
(CLS),
M. J.Rouault
and B. Backeberg
(NTC)
Project website: http://www.niersc.spb.ru/oscsar/
Abstract
Doppler-derived ocean surface velocities manifest the intensity of the
greater Agulhas Current and demonstrate the capability to use the European Space Agency´s (ESA) Envisat Advanced Synthetic Aperture
Radar (ASAR) as a “speed-gun” in space. Building on more than 1200
synoptic wide coverage acquisitions since 2007, new high-resolution
gridded maps (12.5 km x 12.5 km) of the greater Agulhas Current have
been routinely produced. This Doppler based method, in combination
with surface drifter data, satellite altimetry and sea surface temperature
measurements, can resolve circulation patterns that have important
implications for oceanographic and air-sea interaction research.
!
!
Introduction
Regular access to Envisat ASAR Wide Swath Mode (WSM) Doppler
information has led to the creation of a new comprehensive data set
of Doppler anomalies since mid 2007. These anomalies are associated
with the overall mean motion of radar-detected scatter velocities .
(Chapron et al., 2005). Measurements are thus directed along the range
direction and encompass both the desired ocean surface current and an
overall bulk velocity, specifically associated to radar detected surface
wave motions. Mostly governed by short scale surface elements, the latter
contribution is strongly controlled by the wind magnitude and direction.
In combination with reliable estimates of the local wind speed and
direction it is accordingly possible to quantify and systematically remove
the expected wind contribution as demonstrated by Mouche et al., 2012).
The residual Doppler anomalies associated with the surface current can
thus be determined with an r.m.s. error of 5 Hz (equivalent to 0.2 m/s) at a
spatial resolution of 8 km (along-track) by 4 km (across-track)
(Johannessen et al., 2008). The retrieval accuracy of this new emerging
capability to map surface currents is largely improved by averaging
individual swath acquisitions to produce gridded fields as demonstrated
in Figure 1.
E
!
!
Figure 3. (top left) Zonal geostrophic surface current derived from
C
altimetry
mean sea level anomaly and RIO09 MDT combining altimetry
derived mean sea surface, in-situ data and geoid model from the period
!
1992 to 2009; (top right) Zonal surface velocities derived from the Doppler
method from the period mid 2007 to fall 2010; (lower) Comparison of zonal
velocities across the longitude section at 27° E. (Unit in m/s).
Figure 2. (top panel) Surface drifter trajectories from 7 buoys
in the period 27 April 2003 to 30 August 2008 overlaid the mean
range Doppler velocities shown in Figure 1. The color bar to the
right represents the range Doppler velocity in m/s. (lowerBpanel)
Scatter plot of Envisat ASAR range Doppler velocities and range
component of surface drifter from 17 May to 14 August 2008. Red
(blue) symbols mark collocations within 3 (24) hours separations.
mean eastward oriented range Doppler velocities. By selecting
drifter data near coincidently available with the individual
ASAR acquisitions the scatter plot shown in Figure 2 (lower
panel) is obtained. Based on these 80 collocations within 24
hours and 14 within 3 hours the range directed velocities are in
good agreement. This implies that the mean Doppler surface
velocity is representing the speed of the upper layer (~10 m) as
the surface drifters are affected by the sub-merged anchored sail.
Summary
The greater Agulhas Current, with its intense dynamics and manifestation
of surface features is an excellent natural laboratory for examination and
assessment of the new emerging capability to establish long time series
of the range Doppler velocity. This is documented by the quality and
persistence of the retrievals depicting a mean speed of about 1.75 m/s
for the Agulhas Current core dropping to about 0.8 m/s for the return
current. Hence, combined high-resolution satellite based SLA, SST and
range Doppler velocity products together with surface drifter data
demonstrate valuable capacity for routine monitoring of seasonal to
interannual current variability and mass transports of the greater Agulhas
Current. The same analyses have been conducted for the Gulf Stream
region and the North-Atlantic Norwegian slope Current with comparable
rea of benefit
Products
Inte rmedia te user
robust
and
promising
results
(Hansen
et al., 2011). l
Climate
Comp rehensive and
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Figure 1. Mean range Doppler velocity in the Agulhas Current
derived from about 700 ascending Envisat ASAR WSM acquisitions
from mid 2007 to fall 2010. The color bar to the right represents the
range Doppler velocity in m/s.
Comparison and Assessment
This distinct manifestation of the mean Agulhas Current is certainly
striking. It inevitably suggests that the repeat use of Doppler observations
can help to monitor dynamics of intense current regimes, provided the
wind contribution is reliably removed as also emphaized by Rouault et al.,
2010. Overlaying the mean SST field (obtained from the Odyssea products,
(www.mersea.eu.org) on the mean range Doppler velocity field for the
same averaging period reveals good pattern agreement and consistency.
The structure of SST maxima are clearly collocated and oriented with the
mean range Doppler velocity maxima from the separation point towards
the retroflection region and back along the Agulahs Return Current. These
results can be further assessed using surface drifter data
(http://www.aoml.noaa.gov/phod/dac/gdp.html). In Figure 2 (top panel)
the drift trajectories of 7 buoys from April 27th 2003 to August 30th 2008
are overlaid the mosaic range Doppler velocity field shown in Figure 1.
Only these 7 buoys, out of a total of several hundreds, remained trapped
in the core of the Agulhas Current from about 30° E to 22° E, and transected
across the meridional section at 26°E. In spite of the temporal sampling
differences the trajectory converges and follows very clearly the pathway
of the core of the Agulhas Current as depicted by the range Doppler
velocities.
The trajectories associated with the easterly return current picked up
by 4 of the 7 drifters are more meandering and dispersive. Although these
drifters enter into the return current at different locations they are still
following a pathway that to some extent agrees reasonably well with the
!
The strong and persistent manifestation of the Agulhas
Current in the mean radial Doppler velocity is clearly in
favorable agreement with the evidence of topographic
steering of the Agulhas Current between 28° to 22° E as
reported by Gründlingh (1983). This can further be assessed
by exploring the mean zonal Doppler velocity in relation to
the strength and
! pattern of mean dynamic topography (MDT)
from AVISO (ref - RIO09 MDT combining altimetry derived
mean sea surface, in-situ data and geoid model from the
period 1992 to 2009). The MDT is the oceanic relief corresponding to ocean circulation in geostrophic balance on a
rotating Earth. In the southern hemisphere the ocean current
will be directed along constant isolines of MDT with decreasing
height to the right. The map of the zonal surface geostrophic
currents (top panels of Figure 3) derived from Rio09 (left) at a
resolution of 25 km are compared to the zonal surface velocity
map at a grid of 12.5 km x 12.5 km derived from the Doppler
method (right) . Maximum zonal speeds of about 1.3 m/s
are found and the separation from the coast occur in the same
place and with the distinct bending of the main flow direction
located at about 23° E. Note that the Doppler based map is
constructed from about two years of data with a spatial
resolution twice that of the AVISO MDT.
The zonal speed across 26.875° E (bottom right panel of
Figure 3) shows the distinct presence of the Agulhas Current
centered at 33.5° to 35° N, while the broader dome structures
at 37° N represents the location of the Agulhas Return Current.
In accordance with the maximum zonal speed of about 1.3 m/s
the meridional slope of the MDT across the Agulhas Current
(from 33.85° to 34.10° N) reaches more than 0.3 m over 25 km.
This result clearly demonstrates that the Doppler based method
is useful for resolving the MDT slope of intense topographically
steered currents with a resolution better than 25 km.
Contact: [email protected], telephone: + 47 93249414
Policy m
This
study
has
supported
by the ESA
Fishery,
E
Fishery
resea rch
andbeen
manag
emen
t advic e
contract no. 18753/NL/CB and
the
Norwegian
Research Council
Acknowledgement:
contract no. 155835/700. We are also grateful to M.-H. Rio and the French
Space Agency CNES which gave us access to the CNES-CLS09 Mean
Dynamic Topography computed in the framework of the SLOOP project.
W
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