13 Use of SAR data to Monitor the Greater Agulhas Current
261
close to the coast, and the altimetry measurements are limited. Although a dedicated
validation campaign is yet to be executed, the long-term persistence displayed in the
Doppler velocity maps adds significant confidence in the retrieval method. When
compared and assessed against other direct and indirect estimates of the strength of
the greater Agulhas Current it is concluded that:
• trajectories and speed derived from surface drifter data provide an excellent source
of data for validation of the range Doppler velocity field;
• the agreement between the zonal geostrophic current estimates derived from Rio09
MDTs and ASAR range velocities is very promising in view of both different
sampling time and spatial resolution;
• there is a strong consistency between the structure and pathway of the range
Doppler velocity field and the SST field which moreover implies that the region
may be subject to abrupt anomalies in wind stress and momentum exchange;
• the strength and pathway of the surface velocities of the HYCOM simulation
indicates that the model underestimates the mean velocity in the Agulhas Current,
while the topographic steering effect of the Agulhas Current, especially in the
southern region is not clearly depicted.
Routine provision of corrected range Doppler surface velocities, together with contemporary altimetry combined with new MDT field, surface drifter data and SST
maps provide an extended and more comprehensive data set for studies and monitoring of the upper ocean dynamics and its variability in intense current regimes.
Combined with in situ data the volume and heat transport estimations can be
strengthen. It will also impose new invaluable possibilities for model validation
and assimilation, not just how well the geostrophic current and transports are reproduced, but more importantly how their internal vorticity and divergence field
maintain ageostrophic velocity and vertical motions. In agreement with Ljuteharms
and Bornman (2010), this routine monitoring effort should stimulate scientists and
also broaden the base of understanding of the marine environment and ecosystems
to inform decision makers.
Acknowledgements This work was supported by the Research Council of Norway, project number
177441/V30 and by ESA through the study contract no. 18709/05/I-LG. ESA is also acknowledged
for implementing systematic ASAR wide swath acquisitions over the greater Agulhas Current
making this study possible. We are also grateful to the Global Drifter Program at their server at
AOML in Miami http://www.aoml.noaa.gov/phod/dac/gdp.html and to CLS for their support to
the range Doppler analyses. The modelling work presented in the paper was supported by the
Mohn-Sverdrup Center for Global Ocean Studies and Operational Oceanography, through a private
donation from Trond Mohn C/O Frank Mohn AS, Bergen, Norway and the National Research
Foundation, South Africa, while the CPU time was covered by the Norwegian Super-computing
project. Dr. Bjørn Backeberg is supported by the Nansen-Tutu Centre for Marine Environmental
Research at University of Cape Town, Cape Town, South Africa.
261
close to the coast, and the altimetry measurements are limited. Although a dedicated
validation campaign is yet to be executed, the long-term persistence displayed in the
Doppler velocity maps adds significant confidence in the retrieval method. When
compared and assessed against other direct and indirect estimates of the strength of
the greater Agulhas Current it is concluded that:
• trajectories and speed derived from surface drifter data provide an excellent source
of data for validation of the range Doppler velocity field;
• the agreement between the zonal geostrophic current estimates derived from Rio09
MDTs and ASAR range velocities is very promising in view of both different
sampling time and spatial resolution;
• there is a strong consistency between the structure and pathway of the range
Doppler velocity field and the SST field which moreover implies that the region
may be subject to abrupt anomalies in wind stress and momentum exchange;
• the strength and pathway of the surface velocities of the HYCOM simulation
indicates that the model underestimates the mean velocity in the Agulhas Current,
while the topographic steering effect of the Agulhas Current, especially in the
southern region is not clearly depicted.
Routine provision of corrected range Doppler surface velocities, together with contemporary altimetry combined with new MDT field, surface drifter data and SST
maps provide an extended and more comprehensive data set for studies and monitoring of the upper ocean dynamics and its variability in intense current regimes.
Combined with in situ data the volume and heat transport estimations can be
strengthen. It will also impose new invaluable possibilities for model validation
and assimilation, not just how well the geostrophic current and transports are reproduced, but more importantly how their internal vorticity and divergence field
maintain ageostrophic velocity and vertical motions. In agreement with Ljuteharms
and Bornman (2010), this routine monitoring effort should stimulate scientists and
also broaden the base of understanding of the marine environment and ecosystems
to inform decision makers.
Acknowledgements This work was supported by the Research Council of Norway, project number
177441/V30 and by ESA through the study contract no. 18709/05/I-LG. ESA is also acknowledged
for implementing systematic ASAR wide swath acquisitions over the greater Agulhas Current
making this study possible. We are also grateful to the Global Drifter Program at their server at
AOML in Miami http://www.aoml.noaa.gov/phod/dac/gdp.html and to CLS for their support to
the range Doppler analyses. The modelling work presented in the paper was supported by the
Mohn-Sverdrup Center for Global Ocean Studies and Operational Oceanography, through a private
donation from Trond Mohn C/O Frank Mohn AS, Bergen, Norway and the National Research
Foundation, South Africa, while the CPU time was covered by the Norwegian Super-computing
project. Dr. Bjørn Backeberg is supported by the Nansen-Tutu Centre for Marine Environmental
Research at University of Cape Town, Cape Town, South Africa.