Chapter 13
Use of SAR data to Monitor
the Greater Agulhas Current
Johnny A. Johannessen, Bertrand Chapron, Fabrice Collard
and Björn Backeberg
Abstract The strong and dynamic greater Agulhas Current is known to have significant influence on the local marine environment and ecosystem. It is also considered to
play an important role in the global thermohaline circulation, notably due to the transport of heat and salt from the Indian Ocean into the South Atlantic Ocean. Regular
monitoring of the dominant processes and variability within the greater Agulhas Current is thus highly needed. Recently Doppler-derived ocean surface velocities from
the European Space Agency’s (ESA) Envisat Advanced Synthetic Aperture Radar
(ASAR) have demonstrated abilities to manifest the intensity of surface currents. In
this paper the regular direct surface velocity estimates of the greater Agulhas Current
from the ASAR since summer of 2007 are presented. These estimates are assessed by
direct comparison to other independent estimation of the surface current in the greater
Agulhas current obtained from surface drifter data, radar altimetry and an ocean circulation model. The results are promising and highlight that the Doppler-based ocean
surface velocity retrieval offers a new innovative approach to monitor and advance
the understanding of the dynamic processes of the greater Agulhas Current.
13.1 Introduction
The Agulhas Current is, together with the Gulf Stream and Kuroshio Current, among
the strongest western boundary currents in the world’s oceans (Lutjeharms 2006). It
flows poleward along the eastern coast of southern Africa from about 27
◦ to 40
◦ S,
where it retroflects and returns eastward back into the South Indian Ocean. In the
retroflection region eddies are shed into the South Atlantic. The Agulhas Current core
is steered along the continental shelf break, which in the north is very steep and close
to the coast. In this region the Agulhas Current is rather stable, departing less than
J. A. Johannessen ()
Nansen Environmental and Remote Sensing Center, Bergen, Norway
e-mail: Johnny.Johannessen@nersc.no
B. Chapron · F. Collard
Ifremer, Plouzané, France
B. Backeberg
Nansen-Tutu Centre, University of Cape Town, Cape Town, South Africa
V. Barale, M. Gade (eds.), Remote Sensing of the African Seas,
251
DOI 10.1007/978-94-017-8008-7_13,
© Springer Science+Business Media Dordrecht 2014
Use of SAR data to Monitor
the Greater Agulhas Current
Johnny A. Johannessen, Bertrand Chapron, Fabrice Collard
and Björn Backeberg
Abstract The strong and dynamic greater Agulhas Current is known to have significant influence on the local marine environment and ecosystem. It is also considered to
play an important role in the global thermohaline circulation, notably due to the transport of heat and salt from the Indian Ocean into the South Atlantic Ocean. Regular
monitoring of the dominant processes and variability within the greater Agulhas Current is thus highly needed. Recently Doppler-derived ocean surface velocities from
the European Space Agency’s (ESA) Envisat Advanced Synthetic Aperture Radar
(ASAR) have demonstrated abilities to manifest the intensity of surface currents. In
this paper the regular direct surface velocity estimates of the greater Agulhas Current
from the ASAR since summer of 2007 are presented. These estimates are assessed by
direct comparison to other independent estimation of the surface current in the greater
Agulhas current obtained from surface drifter data, radar altimetry and an ocean circulation model. The results are promising and highlight that the Doppler-based ocean
surface velocity retrieval offers a new innovative approach to monitor and advance
the understanding of the dynamic processes of the greater Agulhas Current.
13.1 Introduction
The Agulhas Current is, together with the Gulf Stream and Kuroshio Current, among
the strongest western boundary currents in the world’s oceans (Lutjeharms 2006). It
flows poleward along the eastern coast of southern Africa from about 27
◦ to 40
◦ S,
where it retroflects and returns eastward back into the South Indian Ocean. In the
retroflection region eddies are shed into the South Atlantic. The Agulhas Current core
is steered along the continental shelf break, which in the north is very steep and close
to the coast. In this region the Agulhas Current is rather stable, departing less than
J. A. Johannessen ()
Nansen Environmental and Remote Sensing Center, Bergen, Norway
e-mail: Johnny.Johannessen@nersc.no
B. Chapron · F. Collard
Ifremer, Plouzané, France
B. Backeberg
Nansen-Tutu Centre, University of Cape Town, Cape Town, South Africa
V. Barale, M. Gade (eds.), Remote Sensing of the African Seas,
251
DOI 10.1007/978-94-017-8008-7_13,
© Springer Science+Business Media Dordrecht 2014