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.
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Notes
- 1.
Onboard the Envisat orbital platform of the European Space Agency (ESA).
- 2.
Rain impacts the roughness and the local wind speed at a scale not resolved by Numerical Weather Prediction (NWP) models.
- 3.
Available at http://www.aoml.noaa.gov/phod/dac/gdp.html.
- 4.
See the Centre National d’Etudes Spatiales (CNES) website at www.aviso.oceanobs.com.
- 5.
Rio09 is the shorthand for Mean Dynamic Topography, after Rio et al. 2011. When surface geostrophic current is expressed as Rio09, it is implied that the mean dynamic topography has been inverted to surface geostrophic current assuming geostrophic balance.
- 6.
Available at the Centre ERS d’Archivage et de Traitement (CERSAT), i.e. the French ERS Processing and Archiving Facility. See http://cersat.ifremer.fr/.
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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.
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Johannessen, J., Chapron, B., Collard, F., Backeberg, B. (2014). Use of SAR data to Monitor the Greater Agulhas Current. In: Barale, V., Gade, M. (eds) Remote Sensing of the African Seas. Springer, Dordrecht. https://doi.org/10.1007/978-94-017-8008-7_13
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