246
M. Krug et al.
ability to image the Agulhas Current from space. The SEVIRI SST imagery also
provides an alternative to altimetry for tracking meso to sub-mesoscale features
of the circulation, particularly in the northern Agulhas Current region where SSH
observations from space are seriously compromised due to the proximity of the
current to the coast. In addition, new generation of SST datasets have emerged
under the GHRSST initiative. The currently available GHRSST dataset merge IR
and microwave SST observations to improve the resolution and coverage of SST
observations. Future research to validate and utilise these high resolution merged
SST products could help improve our understanding of the Agulhas Current.
But cloud contamination and inadequate cloud masking procedures continue to
hamper our ability to monitor the variability of the Agulhas Current from SST observations. The northern Agulhas Current and the coastal regions near Port Elizabeth
remain poorly sampled by IR sensors and can not be resolved in Microwave SST
imagery. Comparisons between the reprocessed MODIS and Pathfinder v5.2 datasets
in Section 3 highlight some of the limitation of using global based cloud masking
algorithm to assess long-term changes in the Agulhas Current. The better climatology and coverage of the reprocessed MODIS SST dataset, when compared to the
Pathfinder v5.2 SST, clearly showed that regional-focused cloud masking procedures
could significantly improve the quality of future SST products in the Agulhas Current
region.
Altimetry remains very valuable for tracking deep-sea eddies and investigating
their evolution at the seaward edge of the Agulhas Current. However in the northern
Agulhas, the proximity of the current to the coast continues to challenge satellitebased observations of the Agulhas Current from altimeters. Projects such as the
ESA-funded COASTALT and CNES-funded PISTACH, aiming at recovering useful
altimetric measurements in the coastal zone, have demonstrated that is it possible to
improve the quality of altimeter-based observations near the coast with anticipated
benefits in the northern Agulhas and the coastal and shelf regions of the southern
Agulhas. Surface current information derived from SARs could also contribute significantly to improving our knowledge of the northern Agulhas Current in the future.
The principle of surface current measurements from SARs involves the extraction
of a line of sight velocity from information contained in the frequency spectrum of
the returned radar echoes. The line of sight velocity can then be projected onto a
horizontal plane to provide a range-directed surface current velocity. Between July
2007 and April 2012, maps of range-directed surface current velocities in the Agulhas
Current region were systematically recovered from the now defunct Envisat’s ASAR,
under the ESA funded SAR ocean wind-wave-current project (Collard et al. 2008;
Johannessen et al. 2008). An assessment of the ASAR surface current velocities in
the Agulhas Current region (Rouault et al. 2010) showed that the synoptic nature
and relatively high resolution of ASAR acquisitions make the ASAR derived current
velocities a good complement to altimetry for the study of sub-mesoscale processes
in the Agulhas Current. SAR derived ocean surface currents will form part of the data
stream routinely provided within the ESA driven Sentinel-1 satellite mission, which
is due to be launched in 2013. With the planned deployments of SAR sensors in future Sentinel missions and the ongoing SAR observations available from Radarsat,
the future of the SAR-derived ocean surface currents from space looks promising.
M. Krug et al.
ability to image the Agulhas Current from space. The SEVIRI SST imagery also
provides an alternative to altimetry for tracking meso to sub-mesoscale features
of the circulation, particularly in the northern Agulhas Current region where SSH
observations from space are seriously compromised due to the proximity of the
current to the coast. In addition, new generation of SST datasets have emerged
under the GHRSST initiative. The currently available GHRSST dataset merge IR
and microwave SST observations to improve the resolution and coverage of SST
observations. Future research to validate and utilise these high resolution merged
SST products could help improve our understanding of the Agulhas Current.
But cloud contamination and inadequate cloud masking procedures continue to
hamper our ability to monitor the variability of the Agulhas Current from SST observations. The northern Agulhas Current and the coastal regions near Port Elizabeth
remain poorly sampled by IR sensors and can not be resolved in Microwave SST
imagery. Comparisons between the reprocessed MODIS and Pathfinder v5.2 datasets
in Section 3 highlight some of the limitation of using global based cloud masking
algorithm to assess long-term changes in the Agulhas Current. The better climatology and coverage of the reprocessed MODIS SST dataset, when compared to the
Pathfinder v5.2 SST, clearly showed that regional-focused cloud masking procedures
could significantly improve the quality of future SST products in the Agulhas Current
region.
Altimetry remains very valuable for tracking deep-sea eddies and investigating
their evolution at the seaward edge of the Agulhas Current. However in the northern
Agulhas, the proximity of the current to the coast continues to challenge satellitebased observations of the Agulhas Current from altimeters. Projects such as the
ESA-funded COASTALT and CNES-funded PISTACH, aiming at recovering useful
altimetric measurements in the coastal zone, have demonstrated that is it possible to
improve the quality of altimeter-based observations near the coast with anticipated
benefits in the northern Agulhas and the coastal and shelf regions of the southern
Agulhas. Surface current information derived from SARs could also contribute significantly to improving our knowledge of the northern Agulhas Current in the future.
The principle of surface current measurements from SARs involves the extraction
of a line of sight velocity from information contained in the frequency spectrum of
the returned radar echoes. The line of sight velocity can then be projected onto a
horizontal plane to provide a range-directed surface current velocity. Between July
2007 and April 2012, maps of range-directed surface current velocities in the Agulhas
Current region were systematically recovered from the now defunct Envisat’s ASAR,
under the ESA funded SAR ocean wind-wave-current project (Collard et al. 2008;
Johannessen et al. 2008). An assessment of the ASAR surface current velocities in
the Agulhas Current region (Rouault et al. 2010) showed that the synoptic nature
and relatively high resolution of ASAR acquisitions make the ASAR derived current
velocities a good complement to altimetry for the study of sub-mesoscale processes
in the Agulhas Current. SAR derived ocean surface currents will form part of the data
stream routinely provided within the ESA driven Sentinel-1 satellite mission, which
is due to be launched in 2013. With the planned deployments of SAR sensors in future Sentinel missions and the ongoing SAR observations available from Radarsat,
the future of the SAR-derived ocean surface currents from space looks promising.