258
J. A. Johannessen et al.
quite comparable. However, in contrast to the comparison for the Agulhas Current
the Rio09 surface geostrophic current is stronger than the ASAR based radial velocity for the Agulhas Return Current, reaching up to a zonal speed of 0.85 and 0.70 m/s
respectively. This is probably resulting from more variability in the return current
that increases the magnitude of the SLA and hence the surface geostrophic current.
Finally the mean width of the Agulhas Current core is also fairly comparable being
close to about 100 km. As the time integration of the two derived velocity fields are
rather different (17 versus 2 years) the agreement suggest that the representation of
the mean velocities of the greater Agulhas Current is rather robust.
13.3.3 Sea Surface Temperature Field
The surface thermal expression of the Agulhas Current as portrayed in cloud free
infrared satellite images is characterized by a very sharp and rectilinear shore-ward
boundary, extending southward from the separation at about 33
◦ S with a Sea Surface
Temperature (SST) change ranging from 4 to 6
◦ C (Lutjeharms 2006). Overlaying
the mean SST field, obtained from the ODYSSEA global SST analysis products
6 ,
on the mean range Doppler velocity field (Fig. 13.2) averaged for the same period
reveals good agreement and consistency, as seen in Fig. 13.5.
The structure of SST maxima are clearly collocated and oriented with the mean
range Doppler velocity maxima from the separation point to about 22
◦ E. The gradual
southward turning and spreading of the SST field southwestward from this region
is also consistent with the pattern in the velocity map, as is the meandering SST
contours representing the Agulhas Return Current eastward from the retroflection
point to about 28
◦ E. The combined effect of the strong surface current and SST front
implies that significant modification of the surface stress (up to 25–30 %) will occur
across this sharp and coupled front in surface current and temperature in agreement
with Chelton et al. 2004.
13.3.4 Ocean Model Field
The ocean general circulation model used in this study is the Hybrid Coordinate
Ocean Model (HYCOM). HYCOM combines the optimal features of isopycniccoordinate and fixed-grid ocean circulation models in one framework (Bleck 2002).
The name “hybrid” is derived from the models ability to efficiently change from
isopycnal to Cartesian coordinates. The adaptive (hybrid) vertical grid conveniently
resolves regions of vertical density gradients, such as the thermocline and surface
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/.
J. A. Johannessen et al.
quite comparable. However, in contrast to the comparison for the Agulhas Current
the Rio09 surface geostrophic current is stronger than the ASAR based radial velocity for the Agulhas Return Current, reaching up to a zonal speed of 0.85 and 0.70 m/s
respectively. This is probably resulting from more variability in the return current
that increases the magnitude of the SLA and hence the surface geostrophic current.
Finally the mean width of the Agulhas Current core is also fairly comparable being
close to about 100 km. As the time integration of the two derived velocity fields are
rather different (17 versus 2 years) the agreement suggest that the representation of
the mean velocities of the greater Agulhas Current is rather robust.
13.3.3 Sea Surface Temperature Field
The surface thermal expression of the Agulhas Current as portrayed in cloud free
infrared satellite images is characterized by a very sharp and rectilinear shore-ward
boundary, extending southward from the separation at about 33
◦ S with a Sea Surface
Temperature (SST) change ranging from 4 to 6
◦ C (Lutjeharms 2006). Overlaying
the mean SST field, obtained from the ODYSSEA global SST analysis products
6 ,
on the mean range Doppler velocity field (Fig. 13.2) averaged for the same period
reveals good agreement and consistency, as seen in Fig. 13.5.
The structure of SST maxima are clearly collocated and oriented with the mean
range Doppler velocity maxima from the separation point to about 22
◦ E. The gradual
southward turning and spreading of the SST field southwestward from this region
is also consistent with the pattern in the velocity map, as is the meandering SST
contours representing the Agulhas Return Current eastward from the retroflection
point to about 28
◦ E. The combined effect of the strong surface current and SST front
implies that significant modification of the surface stress (up to 25–30 %) will occur
across this sharp and coupled front in surface current and temperature in agreement
with Chelton et al. 2004.
13.3.4 Ocean Model Field
The ocean general circulation model used in this study is the Hybrid Coordinate
Ocean Model (HYCOM). HYCOM combines the optimal features of isopycniccoordinate and fixed-grid ocean circulation models in one framework (Bleck 2002).
The name “hybrid” is derived from the models ability to efficiently change from
isopycnal to Cartesian coordinates. The adaptive (hybrid) vertical grid conveniently
resolves regions of vertical density gradients, such as the thermocline and surface
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/.