252
J. A. Johannessen et al.
15 km from its mean path (Gründlingh 1983). Observations from a current meter
mooring near 31
◦ S have shown that the core of the current lies within 31 km off the
coast almost 80 % of the time, displaying surface currents up to 200 cm s
−1 (Bryden
et al. 2005). Southward from 35
◦ S the continental shelf widens forcing the current
to separate from the coast and flow southwestward along the Agulhas Bank. In this
region the current is becoming increasingly unstable exhibiting numerous meanders,
plumes and eddies (Lutjeharms et al. 1989).
In recent years the importance of the greater Agulhas Current has become recognized (Lutjeharms and Bornman 2010). It plays a significant role in the Indo-Atlantic
inter-ocean exchange and global thermohaline circulation (Lutjeharms 2006). Using
in situ current meter measurements, Bryden et al. (2005) calculated the average poleward volume transport of the Agulhas Current (from 5 March–27 November 1995)
to be about 70+/−22 Sv (1 Sv = 10
6 m
3 s
−1 ). Biastoch et al. (2009) associated the
effect of Agulhas leakage dynamics of inter-basin heat and volume fluxes at decadal
timescales to the decadal variability in the Atlantic overturning circulation. Recently
Rouault et al. (2009) pointed out an augmentation of the transport since the 1980s
in response to an increase in wind stress curl in the South Indian Ocean. Yet quantitative knowledge of the seasonal to inter-annual current and transport variability
in the greater Agulhas Current regime is incomplete and not adequate to understand
the spatial and temporal variability and eddy shedding (leakage) processes into the
South Atlantic Ocean.
Modeling the greater Agulhas Current system is challenging, in particular in the
very energetic and complex retroflection region, and validation of the model fields
often lack in situ data with adequate spatial and temporal resolution. In this regard
the regular monitoring of the surface geostrophic current from satellite altimetry has
played a very important role both for validation and data assimilation, except near
shore where the altimeter measurements are hampered by uncertainties in the correction terms. This can be compensated for by using the Advanced Synthetic Aperture
Radar (ASAR)
1 range Doppler signals following the approach outlined by Chapron
et al. (2005) and further applied by Johannessen et al. (2008) and Rouault et al. (2010).
In this paper we investigate and assess the surface velocity estimates of the greater
Agulhas Current from the Doppler based method, in combination with surface drifter
data, satellite altimetry, sea surface temperature measurements, and the simulated
surface velocity field from the Hybrid Coordinate Ocean Model (HYCOM).
13.2 Retrieving Surface Velocities from ASAR
Since mid 2007 regular access to Envisat ASAR Wide Swath Mode (WSM) Doppler
grid information has led to the creation of a comprehensive data set of Doppler
anomalies including more than 1,000 synoptic wide coverage acquisitions with a
high-resolution grid (12.5 km × 12.5 km) of the greater Agulhas Current. These data
1 Onboard the Envisat orbital platform of the European Space Agency (ESA).
J. A. Johannessen et al.
15 km from its mean path (Gründlingh 1983). Observations from a current meter
mooring near 31
◦ S have shown that the core of the current lies within 31 km off the
coast almost 80 % of the time, displaying surface currents up to 200 cm s
−1 (Bryden
et al. 2005). Southward from 35
◦ S the continental shelf widens forcing the current
to separate from the coast and flow southwestward along the Agulhas Bank. In this
region the current is becoming increasingly unstable exhibiting numerous meanders,
plumes and eddies (Lutjeharms et al. 1989).
In recent years the importance of the greater Agulhas Current has become recognized (Lutjeharms and Bornman 2010). It plays a significant role in the Indo-Atlantic
inter-ocean exchange and global thermohaline circulation (Lutjeharms 2006). Using
in situ current meter measurements, Bryden et al. (2005) calculated the average poleward volume transport of the Agulhas Current (from 5 March–27 November 1995)
to be about 70+/−22 Sv (1 Sv = 10
6 m
3 s
−1 ). Biastoch et al. (2009) associated the
effect of Agulhas leakage dynamics of inter-basin heat and volume fluxes at decadal
timescales to the decadal variability in the Atlantic overturning circulation. Recently
Rouault et al. (2009) pointed out an augmentation of the transport since the 1980s
in response to an increase in wind stress curl in the South Indian Ocean. Yet quantitative knowledge of the seasonal to inter-annual current and transport variability
in the greater Agulhas Current regime is incomplete and not adequate to understand
the spatial and temporal variability and eddy shedding (leakage) processes into the
South Atlantic Ocean.
Modeling the greater Agulhas Current system is challenging, in particular in the
very energetic and complex retroflection region, and validation of the model fields
often lack in situ data with adequate spatial and temporal resolution. In this regard
the regular monitoring of the surface geostrophic current from satellite altimetry has
played a very important role both for validation and data assimilation, except near
shore where the altimeter measurements are hampered by uncertainties in the correction terms. This can be compensated for by using the Advanced Synthetic Aperture
Radar (ASAR)
1 range Doppler signals following the approach outlined by Chapron
et al. (2005) and further applied by Johannessen et al. (2008) and Rouault et al. (2010).
In this paper we investigate and assess the surface velocity estimates of the greater
Agulhas Current from the Doppler based method, in combination with surface drifter
data, satellite altimetry, sea surface temperature measurements, and the simulated
surface velocity field from the Hybrid Coordinate Ocean Model (HYCOM).
13.2 Retrieving Surface Velocities from ASAR
Since mid 2007 regular access to Envisat ASAR Wide Swath Mode (WSM) Doppler
grid information has led to the creation of a comprehensive data set of Doppler
anomalies including more than 1,000 synoptic wide coverage acquisitions with a
high-resolution grid (12.5 km × 12.5 km) of the greater Agulhas Current. These data
1 Onboard the Envisat orbital platform of the European Space Agency (ESA).