12 Observing the Agulhas Current With Sea Surface Temperature . . .
243
12.4 Combining SSH and SST Observations to Better Resolve
the Circulation of the Northern Agulhas Current
Through its source regions, the Agulhas Current is connected to the variability of
the Indian Ocean. Changes in the Indonesian throughflow as well as wind driven
variability over the South Indian Ocean are transmitted to the source regions of the
Agulhas Current through the South Equatorial Current (SEC) Ridderinkhof et al.
2010). Upon reaching the eastern shore of Madagascar at 15
◦ E, the SEC branches
into a northern and southern flow, feeding two main sources for the Agulhas Current:
the Mozambique and the East Madagascar currents. In the Mozambique channel, the
flow is dominated by the passage of large anticyclonic eddies propagating southwards
towards the Agulhas Current (de Ruijter et al. 2002). These large anticyclonic eddies
occur 5 to 6 times per year (van der Werf et al. 2010), they have spatial scales of
300 to 350 km and move downstream at speeds of 3 to 6 km/day. The contribution
from the SEC to the Agulhas Current occurs in the form of cyclonic and anticyclonic
eddies which tend to drift south-westward towards the northern Agulhas Current
(Biastoch et al. 1999; de Ruijter et al. 2004). Analyses of altimeter data indicate
that on average 4 anticyclonic eddies per year occur south of Madagascar (Schouten
et al. 2003), with the eddy frequency increasing during negative phases of the Indian
Ocean Dipole and El Niño cycles (de Ruijter et al. 2004). Previous observational
and numerical modelling studies have shown that anticyclonic eddies reaching the
eastern boundary of the Agulhas Current can trigger the formation of large offshore
meanders in the Natal Bight region (Schouten et al. 2002; Tsugawa and Hasumi
2010). These large offshore meanders, named Natal Pulses due to their region of
origin (Lutjeharms and Roberts 1988) are major drivers of variability in the Agulhas
Current (Bryden et al. 2005; Lutjeharms 2006).
Altimetry and SST imagery can be combined to follow the trajectory of anticyclonic eddies in the southwest Indian Ocean and study their interaction as they reach
the eastern edge of the northern Agulhas Current. In the next section, a case study of
Natal Pulse’s inception by an anticyclonic eddy is provided. The path and property of
the anticyclonic eddy are extracted from the Chelton et al. (2011) global database of
oceanic eddies. Daily composites of OSI-SAF SST are used to follow the evolution
and fate of the Natal Pulse as it progresses downstream.
On 18 July 2007, an anticyclonic eddy is detected to the south east of Madagascar,
at 48.5
◦ E—27
◦ S. The anticyclone then propagates westwards with a mean speed of
about 8 km/day, until it reaches the eastern boundary of the northern Agulhas Current around 15 February 2008 (Fig. 12.5a). On15 February 2008 the anticyclonic
eddy is centred at 34.5
◦ E—28.8
◦ S, it has diameter of about 230 km and maximum
geostrophic rotational velocities of about 65 cm/s. Interactions between the eddy
and the eastern edge of the Agulhas Current induce the development of a small
perturbation at the inshore border of the Agulhas Current. This small perturbation
in the Agulhas Current is captured in the daily composite map of the OSI-SAF
SST (Fig. 12.5b) and is centred at 32.5
◦ E—29.2
◦ S. Due to its small spatial scale,
this offshore perturbation is not successfully identified in the AVISO map of absolute geostrophic currents. By 10 March 2008, the small perturbation initiated on
243
12.4 Combining SSH and SST Observations to Better Resolve
the Circulation of the Northern Agulhas Current
Through its source regions, the Agulhas Current is connected to the variability of
the Indian Ocean. Changes in the Indonesian throughflow as well as wind driven
variability over the South Indian Ocean are transmitted to the source regions of the
Agulhas Current through the South Equatorial Current (SEC) Ridderinkhof et al.
2010). Upon reaching the eastern shore of Madagascar at 15
◦ E, the SEC branches
into a northern and southern flow, feeding two main sources for the Agulhas Current:
the Mozambique and the East Madagascar currents. In the Mozambique channel, the
flow is dominated by the passage of large anticyclonic eddies propagating southwards
towards the Agulhas Current (de Ruijter et al. 2002). These large anticyclonic eddies
occur 5 to 6 times per year (van der Werf et al. 2010), they have spatial scales of
300 to 350 km and move downstream at speeds of 3 to 6 km/day. The contribution
from the SEC to the Agulhas Current occurs in the form of cyclonic and anticyclonic
eddies which tend to drift south-westward towards the northern Agulhas Current
(Biastoch et al. 1999; de Ruijter et al. 2004). Analyses of altimeter data indicate
that on average 4 anticyclonic eddies per year occur south of Madagascar (Schouten
et al. 2003), with the eddy frequency increasing during negative phases of the Indian
Ocean Dipole and El Niño cycles (de Ruijter et al. 2004). Previous observational
and numerical modelling studies have shown that anticyclonic eddies reaching the
eastern boundary of the Agulhas Current can trigger the formation of large offshore
meanders in the Natal Bight region (Schouten et al. 2002; Tsugawa and Hasumi
2010). These large offshore meanders, named Natal Pulses due to their region of
origin (Lutjeharms and Roberts 1988) are major drivers of variability in the Agulhas
Current (Bryden et al. 2005; Lutjeharms 2006).
Altimetry and SST imagery can be combined to follow the trajectory of anticyclonic eddies in the southwest Indian Ocean and study their interaction as they reach
the eastern edge of the northern Agulhas Current. In the next section, a case study of
Natal Pulse’s inception by an anticyclonic eddy is provided. The path and property of
the anticyclonic eddy are extracted from the Chelton et al. (2011) global database of
oceanic eddies. Daily composites of OSI-SAF SST are used to follow the evolution
and fate of the Natal Pulse as it progresses downstream.
On 18 July 2007, an anticyclonic eddy is detected to the south east of Madagascar,
at 48.5
◦ E—27
◦ S. The anticyclone then propagates westwards with a mean speed of
about 8 km/day, until it reaches the eastern boundary of the northern Agulhas Current around 15 February 2008 (Fig. 12.5a). On15 February 2008 the anticyclonic
eddy is centred at 34.5
◦ E—28.8
◦ S, it has diameter of about 230 km and maximum
geostrophic rotational velocities of about 65 cm/s. Interactions between the eddy
and the eastern edge of the Agulhas Current induce the development of a small
perturbation at the inshore border of the Agulhas Current. This small perturbation
in the Agulhas Current is captured in the daily composite map of the OSI-SAF
SST (Fig. 12.5b) and is centred at 32.5
◦ E—29.2
◦ S. Due to its small spatial scale,
this offshore perturbation is not successfully identified in the AVISO map of absolute geostrophic currents. By 10 March 2008, the small perturbation initiated on