12 Observing the Agulhas Current With Sea Surface Temperature . . .
235
The Agulhas Current is the strongest western boundary current of the southern
hemisphere and a major component of the global climate (Beal et al. 2011). On a
regional scale, the Agulhas Current directly influences the oceanography of the continental shelves through a range of meso and sub-meso scale processes such as the shedding of rings, eddies or filaments, as well as the meandering or intrusion of the current
onto the shelf (Lutjeharms 2006). Despite its importance at both the regional and
global scales, the Agulhas Current remains poorly sampled. Only recently was an annual cycle observed in the Agulhas Current (Krug and Tournadre, 2012) and the longterm variability of the Agulhas Current remains largely unknown (Lutjeharms 2006).
Satellite remote sensing provides a powerful mean of observing the Agulhas
Current over the time-scale required to resolve its seasonal and inter-annual variability. Over the last two decades, SST and SSH observations from space have been
widely used by oceanographers to monitor change. Altimetry and Infra-Red (IR)
measurements from space are mature techniques which provide extended observational records (with close to 20 and 30 years of measurements available for SSH and
SST, respectively). Altimetry, through the use of the geostrophic approximation, has
been used with great success to estimate the upper transport and magnitude of the
world’s major ocean currents, track eddies or even discover new currents (Siedler
et al. 2006). SST observations through front-detection techniques have been used
to estimate surface current speeds (Emery 2001) and monitor meso-scale activity in
western boundary current regions (Rouault and Penven 2011).
The Agulhas Current system is a challenging environment for remote sensing
observations as it is affected by a wide range of processes which originate near the
coast or the open ocean and occur over synoptic to inter-annual time-scales. Over the
Indian Ocean, large scale processes such as changes in the wind circulation, westward
propagating Rossby waves or offshore eddies originating from the Mozambique
or Madagascar regions constitute potential drivers of variability for the Agulhas
Current. Variability intrinsic to the Agulhas Current also occurs on a wide range of
scales from small filaments, shear-edge eddies or meanders to the shedding of large
Agulhas Rings at the Retroflection (Lutjeharms 2006). Cloud formation over the
Agulhas Current severely restricts observations from IR SST sensors. Altimeters or
microwave SST sensors which can see through clouds, suffer from the proximity of
the current to the coast over large regions of the Agulhas Current due to factors such
as land contamination or atmospheric errors (Vignudelli et al. 2011).
In this paper we highlight some of the challenges of SST and altimetry observations in the Agulhas Current region and illustrate how the two measurements
techniques can be combined to improve our understanding of the Agulhas Current
dynamics. A presentation of the datasets used is given in Sect. 2. Section 3 discusses
some of the limitations and advantages of IR and microwave observations in the
Agulhas Current region while Sect. 4 provides an example of synergetic use of altimetry and SST to monitor the Agulhas Current. The conclusions to this Chapter
are presented in Sect. 5 together with some perspectives for future observation of the
Agulhas Current system from space.
235
The Agulhas Current is the strongest western boundary current of the southern
hemisphere and a major component of the global climate (Beal et al. 2011). On a
regional scale, the Agulhas Current directly influences the oceanography of the continental shelves through a range of meso and sub-meso scale processes such as the shedding of rings, eddies or filaments, as well as the meandering or intrusion of the current
onto the shelf (Lutjeharms 2006). Despite its importance at both the regional and
global scales, the Agulhas Current remains poorly sampled. Only recently was an annual cycle observed in the Agulhas Current (Krug and Tournadre, 2012) and the longterm variability of the Agulhas Current remains largely unknown (Lutjeharms 2006).
Satellite remote sensing provides a powerful mean of observing the Agulhas
Current over the time-scale required to resolve its seasonal and inter-annual variability. Over the last two decades, SST and SSH observations from space have been
widely used by oceanographers to monitor change. Altimetry and Infra-Red (IR)
measurements from space are mature techniques which provide extended observational records (with close to 20 and 30 years of measurements available for SSH and
SST, respectively). Altimetry, through the use of the geostrophic approximation, has
been used with great success to estimate the upper transport and magnitude of the
world’s major ocean currents, track eddies or even discover new currents (Siedler
et al. 2006). SST observations through front-detection techniques have been used
to estimate surface current speeds (Emery 2001) and monitor meso-scale activity in
western boundary current regions (Rouault and Penven 2011).
The Agulhas Current system is a challenging environment for remote sensing
observations as it is affected by a wide range of processes which originate near the
coast or the open ocean and occur over synoptic to inter-annual time-scales. Over the
Indian Ocean, large scale processes such as changes in the wind circulation, westward
propagating Rossby waves or offshore eddies originating from the Mozambique
or Madagascar regions constitute potential drivers of variability for the Agulhas
Current. Variability intrinsic to the Agulhas Current also occurs on a wide range of
scales from small filaments, shear-edge eddies or meanders to the shedding of large
Agulhas Rings at the Retroflection (Lutjeharms 2006). Cloud formation over the
Agulhas Current severely restricts observations from IR SST sensors. Altimeters or
microwave SST sensors which can see through clouds, suffer from the proximity of
the current to the coast over large regions of the Agulhas Current due to factors such
as land contamination or atmospheric errors (Vignudelli et al. 2011).
In this paper we highlight some of the challenges of SST and altimetry observations in the Agulhas Current region and illustrate how the two measurements
techniques can be combined to improve our understanding of the Agulhas Current
dynamics. A presentation of the datasets used is given in Sect. 2. Section 3 discusses
some of the limitations and advantages of IR and microwave observations in the
Agulhas Current region while Sect. 4 provides an example of synergetic use of altimetry and SST to monitor the Agulhas Current. The conclusions to this Chapter
are presented in Sect. 5 together with some perspectives for future observation of the
Agulhas Current system from space.