12 Observing the Agulhas Current With Sea Surface Temperature . . .
245
0.5 m in SSHA between 31.5
◦ S and 32
◦ S, i.e. over a distance of about 50 km, corresponds to a geostrophic current anomaly of about 1 m/s, directed SW. It is worth
noting that between 31
◦ S and 31.5
◦ S, the along-track profile shows a distinct reversal
in the direction of the current, with the flow switching from a SW direction away
from the coast to a NE flowing current closer to the coast. The presence of a NE
flow near the coast is not apparent in the interpolated map. On 10 March 2008, the
anticyclonic eddy responsible for triggering the Natal Pulse is located at 33
◦ E—
31.3
◦ S and has a diameter of about 200 km. Strong anticyclonic flow is observed
within the anticyclonic eddy with maximum geostrophic rotational speed reaching
about 80 cm/s. Between 18 and 20 March 2008 (not shown in figures), interactions
between the trailing edge of the Natal Pulse and the topography induce an instability
upstream of the Natal Pulse in the shape of a smaller secondary offshore meander.
The development of such upstream instabilities during the southward progression of
Natal Pulses has been described in detail in the study of Rouault and Penven (2011).
SST imagery on 4 April 2008 (Fig. 12.5d) shows that the Natal Pulse has progressed
south to 28
◦ E—34
◦ S with the secondary instability located just south of Port Edward
(29.8
◦ E—32
◦ S). A decrease of about 50 % in the spatial extent of the Natal Pulse
is observed between 15 February 2008 and 10 March 2008. Subsequent SST maps
(not shown here) are not able to highlight the presence of the Natal Pulse and its
upstream generated perturbation after 14 April 2008.
The case study presented here shows how altimetry can be used to track deep
sea eddies and study their interaction with western boundary currents such as the
Agulhas Current. Here we provide additional support to the study of Schouten et al.
(2002), which linked Natal Pulses to the far eddy field. The inception of a Natal Pulse
by an anticyclonic eddy follows a process similar to that described by Tsugawa and
Hasumi (2010) in their modelling study. Closer to the shore the higher resolution and
frequency acquisitions afforded by IR SST imagery proves useful when following
the evolution of Natal Pulses. The initial growth of the Natal Pulse, its interaction
with the coastal and shelf waters and its subsequent dissipation add weight to the
hypothesis put forward by Rouault and Penven (2011), that part of the variability
observed in the northern Agulhas Current is lost downstream.
12.5 Conclusions and Perspectives
Over the last 2 decades, satellite remote sensing observations have provided a costeffective alternative to in situ measurements for many regions of the world’s ocean.
The sampling capabilities of the Agulhas Current from space have significantly improved in recent years due to an increase in satellite spatial and temporal coverage,
the emergence of new remote sensing methods and the use of more robust algorithms
for the derivation of ocean properties.
The launch of the MSG-2 geostationary satellite has provided a major boost for
IR SST observations over the Agulhas Current region. High frequency acquisitions
from the SEVIRI sensor onboard the MSG-2 satellite have markedly improved our
245
0.5 m in SSHA between 31.5
◦ S and 32
◦ S, i.e. over a distance of about 50 km, corresponds to a geostrophic current anomaly of about 1 m/s, directed SW. It is worth
noting that between 31
◦ S and 31.5
◦ S, the along-track profile shows a distinct reversal
in the direction of the current, with the flow switching from a SW direction away
from the coast to a NE flowing current closer to the coast. The presence of a NE
flow near the coast is not apparent in the interpolated map. On 10 March 2008, the
anticyclonic eddy responsible for triggering the Natal Pulse is located at 33
◦ E—
31.3
◦ S and has a diameter of about 200 km. Strong anticyclonic flow is observed
within the anticyclonic eddy with maximum geostrophic rotational speed reaching
about 80 cm/s. Between 18 and 20 March 2008 (not shown in figures), interactions
between the trailing edge of the Natal Pulse and the topography induce an instability
upstream of the Natal Pulse in the shape of a smaller secondary offshore meander.
The development of such upstream instabilities during the southward progression of
Natal Pulses has been described in detail in the study of Rouault and Penven (2011).
SST imagery on 4 April 2008 (Fig. 12.5d) shows that the Natal Pulse has progressed
south to 28
◦ E—34
◦ S with the secondary instability located just south of Port Edward
(29.8
◦ E—32
◦ S). A decrease of about 50 % in the spatial extent of the Natal Pulse
is observed between 15 February 2008 and 10 March 2008. Subsequent SST maps
(not shown here) are not able to highlight the presence of the Natal Pulse and its
upstream generated perturbation after 14 April 2008.
The case study presented here shows how altimetry can be used to track deep
sea eddies and study their interaction with western boundary currents such as the
Agulhas Current. Here we provide additional support to the study of Schouten et al.
(2002), which linked Natal Pulses to the far eddy field. The inception of a Natal Pulse
by an anticyclonic eddy follows a process similar to that described by Tsugawa and
Hasumi (2010) in their modelling study. Closer to the shore the higher resolution and
frequency acquisitions afforded by IR SST imagery proves useful when following
the evolution of Natal Pulses. The initial growth of the Natal Pulse, its interaction
with the coastal and shelf waters and its subsequent dissipation add weight to the
hypothesis put forward by Rouault and Penven (2011), that part of the variability
observed in the northern Agulhas Current is lost downstream.
12.5 Conclusions and Perspectives
Over the last 2 decades, satellite remote sensing observations have provided a costeffective alternative to in situ measurements for many regions of the world’s ocean.
The sampling capabilities of the Agulhas Current from space have significantly improved in recent years due to an increase in satellite spatial and temporal coverage,
the emergence of new remote sensing methods and the use of more robust algorithms
for the derivation of ocean properties.
The launch of the MSG-2 geostationary satellite has provided a major boost for
IR SST observations over the Agulhas Current region. High frequency acquisitions
from the SEVIRI sensor onboard the MSG-2 satellite have markedly improved our