60
N. Hoepffner et al.
All together, these currents contribute to a large extent to shape the global climate
through exchange of heat/temperature and salt with adjacent waters (Tyson et al.
2002). Along the eastern coast of Africa, the seasonally-reversing monsoon winds
over the Indian Ocean translate into the SST fields with a strong South Equatorial Current (22–23
◦ C) flowing westward from June to September (southwest monsoon) and
feeding both the southward currents on either side of Madagascar and the northward
Somali current. During the northeast monsoon (December–February), the flow reverse in the northern Indian Ocean carrying colder water westward and flowing south
along the Somali and Kenya coast until it mixes with the warmer South Equatorial
Current.
The benefit of satellite TIR data was exploited as early as the late 70’s and early
80’s to provide synoptic views of these strategic and dynamically-complex regions,
and in particular the Aguhlas Current system (Lutjeharms 1981; Lutjeharms and
Roberts 1988; see also references in Lutjeharms 2007). A large number of peerreviewed publications and books (e.g. Lutjeharms 2006) were specifically dedicated
to the Agulhas Current in which TIR images have an important role to describe the
surface dynamics. Basically, the current flows southwestward along the southern
African coast carrying warm waters from the Mozambique Channel and eastern
Madagascar down to the southern tip of South Africa, where it get trapped within
the Antarctic Circumpolar Current creating some spectacular mesoscale dynamics
eastward along the Subtropical Convergence (Olson and Evans 1986). In Fig. 3.2, a
monthly time series of MODIS SST data for 2005 has been analyzed using a threshold
value in order to mask temperatures below 20 or 22
◦ C depending on the season. As
a result, the sequence of images show the variability of the Agulhas flow, collecting
waters from the South Equatorial Current on either side of Madagascar into a jet-like
structure flowing along the South African coast until it retroflects to the east offshore
Port Elizabeth, creating meanders and eddies as it collides with the south Atlantic
waters.
Using TIR data from Meteosat II, Lutjeharms (1988) was also able to observe the
shedding of a warm eddy detached from the Agulhas Current and crossing the SubTropical Convergence and contributing to meridional heat transport at the border of
the Southern Ocean. The TIR images also revealed some Agulhas eddies formed at
the tip of South Africa and propagating into the Atlantic ocean. The variability of this
so-called Agulhas leakage could have an important impact on the overall Atlantic
overturning circulation (Whittle et al. 2008; Beal et al. 2011).
The stability of the Agulhas Current along the steep continental shelf is occasionally perturbed by transient events, so-called Natal pulses, generated by
slight changes in the shelf width off Natal Bight (Lutjeharms and Roberts 1988;
Lutjeharms et al. 2003; Bryden et al. 2005; Lutjeharms 2006). These instabilities are
described as large solitary meanders associated with cold-water core readily observable within the warmer Agulhas Current from satellite TIR images. Using 6 years of
data collected from the Spinning Enhanced Visible and Infra-Red Imager (SEVIRI),
onboard the MSG geostationary satellite, Rouault and Penven (2011) provided a
detailed analysis of these features occurring on average at a rate of 1.6 pulses per
year, i.e. significantly less than previous estimates based on field observations and/or
N. Hoepffner et al.
All together, these currents contribute to a large extent to shape the global climate
through exchange of heat/temperature and salt with adjacent waters (Tyson et al.
2002). Along the eastern coast of Africa, the seasonally-reversing monsoon winds
over the Indian Ocean translate into the SST fields with a strong South Equatorial Current (22–23
◦ C) flowing westward from June to September (southwest monsoon) and
feeding both the southward currents on either side of Madagascar and the northward
Somali current. During the northeast monsoon (December–February), the flow reverse in the northern Indian Ocean carrying colder water westward and flowing south
along the Somali and Kenya coast until it mixes with the warmer South Equatorial
Current.
The benefit of satellite TIR data was exploited as early as the late 70’s and early
80’s to provide synoptic views of these strategic and dynamically-complex regions,
and in particular the Aguhlas Current system (Lutjeharms 1981; Lutjeharms and
Roberts 1988; see also references in Lutjeharms 2007). A large number of peerreviewed publications and books (e.g. Lutjeharms 2006) were specifically dedicated
to the Agulhas Current in which TIR images have an important role to describe the
surface dynamics. Basically, the current flows southwestward along the southern
African coast carrying warm waters from the Mozambique Channel and eastern
Madagascar down to the southern tip of South Africa, where it get trapped within
the Antarctic Circumpolar Current creating some spectacular mesoscale dynamics
eastward along the Subtropical Convergence (Olson and Evans 1986). In Fig. 3.2, a
monthly time series of MODIS SST data for 2005 has been analyzed using a threshold
value in order to mask temperatures below 20 or 22
◦ C depending on the season. As
a result, the sequence of images show the variability of the Agulhas flow, collecting
waters from the South Equatorial Current on either side of Madagascar into a jet-like
structure flowing along the South African coast until it retroflects to the east offshore
Port Elizabeth, creating meanders and eddies as it collides with the south Atlantic
waters.
Using TIR data from Meteosat II, Lutjeharms (1988) was also able to observe the
shedding of a warm eddy detached from the Agulhas Current and crossing the SubTropical Convergence and contributing to meridional heat transport at the border of
the Southern Ocean. The TIR images also revealed some Agulhas eddies formed at
the tip of South Africa and propagating into the Atlantic ocean. The variability of this
so-called Agulhas leakage could have an important impact on the overall Atlantic
overturning circulation (Whittle et al. 2008; Beal et al. 2011).
The stability of the Agulhas Current along the steep continental shelf is occasionally perturbed by transient events, so-called Natal pulses, generated by
slight changes in the shelf width off Natal Bight (Lutjeharms and Roberts 1988;
Lutjeharms et al. 2003; Bryden et al. 2005; Lutjeharms 2006). These instabilities are
described as large solitary meanders associated with cold-water core readily observable within the warmer Agulhas Current from satellite TIR images. Using 6 years of
data collected from the Spinning Enhanced Visible and Infra-Red Imager (SEVIRI),
onboard the MSG geostationary satellite, Rouault and Penven (2011) provided a
detailed analysis of these features occurring on average at a rate of 1.6 pulses per
year, i.e. significantly less than previous estimates based on field observations and/or