11 Satellite Observations of Oceanic Eddies Around Africa
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eddy, it is associated with upwelling of cold water. Cold eddies are prone to carry
with them much biota which are sources of surface active material, which ascends
to the sea surface and forms there biogenic surface films.
11.4 Infrared and Optical Images
Oceanic eddies can also be detected from space by infrared and optical sensors
provided that there is no or only little cloud coverage. Infrared sensors measure
SST and optical sensors the ocean color from which the CHL distribution is derived.
Figure 11.6 shows a reflection map derived from data acquired by the MODIS sensor
onboard the Terra satellite in the 555 nm band at 0910 UTC on 26 December 2011.
It shows an anti-cyclonic mesoscale eddy (rotating anticlockwise in the southern
hemisphere) with a diameter of approximately 150 km located 800 km south of South
Africa. The eddy becomes visible in light blue due to high scattering by phytoplankton
(probably coccolithophorids). Observations and computer simulations have shown
that such eddies are frequently encountered in this ocean region and that they spin
off from the Agulhas current.
Figure 11.7 shows a map of the CHL distribution in the waters west of the coasts
of Mauretania, Senegal and Guinea. It was derived from data acquired by the MODIS
sensor onboard the Aqua satellite at 1440 UTC on 16 November 2006. Visible is a
large-scale anti-cyclonic eddy (which rotates clockwise in the northern hemisphere)
by means of a band of high CHL concentration (brownish colors). It is located west of
the coast of Senegal/Guinea and has a diameter of approximately 150 km. This band
of high CHL concentration forming the rim of the eddy is connected with the area of
high CHL concentration at the coast of Guinea. This suggests that the anticyclonic
eddy has drawn its high CHL concentrated in the spiral arm from the upwelling area at
the coast of Guinea. Note that the CHL concentration in the center of the eddy is very
low. Such anti-cyclonic eddies with high CHL concentration at the rim of anticyclonic
eddies have also been observed in other parts of the World’s ocean, among them in the
Oyahio Current region in the western North Pacific. Kusakabe et al. (2002) observed
that boundaries of anticyclonic eddies in the Oyashio Current region contain coastal
Oyashio waters with high biological activity, like in our case (Fig. 11.7), where the
rim of the anticyclonic eddy contains waters with high biological activity.
Figures 11.8, 11.9 and 11.10 show SST and CHL maps based on MODIS data on
which signatures of small-scale cyclonic eddies are visible. All of them are located
off the coast of Senegal and were captured between November and December when
upwelling starts at the Senegalese coast. In Sect. 11.5 we shall show more images of
this type. The two SST maps depicted in Fig. 11.8 have been derived from MODIS
data acquired at 1440 UTC on 7 November 2006 and at 1440 UTC on 16 November
2006, respectively. They show signatures of a small-scale eddy that was generated
at Cap-Vert (Fig. 11.8a), presumably by flow separation (see Sect. 11.5), which then
propagated northwestward into the Atlantic. Also visible are on Fig. 11.8a, west of
this eddy, signatures of several very small-scale eddies. They are not visible anymore
on Fig. 11.8b, presumably because they have dissipated.
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eddy, it is associated with upwelling of cold water. Cold eddies are prone to carry
with them much biota which are sources of surface active material, which ascends
to the sea surface and forms there biogenic surface films.
11.4 Infrared and Optical Images
Oceanic eddies can also be detected from space by infrared and optical sensors
provided that there is no or only little cloud coverage. Infrared sensors measure
SST and optical sensors the ocean color from which the CHL distribution is derived.
Figure 11.6 shows a reflection map derived from data acquired by the MODIS sensor
onboard the Terra satellite in the 555 nm band at 0910 UTC on 26 December 2011.
It shows an anti-cyclonic mesoscale eddy (rotating anticlockwise in the southern
hemisphere) with a diameter of approximately 150 km located 800 km south of South
Africa. The eddy becomes visible in light blue due to high scattering by phytoplankton
(probably coccolithophorids). Observations and computer simulations have shown
that such eddies are frequently encountered in this ocean region and that they spin
off from the Agulhas current.
Figure 11.7 shows a map of the CHL distribution in the waters west of the coasts
of Mauretania, Senegal and Guinea. It was derived from data acquired by the MODIS
sensor onboard the Aqua satellite at 1440 UTC on 16 November 2006. Visible is a
large-scale anti-cyclonic eddy (which rotates clockwise in the northern hemisphere)
by means of a band of high CHL concentration (brownish colors). It is located west of
the coast of Senegal/Guinea and has a diameter of approximately 150 km. This band
of high CHL concentration forming the rim of the eddy is connected with the area of
high CHL concentration at the coast of Guinea. This suggests that the anticyclonic
eddy has drawn its high CHL concentrated in the spiral arm from the upwelling area at
the coast of Guinea. Note that the CHL concentration in the center of the eddy is very
low. Such anti-cyclonic eddies with high CHL concentration at the rim of anticyclonic
eddies have also been observed in other parts of the World’s ocean, among them in the
Oyahio Current region in the western North Pacific. Kusakabe et al. (2002) observed
that boundaries of anticyclonic eddies in the Oyashio Current region contain coastal
Oyashio waters with high biological activity, like in our case (Fig. 11.7), where the
rim of the anticyclonic eddy contains waters with high biological activity.
Figures 11.8, 11.9 and 11.10 show SST and CHL maps based on MODIS data on
which signatures of small-scale cyclonic eddies are visible. All of them are located
off the coast of Senegal and were captured between November and December when
upwelling starts at the Senegalese coast. In Sect. 11.5 we shall show more images of
this type. The two SST maps depicted in Fig. 11.8 have been derived from MODIS
data acquired at 1440 UTC on 7 November 2006 and at 1440 UTC on 16 November
2006, respectively. They show signatures of a small-scale eddy that was generated
at Cap-Vert (Fig. 11.8a), presumably by flow separation (see Sect. 11.5), which then
propagated northwestward into the Atlantic. Also visible are on Fig. 11.8a, west of
this eddy, signatures of several very small-scale eddies. They are not visible anymore
on Fig. 11.8b, presumably because they have dissipated.