11 Satellite Observations of Oceanic Eddies Around Africa
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confined to the upper layer of the ocean (Eldevik and Dysthe 2002). Furthermore,
they are a source of kinetic energy for the larger scale flow. Like mesoscale eddies,
also small-scale eddies can be generated by several processes, such as (1) interaction
of large-scale currents with bottom topography, islands or headlands, (2) barotropic
or baroclinic instability of currents and fronts, or (3) by atmospheric forcing (vorticity input from wind stress). Recently Golitsyn (2012) suggested that the majority
of the small-scale eddies are generated by convection in the water caused by surface
cooling.
Another detailed study of small-scale eddies was recently carried out by Karimova
(2012). She analyzed over 2000 SAR images acquired between 2009 and 2010 by
the European Remote Satellites ERS-1 and ERS-2 and the European Envisat satellite
over the Baltic Sea, the Black Sea, and the Caspian Sea. She detected on them more
than 14,000 radar signatures of vortical structures with diameters between 1 and
75 km. About 99 % of them had diameters in the range 1–20 km and 98 % had a
cyclonic rotation.
In this paper we present several satellite images showing sea surface signatures
of meso- and small-scale eddies around Africa. Prominent areas where such eddies
are encountered are the upwelling regions off West Africa and the Agulhas Current
at the east coast of Africa which frequently sheds eddies.
This paper is organized as follows: In Sect. 11.2 we present three photographs
taken by a handheld camera from Space Shuttle over areas of the Mediterranean
close to the Egyptian coast, On these photographs, eddies are imaged in the sunglint.
They become visible on these photographs via surface films which trace the eddy’s
surface current field and modify the reflected light. In Sects. 11.3 and 11.4 we present
SAR and optical and infrared images, respectively, showing signatures of eddies in
the waters surrounding Africa. In Sect. 11.5 we present results from a case study
showing the propagation of a small-scale eddy from its birth place in the upwelling
region near Cap-Vert, Senegal, into the open North Atlantic. It transported nutrients
from the Senegal upwelling region westward into the oligotrophic North Atlantic,
where it gave rise to enhanced CHL concentration. Finally, in Sect. 11.6 we discuss
and summarize the results.
11.2 Sunglint Images
Sunglint refers to the reflection of incoming solar radiation from the sea surface
observed by optical (visible/near-infrared) sensors under specular or near-specular
conditions, where the sunlight’s angle of incidence equals the angle of reflection to
the observer. On a flat, undisturbed sea surface the sun’s reflection would simply
appear as a mirror image of its disk in the sky. However, the surface of the ocean
is almost always roughened due the action of the wind such that the “mirror” is
broken into many small facets tilted at a variety of different angles and directions.
This sea surface roughening makes it possible to observe specular reflected sunlight
by suitably tilted facets even when its viewing angle is not directed at the specular
207
confined to the upper layer of the ocean (Eldevik and Dysthe 2002). Furthermore,
they are a source of kinetic energy for the larger scale flow. Like mesoscale eddies,
also small-scale eddies can be generated by several processes, such as (1) interaction
of large-scale currents with bottom topography, islands or headlands, (2) barotropic
or baroclinic instability of currents and fronts, or (3) by atmospheric forcing (vorticity input from wind stress). Recently Golitsyn (2012) suggested that the majority
of the small-scale eddies are generated by convection in the water caused by surface
cooling.
Another detailed study of small-scale eddies was recently carried out by Karimova
(2012). She analyzed over 2000 SAR images acquired between 2009 and 2010 by
the European Remote Satellites ERS-1 and ERS-2 and the European Envisat satellite
over the Baltic Sea, the Black Sea, and the Caspian Sea. She detected on them more
than 14,000 radar signatures of vortical structures with diameters between 1 and
75 km. About 99 % of them had diameters in the range 1–20 km and 98 % had a
cyclonic rotation.
In this paper we present several satellite images showing sea surface signatures
of meso- and small-scale eddies around Africa. Prominent areas where such eddies
are encountered are the upwelling regions off West Africa and the Agulhas Current
at the east coast of Africa which frequently sheds eddies.
This paper is organized as follows: In Sect. 11.2 we present three photographs
taken by a handheld camera from Space Shuttle over areas of the Mediterranean
close to the Egyptian coast, On these photographs, eddies are imaged in the sunglint.
They become visible on these photographs via surface films which trace the eddy’s
surface current field and modify the reflected light. In Sects. 11.3 and 11.4 we present
SAR and optical and infrared images, respectively, showing signatures of eddies in
the waters surrounding Africa. In Sect. 11.5 we present results from a case study
showing the propagation of a small-scale eddy from its birth place in the upwelling
region near Cap-Vert, Senegal, into the open North Atlantic. It transported nutrients
from the Senegal upwelling region westward into the oligotrophic North Atlantic,
where it gave rise to enhanced CHL concentration. Finally, in Sect. 11.6 we discuss
and summarize the results.
11.2 Sunglint Images
Sunglint refers to the reflection of incoming solar radiation from the sea surface
observed by optical (visible/near-infrared) sensors under specular or near-specular
conditions, where the sunlight’s angle of incidence equals the angle of reflection to
the observer. On a flat, undisturbed sea surface the sun’s reflection would simply
appear as a mirror image of its disk in the sky. However, the surface of the ocean
is almost always roughened due the action of the wind such that the “mirror” is
broken into many small facets tilted at a variety of different angles and directions.
This sea surface roughening makes it possible to observe specular reflected sunlight
by suitably tilted facets even when its viewing angle is not directed at the specular