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S. S. Karimova and M. Gade
“black” eddies appear on SAR imagery, as a consequence, the diameter of the turbulent rotational flow should be smaller, which was indeed observed. The histogram
for all detected eddies (lower panel in Fig. 18.4) is broader, and the total mean eddy
diameter was found to be 8.3 km.
18.3.2 Spatial Distribution
Figure 18.5 shows the spatial distributions of the detected “black” eddies (Panel a)
and “white” eddies (Panel b) for the entire period 2006–2011. In order to avoid
a confusion with the small circles (denoting eddy locations) we have removed all
islands from the underlying map.
“Black” eddies were mostly found in the northern part of the Red Sea, with
highest densities along the coasts. Again, since lower wind speeds are needed for
the visualization of “black” eddies, more of them should be found in coastal zones,
where the interaction with, and the sheltering by, the land reduces the mean wind
speed. Moreover, in addition to the constraints given by the SAR imaging of surface
films, both the shallow bathymetry in coastal zones and the closer distance to the
shore should have an impact on the generation of small eddies and, thus, on their
size distribution in those areas. In this respect, it is interesting to note that the overall
availability of chlorophyll-like pigments in the northern part of the Red Sea is lower
than in its southern part (Barale and Gade 2014). Therefore, a simple relationship
between the occurrence of “black” eddies and the biological productivity (and, thus,
the availability of surfactants at the sea surface) cannot be inferred.
In contrast to the “black” eddies we found “white” eddies mostly offshore, i.e.
in the open part of the Red Sea (Fig. 18.5b). Again, the overall higher (mean) wind
speed on the open sea supports the visualization of “white” eddies, since the visibility
of surfactants on SAR imagery is reduced. Moreover, most of the “white” eddies
were found in the northern part of the Red Sea. At a first glance, this may be due to the
higher availability of SAR images from the northern part (see Fig. 18.1); however, we
also found only very few “white” eddies at the southern tip of the Red Sea, close to the
Strait of Bal-el-Mandeb, where more SAR imagery was available again. Therefore,
it is unlikely that the heterogeneous distribution of the “white” eddies (and of the
“black” eddies as well) is just a manifestation of the heterogeneous coverage of the
total area by SAR imagery. Instead, hydrodynamic differences, in combination with
a different bathymetry and wind climatology in either parts of the basin, must be
responsible for the observed differences in eddy density.
In order to support the statement that the spatial eddy distribution does not simply
depend on the SAR image coverage, we have calculated normalized densities, i.e. the
number of (detected) eddies per resolution cell of size 0.2
◦
× 0.2
◦ , for convenience
multiplied by 100. The respective maps are shown in Fig. 18.6 and demonstrate that
the general distribution of both “black” and “white” eddies is looking the same, with
or without normalization. We note, however, that the local density of “black” eddies
S. S. Karimova and M. Gade
“black” eddies appear on SAR imagery, as a consequence, the diameter of the turbulent rotational flow should be smaller, which was indeed observed. The histogram
for all detected eddies (lower panel in Fig. 18.4) is broader, and the total mean eddy
diameter was found to be 8.3 km.
18.3.2 Spatial Distribution
Figure 18.5 shows the spatial distributions of the detected “black” eddies (Panel a)
and “white” eddies (Panel b) for the entire period 2006–2011. In order to avoid
a confusion with the small circles (denoting eddy locations) we have removed all
islands from the underlying map.
“Black” eddies were mostly found in the northern part of the Red Sea, with
highest densities along the coasts. Again, since lower wind speeds are needed for
the visualization of “black” eddies, more of them should be found in coastal zones,
where the interaction with, and the sheltering by, the land reduces the mean wind
speed. Moreover, in addition to the constraints given by the SAR imaging of surface
films, both the shallow bathymetry in coastal zones and the closer distance to the
shore should have an impact on the generation of small eddies and, thus, on their
size distribution in those areas. In this respect, it is interesting to note that the overall
availability of chlorophyll-like pigments in the northern part of the Red Sea is lower
than in its southern part (Barale and Gade 2014). Therefore, a simple relationship
between the occurrence of “black” eddies and the biological productivity (and, thus,
the availability of surfactants at the sea surface) cannot be inferred.
In contrast to the “black” eddies we found “white” eddies mostly offshore, i.e.
in the open part of the Red Sea (Fig. 18.5b). Again, the overall higher (mean) wind
speed on the open sea supports the visualization of “white” eddies, since the visibility
of surfactants on SAR imagery is reduced. Moreover, most of the “white” eddies
were found in the northern part of the Red Sea. At a first glance, this may be due to the
higher availability of SAR images from the northern part (see Fig. 18.1); however, we
also found only very few “white” eddies at the southern tip of the Red Sea, close to the
Strait of Bal-el-Mandeb, where more SAR imagery was available again. Therefore,
it is unlikely that the heterogeneous distribution of the “white” eddies (and of the
“black” eddies as well) is just a manifestation of the heterogeneous coverage of the
total area by SAR imagery. Instead, hydrodynamic differences, in combination with
a different bathymetry and wind climatology in either parts of the basin, must be
responsible for the observed differences in eddy density.
In order to support the statement that the spatial eddy distribution does not simply
depend on the SAR image coverage, we have calculated normalized densities, i.e. the
number of (detected) eddies per resolution cell of size 0.2
◦
× 0.2
◦ , for convenience
multiplied by 100. The respective maps are shown in Fig. 18.6 and demonstrate that
the general distribution of both “black” and “white” eddies is looking the same, with
or without normalization. We note, however, that the local density of “black” eddies