18 Eddies in the Red Sea as seen by Satellite SAR Imagery
377
18.5 Conclusions
We used more than 500 Envisat ASAR WS images acquired in 2006–2011 over
the Red Sea to study the spatial and temporal distribution of eddies at sub-meso-,
meso-, and basin-scale. Most of the images covered the northern part of the basin
and therefore, most of the eddies detected were also located there.
However, the normalized densities (i.e. the numbers of detected eddies per image
resolution cell) show evidence that the observed heterogeneity is not simply due to
the inhomogeneous coverage by SAR imagery, but that a general trend exists of a
greater number of sub-mesoscale eddies in the northern part of the basin (north of
20
◦ N).
In total more than 1,000 sub-mesoscale eddies were found, which is about two
eddies per SAR image. This is generally less than previous studies revealed for the
Baltic, Black, and Caspian seas. We hypothesize that this finding is linked to the
greater depth of the upper mixed layer in the Red Sea (about 100 m), but also to the
overall shape of the (narrow) basin and to the different circulation within the basin.
“Black” eddies, i.e. eddies manifesting in SAR images through the accumulation
of surfactants along the shear lines, were found mostly in coastal areas, while “white”
eddies, i.e. manifesting through wave-current interactions, were found further offshore. In general, “black” eddies visualize in SAR images at lower wind speeds,
since the surfactants start to disrupt when the wind speed increases.
We also found about 50 meso- and basin-scale eddies with diameters up to approximately 200 km. Their rotation was both cyclonic and anti-cyclonic; however,
most of the basin-scale eddies (with diameters exceeding 100 km) were found to
be anti-cyclonic, which is in contrast to the smaller, sub-mesoscale eddies. Those
basin-scale eddies were mainly found in SAR images acquired in spring, and they
were located between 21 and 24
◦ N, which supports the hypothesis of Quadfasel and
Baudner (1993) that wind forcing and interactions with the local topography in that
area are the main generation mechanisms.
SAR imagery of six consecutive years is certainly not sufficient for the derivation
of complete climatologies, particularly if the entire basin was not covered homogeneously. However, our results indicate very well that the systematic analyses of
SAR imagery with respect to the detection of sub-mesoscale, mesoscale, and basinscale eddies has great potential to further the knowledge about the hydrodynamics
in certain sea areas or in enclosed or semi-enclosed seas.
Acknowledgments This work was supported within the framework of the Federal Target Program
“Scientific and scientific-pedagogical personnel of innovative Russia” in 2009–2013. The data used
for the present study were kindly provided by ESA under project ID 14120 (SESAMeSEA).
377
18.5 Conclusions
We used more than 500 Envisat ASAR WS images acquired in 2006–2011 over
the Red Sea to study the spatial and temporal distribution of eddies at sub-meso-,
meso-, and basin-scale. Most of the images covered the northern part of the basin
and therefore, most of the eddies detected were also located there.
However, the normalized densities (i.e. the numbers of detected eddies per image
resolution cell) show evidence that the observed heterogeneity is not simply due to
the inhomogeneous coverage by SAR imagery, but that a general trend exists of a
greater number of sub-mesoscale eddies in the northern part of the basin (north of
20
◦ N).
In total more than 1,000 sub-mesoscale eddies were found, which is about two
eddies per SAR image. This is generally less than previous studies revealed for the
Baltic, Black, and Caspian seas. We hypothesize that this finding is linked to the
greater depth of the upper mixed layer in the Red Sea (about 100 m), but also to the
overall shape of the (narrow) basin and to the different circulation within the basin.
“Black” eddies, i.e. eddies manifesting in SAR images through the accumulation
of surfactants along the shear lines, were found mostly in coastal areas, while “white”
eddies, i.e. manifesting through wave-current interactions, were found further offshore. In general, “black” eddies visualize in SAR images at lower wind speeds,
since the surfactants start to disrupt when the wind speed increases.
We also found about 50 meso- and basin-scale eddies with diameters up to approximately 200 km. Their rotation was both cyclonic and anti-cyclonic; however,
most of the basin-scale eddies (with diameters exceeding 100 km) were found to
be anti-cyclonic, which is in contrast to the smaller, sub-mesoscale eddies. Those
basin-scale eddies were mainly found in SAR images acquired in spring, and they
were located between 21 and 24
◦ N, which supports the hypothesis of Quadfasel and
Baudner (1993) that wind forcing and interactions with the local topography in that
area are the main generation mechanisms.
SAR imagery of six consecutive years is certainly not sufficient for the derivation
of complete climatologies, particularly if the entire basin was not covered homogeneously. However, our results indicate very well that the systematic analyses of
SAR imagery with respect to the detection of sub-mesoscale, mesoscale, and basinscale eddies has great potential to further the knowledge about the hydrodynamics
in certain sea areas or in enclosed or semi-enclosed seas.
Acknowledgments This work was supported within the framework of the Federal Target Program
“Scientific and scientific-pedagogical personnel of innovative Russia” in 2009–2013. The data used
for the present study were kindly provided by ESA under project ID 14120 (SESAMeSEA).