358
S. S. Karimova and M. Gade
number, location, and sign of vorticity (cyclonic or anti-cyclonic). Ultimately, there
is still a lack of systematic analyses of the occurrence of mesoscale and even submesoscale eddies (i.e. turbulent vortical structures in the upper water layer with
diameters smaller than, or up to, the baroclinic Rossby radius).
This paper aims at providing some statistical information on the spatial and temporal distribution of sub-mesoscale to basin-scale eddies in the Red Sea, as gained
from high-resolution satellite data. Images obtained at infrared (IR) and visible (VIS)
ranges of the electromagnetic spectrum can be used for the detection of mesoscale
eddies (Alpers et al. 2013). Moreover, synthetic aperture radar (SAR), not only because of its high ground resolution, but also because of its independence of daylight
and cloud conditions, offers great opportunities for studies of the surface ocean circulation at meso- and sub-mesoscales. Consequently, Karimova (2012) used a large
number of Synthetic Aperture Radar (SAR) data of the Baltic, Black, and Caspian
Seas to derive statistics on the occurrence of sub-mesoscale eddies. Those investigations are adopted herein and are extended towards the analysis of Red Sea surface
dynamics based on satellite imagery.
18.2 Data and Methods
The present study is mainly based on the analysis of 492 Envisat Advanced SAR
(ASAR) Wide Swath (WS) imagery acquired over the Red Sea during the whole
years 2006–2011. The spatial resolution of the ASAR WS imagery is about 150 m,
thus allowing for a detection of features, whose sizes are on the order of a few
kilometers and below. Figure 18.1 shows the areal coverage of the Red Sea region
with ASAR images for the whole observation period. For the northern part of the
Red Sea (north of 20
◦ N) more than 100 ASAR scenes are available at any given
location, whereas some areas in the southern part were imaged less than 20 times in
2006–2011.
In general, sub-mesoscale eddies manifest in SAR imagery due to two main
mechanisms, namely the accumulation of surfactants and wave/current interactions
(Karimova 2012). At low to moderate wind speeds (of 3–5 m/s) the former is the main
mechanism and eddies usually appear in SAR images due to the presence of natural
films on the sea surface (Espedal et al. 1998). Surfactants dampen small-scale surface
waves, thereby reducing the radar backscattering from the sea surface (Alpers and
Hühnerfuss 1989), and they often accumulate along the shear lines and convergence
zones, which in turn are associated with the spiral flow (Eldevik and Dysthe 2002).
Thus, the eddies become visible on SAR imagery (Gade et al. 2013). Since this
effect causes vortices to be marked dark, for shortness sake, eddies visualized due to
slicks are hereinafter referred to as “black” eddies. In Fig. 18.2 examples of “black”
eddies in the Red Sea are presented. The image (shown area is 130 × 95 km) was
acquired on June 29, 2006 (i.e. during summer) and shows the north-western coast
of the Red Sea, north of Cape Banas. Starting on the right bottom of the image,
and reaching up into the upper middle, a chain of (cyclonic) “black” eddies can be
inferred, which may result from the interaction of the general flow in this area with
S. S. Karimova and M. Gade
number, location, and sign of vorticity (cyclonic or anti-cyclonic). Ultimately, there
is still a lack of systematic analyses of the occurrence of mesoscale and even submesoscale eddies (i.e. turbulent vortical structures in the upper water layer with
diameters smaller than, or up to, the baroclinic Rossby radius).
This paper aims at providing some statistical information on the spatial and temporal distribution of sub-mesoscale to basin-scale eddies in the Red Sea, as gained
from high-resolution satellite data. Images obtained at infrared (IR) and visible (VIS)
ranges of the electromagnetic spectrum can be used for the detection of mesoscale
eddies (Alpers et al. 2013). Moreover, synthetic aperture radar (SAR), not only because of its high ground resolution, but also because of its independence of daylight
and cloud conditions, offers great opportunities for studies of the surface ocean circulation at meso- and sub-mesoscales. Consequently, Karimova (2012) used a large
number of Synthetic Aperture Radar (SAR) data of the Baltic, Black, and Caspian
Seas to derive statistics on the occurrence of sub-mesoscale eddies. Those investigations are adopted herein and are extended towards the analysis of Red Sea surface
dynamics based on satellite imagery.
18.2 Data and Methods
The present study is mainly based on the analysis of 492 Envisat Advanced SAR
(ASAR) Wide Swath (WS) imagery acquired over the Red Sea during the whole
years 2006–2011. The spatial resolution of the ASAR WS imagery is about 150 m,
thus allowing for a detection of features, whose sizes are on the order of a few
kilometers and below. Figure 18.1 shows the areal coverage of the Red Sea region
with ASAR images for the whole observation period. For the northern part of the
Red Sea (north of 20
◦ N) more than 100 ASAR scenes are available at any given
location, whereas some areas in the southern part were imaged less than 20 times in
2006–2011.
In general, sub-mesoscale eddies manifest in SAR imagery due to two main
mechanisms, namely the accumulation of surfactants and wave/current interactions
(Karimova 2012). At low to moderate wind speeds (of 3–5 m/s) the former is the main
mechanism and eddies usually appear in SAR images due to the presence of natural
films on the sea surface (Espedal et al. 1998). Surfactants dampen small-scale surface
waves, thereby reducing the radar backscattering from the sea surface (Alpers and
Hühnerfuss 1989), and they often accumulate along the shear lines and convergence
zones, which in turn are associated with the spiral flow (Eldevik and Dysthe 2002).
Thus, the eddies become visible on SAR imagery (Gade et al. 2013). Since this
effect causes vortices to be marked dark, for shortness sake, eddies visualized due to
slicks are hereinafter referred to as “black” eddies. In Fig. 18.2 examples of “black”
eddies in the Red Sea are presented. The image (shown area is 130 × 95 km) was
acquired on June 29, 2006 (i.e. during summer) and shows the north-western coast
of the Red Sea, north of Cape Banas. Starting on the right bottom of the image,
and reaching up into the upper middle, a chain of (cyclonic) “black” eddies can be
inferred, which may result from the interaction of the general flow in this area with