212
W. Alpers et al.
waves by surface films. Thus, on this photograph, the eddy pattern is traced out by
dark lines caused by the accumulation of surface films in convergent surface current
regions. Surface films damp the short-scale sea surface waves and thus modify the
reflection of sunlight from the sea surface. Sea surface films are either of natural or
anthropogenic origin.
Natural surface films consists of mineral oil released from oil seeps at the sea
bottom or of surface active material secreted by biota in the water column. In the
latter case, they are called biogenic surface films, which form monomolecular surface
films at the sea surface. They can damp the short surface waves as strongly as mineral
oil films. Anthropogenic or man-made surface films usually consist of oily substances
released from ships (bunker or sludge oil from the ship’s engine room, crude oil from
washing the tanks of tankers), from oil platforms or from land-based industrial plants.
Since it is well known that the biological productivity is very low in the Mediterranean
waters west of the Nile estuary (Siokou-Frangou et al. 2010), we suspect that in this
case not biogenic surface films served as tracer for the eddy’s surface current field,
but anthropogenic oily substances released from ships. Since we are not aware of
any oil seeps in this area, we suspect that the surface films originated from ships.
Since the imaged area is located close to the shipping lane leading to the Suez Canal,
the probability to encounter anthropogenic surface films in this area is quite high.
11.3 Synthetic Aperture Radar Images
Mesoscale and submesoscale eddies become detectable on SAR images by modulation of the short-scale sea surface waves, which causes a modulation of the
backscattered radar power or the Normalized Radar Cross Section (NRCS). Such
modulation can be induced (1) by surface current variations associated with the
eddy, (2) by cold upwelled water in cyclonic eddies or (3) by surface films.
In the first case, the interaction of short surface with a variable current enters into
the SAR imaging mechanism for which theories have been developed (Alpers and
Hennings 1984; Kudryavtsev et al. 2005). Since the variations of the surface currents
associated with eddies are small, also the corresponding radar signatures are small.
This makes it usually difficult to identify sea surface signatures of eddies on SAR
images.
In the second case, the stability of the air-sea interface enters into the SAR imaging
mechanism. If the Sea Surface Temperature (SST) over an eddy is cooler than over the
ambient waters, then the stability of marine boundary layer changes from (usually)
neutrally stable (over the ambient water) to unstable (over the cold eddy) which
causes a reduction of the friction velocity (or wind stress) and thus a reduction of
the capability of the wind to generate short surface waves (see e.g., Large and Pond
1981; Koslov et al. 2011). As a result, the NRCS is reduced over the cold eddies,
typically by 2–3 dB.
In the third case the damping of short surface waves by biogenic or anthropogenic
surface films enters into the SAR imaging mechanism (Valenzuela 1978; Huehnerfuss et al. 1996; Espedal et al. 1998; Alpers and Espedal 2004). This requires the
presence of surface films, which is not always the case. The presence of surface films
W. Alpers et al.
waves by surface films. Thus, on this photograph, the eddy pattern is traced out by
dark lines caused by the accumulation of surface films in convergent surface current
regions. Surface films damp the short-scale sea surface waves and thus modify the
reflection of sunlight from the sea surface. Sea surface films are either of natural or
anthropogenic origin.
Natural surface films consists of mineral oil released from oil seeps at the sea
bottom or of surface active material secreted by biota in the water column. In the
latter case, they are called biogenic surface films, which form monomolecular surface
films at the sea surface. They can damp the short surface waves as strongly as mineral
oil films. Anthropogenic or man-made surface films usually consist of oily substances
released from ships (bunker or sludge oil from the ship’s engine room, crude oil from
washing the tanks of tankers), from oil platforms or from land-based industrial plants.
Since it is well known that the biological productivity is very low in the Mediterranean
waters west of the Nile estuary (Siokou-Frangou et al. 2010), we suspect that in this
case not biogenic surface films served as tracer for the eddy’s surface current field,
but anthropogenic oily substances released from ships. Since we are not aware of
any oil seeps in this area, we suspect that the surface films originated from ships.
Since the imaged area is located close to the shipping lane leading to the Suez Canal,
the probability to encounter anthropogenic surface films in this area is quite high.
11.3 Synthetic Aperture Radar Images
Mesoscale and submesoscale eddies become detectable on SAR images by modulation of the short-scale sea surface waves, which causes a modulation of the
backscattered radar power or the Normalized Radar Cross Section (NRCS). Such
modulation can be induced (1) by surface current variations associated with the
eddy, (2) by cold upwelled water in cyclonic eddies or (3) by surface films.
In the first case, the interaction of short surface with a variable current enters into
the SAR imaging mechanism for which theories have been developed (Alpers and
Hennings 1984; Kudryavtsev et al. 2005). Since the variations of the surface currents
associated with eddies are small, also the corresponding radar signatures are small.
This makes it usually difficult to identify sea surface signatures of eddies on SAR
images.
In the second case, the stability of the air-sea interface enters into the SAR imaging
mechanism. If the Sea Surface Temperature (SST) over an eddy is cooler than over the
ambient waters, then the stability of marine boundary layer changes from (usually)
neutrally stable (over the ambient water) to unstable (over the cold eddy) which
causes a reduction of the friction velocity (or wind stress) and thus a reduction of
the capability of the wind to generate short surface waves (see e.g., Large and Pond
1981; Koslov et al. 2011). As a result, the NRCS is reduced over the cold eddies,
typically by 2–3 dB.
In the third case the damping of short surface waves by biogenic or anthropogenic
surface films enters into the SAR imaging mechanism (Valenzuela 1978; Huehnerfuss et al. 1996; Espedal et al. 1998; Alpers and Espedal 2004). This requires the
presence of surface films, which is not always the case. The presence of surface films