11 Satellite Observations of Oceanic Eddies Around Africa
211
The center point of the photograph (determined to the nearest half degree) is at
31.5
◦ N, 27.0
◦ E. The Space Shuttle was located at the time of the data take at 32.8
◦ N,
29.1
◦ E, where the elevation angle of the sun was 40
◦ . The photograph shows in the
upper section sea surface signatures of a cyclonic eddy in the sunglint region. The
bright lines on the photograph are smooth lines on the sea surface covered by surface
films. Based on the scale of land features, we estimate the diameter of the eddy to
be 35 km.
The photograph depicted in Fig. 11.2 was taken 12 s earlier at 12:02: 43 UTC
(orbit: 33, roll: 35, frame: 94). It also shows a photograph of the Mediterranean Sea
near the Egyptian coast, approximately half a degree further north and one degree
further east than the previous photograph. The center point of the photograph is at
32.0
◦ N, 28.0
◦ E. The Space Shuttle was located at the time of the data take at 33.5
◦ N,
28.5
◦ E, where the elevation angle of the sun was 40
◦ . The photograph shows three
interconnected cyclonic eddies in the sunglint region. This photograph is also shown
in the paper by Munk et al. (2000) and has since then been reproduced in many papers,
e.g., in the paper by Eldevik and Dysthe (2002). Munk et al. (2000) have estimated
the width of the imaged area to be 30 km, from which follows that the diameter of the
largest eddy is about 8 km. They attribute the eddy generation to horizontal current
shear induced by wind shear. Although calm wind conditions prevailed during the
time of the data take, strong winds with speeds up to 10 ms
−1 were blowing the
week before from a north-northwesterly direction over the Mediterranean Sea. An
argument in favor of this generation mechanism is that the centers of the spirals are
aligned approximately in wind direction which should have been also direction of
the shear line.
However, in a recent paper Golitsyn (2012) argues that the majority of the smallscale eddies visible on sunglint and SAR images is not generated by horizontal shear
instability, but by convection in the water caused by surface cooling. Since most of the
observed small-scale eddies are cyclonic, this points to the fact that eddy generation
is affected by the Earth’s rotation. The sinking of cold water leads to convergence
of surface waters, i.e. to a concentration of angular momentum associated with the
Earth’s rotation. Golitsyn (2012) estimated that the time it takes for generating a
small-scale eddy is of the order of hours, and not of 5–15 days as in the case of
eddy generation by shear instability (Munk et al. 2000). According to Golitsyn, most
of the cyclonic eddies in the ocean can be considered as hydrodynamic analogs to
tropical and polar hurricanes in the atmosphere. This would explain the dominance
of cyclonic small-scale eddies in the World’s ocean. On the other hand, small-scale
eddies generated by velocity shear should be rare phenomena, since they require
special wind conditions, like constant winds blowing over the water for a sufficient
long time, which is seldom the case.
Figure 11.3 shows another photograph of the Mediterranean Sea near the Egyptian
coast with the Cape Alan el Rum to the right, which was taken at 1145 UTC on 8
October 1984 (orbit: 49, roll: 38, frame: 65). The center point of the photograph
is at 31.5
◦ N, 27.5
◦ E. The space shuttle was located at the time of the data take at
32.2
◦ N, 28.3
◦ E, where the elevation angle of the sun was 43
◦ . The photograph shows
a cyclonic eddy with a diameter of approximately 40 km in the outer sunglint region,
where the dark lines are smooth lines caused by damping of the short-scale sea surface
211
The center point of the photograph (determined to the nearest half degree) is at
31.5
◦ N, 27.0
◦ E. The Space Shuttle was located at the time of the data take at 32.8
◦ N,
29.1
◦ E, where the elevation angle of the sun was 40
◦ . The photograph shows in the
upper section sea surface signatures of a cyclonic eddy in the sunglint region. The
bright lines on the photograph are smooth lines on the sea surface covered by surface
films. Based on the scale of land features, we estimate the diameter of the eddy to
be 35 km.
The photograph depicted in Fig. 11.2 was taken 12 s earlier at 12:02: 43 UTC
(orbit: 33, roll: 35, frame: 94). It also shows a photograph of the Mediterranean Sea
near the Egyptian coast, approximately half a degree further north and one degree
further east than the previous photograph. The center point of the photograph is at
32.0
◦ N, 28.0
◦ E. The Space Shuttle was located at the time of the data take at 33.5
◦ N,
28.5
◦ E, where the elevation angle of the sun was 40
◦ . The photograph shows three
interconnected cyclonic eddies in the sunglint region. This photograph is also shown
in the paper by Munk et al. (2000) and has since then been reproduced in many papers,
e.g., in the paper by Eldevik and Dysthe (2002). Munk et al. (2000) have estimated
the width of the imaged area to be 30 km, from which follows that the diameter of the
largest eddy is about 8 km. They attribute the eddy generation to horizontal current
shear induced by wind shear. Although calm wind conditions prevailed during the
time of the data take, strong winds with speeds up to 10 ms
−1 were blowing the
week before from a north-northwesterly direction over the Mediterranean Sea. An
argument in favor of this generation mechanism is that the centers of the spirals are
aligned approximately in wind direction which should have been also direction of
the shear line.
However, in a recent paper Golitsyn (2012) argues that the majority of the smallscale eddies visible on sunglint and SAR images is not generated by horizontal shear
instability, but by convection in the water caused by surface cooling. Since most of the
observed small-scale eddies are cyclonic, this points to the fact that eddy generation
is affected by the Earth’s rotation. The sinking of cold water leads to convergence
of surface waters, i.e. to a concentration of angular momentum associated with the
Earth’s rotation. Golitsyn (2012) estimated that the time it takes for generating a
small-scale eddy is of the order of hours, and not of 5–15 days as in the case of
eddy generation by shear instability (Munk et al. 2000). According to Golitsyn, most
of the cyclonic eddies in the ocean can be considered as hydrodynamic analogs to
tropical and polar hurricanes in the atmosphere. This would explain the dominance
of cyclonic small-scale eddies in the World’s ocean. On the other hand, small-scale
eddies generated by velocity shear should be rare phenomena, since they require
special wind conditions, like constant winds blowing over the water for a sufficient
long time, which is seldom the case.
Figure 11.3 shows another photograph of the Mediterranean Sea near the Egyptian
coast with the Cape Alan el Rum to the right, which was taken at 1145 UTC on 8
October 1984 (orbit: 49, roll: 38, frame: 65). The center point of the photograph
is at 31.5
◦ N, 27.5
◦ E. The space shuttle was located at the time of the data take at
32.2
◦ N, 28.3
◦ E, where the elevation angle of the sun was 43
◦ . The photograph shows
a cyclonic eddy with a diameter of approximately 40 km in the outer sunglint region,
where the dark lines are smooth lines caused by damping of the short-scale sea surface