11 Satellite Observations of Oceanic Eddies Around Africa
225
3 mg m
− 3. Our data confirm the presence of this diurnal SST variability in the
Cap-Vert upwelling region.
11.5.4 Synthetic Aperture Radar Image
Figure 11.15 shows a SAR image which was acquired in the Wide Swath Mode
(WSM) by the Advanced SAR (ASAR) onboard the European Envisat satellite at
2317 UTC on 31 October over the waters close to Cap-Vert. Similarly as on the
SAR image shown in Fig. 11.5, also here the eddy becomes visible by reduced
image brightness caused by reduction of the radar backscattering due to the presence
of biogenic surface films. Since it is a cyclonic eddy causing upwelling of cold
nutrient-rich water, very likely it contains much biota which secretes surface active
material such that biogenic surface films were formed.
11.6 Discussion and Summary
In this paper we have presented several images collected from Space Shuttle, the
Terra and Acqua satellites, and the Envisat satellite over the seas surrounding Africa
showing sea surface signatures of mesoscale and small-scale eddies. These images
were acquired in the visible, infrared and microwave (5.3 GHz) bands. The instrument most used from space to study the dynamics of mesoscale eddies is the radar
altimeter. It is a microwave instrument, which yields data (almost) independent of
cloud coverage and independent of the time of the day. However, it cannot resolve
eddies with dimensions much smaller than100 km. Since the study of meso-scale
oceanic phenomena using altimeter data is the subject of another chapter of this
book, we have refrained here from discussing in detail the use of altimeter data in investigating eddies. Here we just want to mention a paper by Chaigneau et al. (2008),
in which mesoscale eddy activity in the Canary upwelling area (10–45
◦ N; 40–50
◦ W)
is investigated using 15 years of satellite altimetry data from several satellites. Restricting the analysis to long-lived eddies having sea surface height anomalies larger
than 2 cm and lifetimes larger than 35 days, they found that, on the average, around
60–100 eddies with diameters of 140–320 km are present on weekly maps and that
4–7 eddies are generated each week in the Canary upwelling area.
Small-scale eddies can only be observed from space only by high resolution
infrared/optical sensors and SAR. Both types of instruments have their limitations.
Infrared and optical sensors are limited by cloud coverage, and spaceborne SARs by
their poor coverage and the weak radar signatures of eddies (exception: when sea
surface is covered partially by surface films and the film is entrained in the surface
current field of the eddy).
As stated before, mesoscale and submesoscale eddies play an important role in
ocean dynamics. In order to get a better understanding of this dynamics, a systemic
225
3 mg m
− 3. Our data confirm the presence of this diurnal SST variability in the
Cap-Vert upwelling region.
11.5.4 Synthetic Aperture Radar Image
Figure 11.15 shows a SAR image which was acquired in the Wide Swath Mode
(WSM) by the Advanced SAR (ASAR) onboard the European Envisat satellite at
2317 UTC on 31 October over the waters close to Cap-Vert. Similarly as on the
SAR image shown in Fig. 11.5, also here the eddy becomes visible by reduced
image brightness caused by reduction of the radar backscattering due to the presence
of biogenic surface films. Since it is a cyclonic eddy causing upwelling of cold
nutrient-rich water, very likely it contains much biota which secretes surface active
material such that biogenic surface films were formed.
11.6 Discussion and Summary
In this paper we have presented several images collected from Space Shuttle, the
Terra and Acqua satellites, and the Envisat satellite over the seas surrounding Africa
showing sea surface signatures of mesoscale and small-scale eddies. These images
were acquired in the visible, infrared and microwave (5.3 GHz) bands. The instrument most used from space to study the dynamics of mesoscale eddies is the radar
altimeter. It is a microwave instrument, which yields data (almost) independent of
cloud coverage and independent of the time of the day. However, it cannot resolve
eddies with dimensions much smaller than100 km. Since the study of meso-scale
oceanic phenomena using altimeter data is the subject of another chapter of this
book, we have refrained here from discussing in detail the use of altimeter data in investigating eddies. Here we just want to mention a paper by Chaigneau et al. (2008),
in which mesoscale eddy activity in the Canary upwelling area (10–45
◦ N; 40–50
◦ W)
is investigated using 15 years of satellite altimetry data from several satellites. Restricting the analysis to long-lived eddies having sea surface height anomalies larger
than 2 cm and lifetimes larger than 35 days, they found that, on the average, around
60–100 eddies with diameters of 140–320 km are present on weekly maps and that
4–7 eddies are generated each week in the Canary upwelling area.
Small-scale eddies can only be observed from space only by high resolution
infrared/optical sensors and SAR. Both types of instruments have their limitations.
Infrared and optical sensors are limited by cloud coverage, and spaceborne SARs by
their poor coverage and the weak radar signatures of eddies (exception: when sea
surface is covered partially by surface films and the film is entrained in the surface
current field of the eddy).
As stated before, mesoscale and submesoscale eddies play an important role in
ocean dynamics. In order to get a better understanding of this dynamics, a systemic