104
L. G.-Weil et al.
with the highest EKE values (> 400 cm
2 s
− 2 ) of 2001, and also with the lowest ones
(∼ 80 cm
2 s
− 2 ) of 2006. In 1998–1999, EKE values were slightly higher than the
climatological record, but according to the available data this can be considered a
“normal” period. The important peculiarity of this period is that it is the first one
when altimeter, ocean color an SST data became available together. In the following,
the mesoscale variability in the Canary Islands area will be detailed during these
years, through the combined use of different RS data.
5.3.2 Observations of Mesoscale Features
Taking advantage on the complementarity of the sampling scheme offered by different RS data, the main mesoscale features generated during 1998–1999 were
studied in some detail. AVHRR-derived BT data were analyzed in combination with
SeaWiFS-derived Chl-a data and SLA data recorded by various altimeters during the
same period. Both BT and Chl-a data show similar patterns next to the area where
mesoscale features are generated (i.e. mesoscale eddies, fronts and coastal upwelling
can be seen in color and infrared images). An initial assessment of dynamical features
was based primarily on AVHRR images, because they are obtained more frequently.
SeaWiFS images were used only when able to add new information, in addition to
that provided by BT patterns. Further, SLA data were used in combination with the
Chl-a and BT data to provide a direct link to ocean dynamics and a better feature
resolution. Mesoscale features are observed in altimeter maps when they reach suitable size and strength, and can be detected even when Chl-a and BT gradients are
too weak to be recorded by a radiometer.
There is ample evidence, including that shown in the present work, that Canary
Islands eddies are generated all year around, which suggests that current flow disturbances are strong enough to spawn these features at any time. Similarly, disturbances
in the form of frontal instabilities and upwelling filaments, usually associated with
mesoscale eddies, are regularly observed next to the African coast.
A complete census of all these mesoscale features is not possible using the chosen
RS data, due to implicit limitations in their sampling characteristics. Thus, the present
study is focused on the most prominent mesoscale features that could be observed
in the satellite data for a period longer than 2 months, and of which both generation
and life history characteristics could be unambiguously established. These coherent,
long-lived features, in fact, are those that have more implications in the transport of
heat, nutrients, carbon and other properties from the shelf to the coastal transition
zone, and from this region to the open ocean.
5.3.2.1 Description of Variability in the Canary Islands Area
Several eddy-like structures appear in the series of BT (preferred here to SST for
the reason explained earlier), Chl-a and SLA images, shown in Fig. 5.3 and following
L. G.-Weil et al.
with the highest EKE values (> 400 cm
2 s
− 2 ) of 2001, and also with the lowest ones
(∼ 80 cm
2 s
− 2 ) of 2006. In 1998–1999, EKE values were slightly higher than the
climatological record, but according to the available data this can be considered a
“normal” period. The important peculiarity of this period is that it is the first one
when altimeter, ocean color an SST data became available together. In the following,
the mesoscale variability in the Canary Islands area will be detailed during these
years, through the combined use of different RS data.
5.3.2 Observations of Mesoscale Features
Taking advantage on the complementarity of the sampling scheme offered by different RS data, the main mesoscale features generated during 1998–1999 were
studied in some detail. AVHRR-derived BT data were analyzed in combination with
SeaWiFS-derived Chl-a data and SLA data recorded by various altimeters during the
same period. Both BT and Chl-a data show similar patterns next to the area where
mesoscale features are generated (i.e. mesoscale eddies, fronts and coastal upwelling
can be seen in color and infrared images). An initial assessment of dynamical features
was based primarily on AVHRR images, because they are obtained more frequently.
SeaWiFS images were used only when able to add new information, in addition to
that provided by BT patterns. Further, SLA data were used in combination with the
Chl-a and BT data to provide a direct link to ocean dynamics and a better feature
resolution. Mesoscale features are observed in altimeter maps when they reach suitable size and strength, and can be detected even when Chl-a and BT gradients are
too weak to be recorded by a radiometer.
There is ample evidence, including that shown in the present work, that Canary
Islands eddies are generated all year around, which suggests that current flow disturbances are strong enough to spawn these features at any time. Similarly, disturbances
in the form of frontal instabilities and upwelling filaments, usually associated with
mesoscale eddies, are regularly observed next to the African coast.
A complete census of all these mesoscale features is not possible using the chosen
RS data, due to implicit limitations in their sampling characteristics. Thus, the present
study is focused on the most prominent mesoscale features that could be observed
in the satellite data for a period longer than 2 months, and of which both generation
and life history characteristics could be unambiguously established. These coherent,
long-lived features, in fact, are those that have more implications in the transport of
heat, nutrients, carbon and other properties from the shelf to the coastal transition
zone, and from this region to the open ocean.
5.3.2.1 Description of Variability in the Canary Islands Area
Several eddy-like structures appear in the series of BT (preferred here to SST for
the reason explained earlier), Chl-a and SLA images, shown in Fig. 5.3 and following