5 Mesoscale Dynamics in the Canary Islands Area . . .
115
Both filaments F2 and F3 seem to be recursive oceanographic features linked to
eddies in the Canary Islands area, as analogous features have been observed jointly
in other occasions (see for example Fig. 11c in Barton et al. 1998). In addition,
not only the eddies participate in filament generation, but they can also operate
in combination with the same filaments to export the coastal upwelled waters at
great distances. RS images show often anticyclonic eddies shedded by Gran Canaria
interacting with a filament similar to F2, entraining cold and high-chlorophyll waters
in their peripheries (Arístegui et al. 1997; Pacheco and Hernández-Guerra 1999).
This situation repeated again in July, 1998 (Figs. 5.3d and 5.10d) and was produced
by the newly generated anticyclonic eddy A1. Occasionally, various eddies may be
involved at the same time in the offshore export of upwelled water. This can be
observed starting in August 1998 (Figs. 5.3f, g and 5.10c), when the eddies A2 and
C2 worked in combination as a gear-like system, spreading coastal water transported
by the filament at a distance of ∼ 350 km from the coast.
5.3.2.3 Eddy–Eddy Interactions
South of the Canary Islands, eddy motion is not only influenced by advection due to
the Canary Current, but also through the interaction with other vortices. Relatively
small cyclonic eddies, like C1 and C2, are driven around stronger anticyclonic vortices, and in the case of C2 even squeezed and distorted, when entrained between A1
and A2, as already seen earlier (Fig. 5.10c).
Similarly, the motion of bigger eddies is also influenced by other factors besides
the Canary Current. Thus, the path of A1 next to its generation area was determined
by the presence offshore of A2 and by the proximity of the African coast. This
produced the arresting or slowing down of eddy A1 (Figs. 5.6a and 5.10c, d). In a
similar way, the free movement of C3 was impeded by the coastal topography and
the blocking effect of A1, located south of C3 (Figs. 5.3e–h and 5.10c, d). When A2
moved westward, at the beginning of September 1998, A1 and C3 started to drift also
from their nearly stationary positions (Fig. 5.6b, c). The movement of these larger
vortices is coupled next to their generation area. Far from the formation region,
eddies move to the west under the combined influence of the Canary Current and the
beta effect, covering about 100 km per month (Figs. 5.5, 5.7, 5.8 and 5.9).
The larger size and longer time span of anticyclonic eddies contribute to the
collision and possible merging of these mesoscale features in Canarian waters. In
October 1998, mature eddy A1 interacted with a new anticyclonic eddy located south
of Tenerife. On October 24, A1, having a mean size of ∼ 135 km, and the younger
eddy, having a mean size of ∼ 90 km, adopted a figure 8 structure (Fig. 5.6c), with
the centres of the eddies separated by 130 km (less than three times the mean radius
of the vortices, which was ∼ 55 km). From October 26 to November 8 there were
no clear AVHRR or SeaWiFS images available. Afterwards, a bigger, reinforced
anticyclonic eddy, with a diameter of ∼ 160 km, was observed, as the result of the
coalescence between the two eddies (Figs. 5.4h and 5.7a). This suggests that vortex
merging is a quick process, which can last less than 10 days. In spite of its short
duration, the process could be detected also in the weekly SLA maps, where the two
approaching eddies appear as an elongated feature (Fig. 5.4g).
115
Both filaments F2 and F3 seem to be recursive oceanographic features linked to
eddies in the Canary Islands area, as analogous features have been observed jointly
in other occasions (see for example Fig. 11c in Barton et al. 1998). In addition,
not only the eddies participate in filament generation, but they can also operate
in combination with the same filaments to export the coastal upwelled waters at
great distances. RS images show often anticyclonic eddies shedded by Gran Canaria
interacting with a filament similar to F2, entraining cold and high-chlorophyll waters
in their peripheries (Arístegui et al. 1997; Pacheco and Hernández-Guerra 1999).
This situation repeated again in July, 1998 (Figs. 5.3d and 5.10d) and was produced
by the newly generated anticyclonic eddy A1. Occasionally, various eddies may be
involved at the same time in the offshore export of upwelled water. This can be
observed starting in August 1998 (Figs. 5.3f, g and 5.10c), when the eddies A2 and
C2 worked in combination as a gear-like system, spreading coastal water transported
by the filament at a distance of ∼ 350 km from the coast.
5.3.2.3 Eddy–Eddy Interactions
South of the Canary Islands, eddy motion is not only influenced by advection due to
the Canary Current, but also through the interaction with other vortices. Relatively
small cyclonic eddies, like C1 and C2, are driven around stronger anticyclonic vortices, and in the case of C2 even squeezed and distorted, when entrained between A1
and A2, as already seen earlier (Fig. 5.10c).
Similarly, the motion of bigger eddies is also influenced by other factors besides
the Canary Current. Thus, the path of A1 next to its generation area was determined
by the presence offshore of A2 and by the proximity of the African coast. This
produced the arresting or slowing down of eddy A1 (Figs. 5.6a and 5.10c, d). In a
similar way, the free movement of C3 was impeded by the coastal topography and
the blocking effect of A1, located south of C3 (Figs. 5.3e–h and 5.10c, d). When A2
moved westward, at the beginning of September 1998, A1 and C3 started to drift also
from their nearly stationary positions (Fig. 5.6b, c). The movement of these larger
vortices is coupled next to their generation area. Far from the formation region,
eddies move to the west under the combined influence of the Canary Current and the
beta effect, covering about 100 km per month (Figs. 5.5, 5.7, 5.8 and 5.9).
The larger size and longer time span of anticyclonic eddies contribute to the
collision and possible merging of these mesoscale features in Canarian waters. In
October 1998, mature eddy A1 interacted with a new anticyclonic eddy located south
of Tenerife. On October 24, A1, having a mean size of ∼ 135 km, and the younger
eddy, having a mean size of ∼ 90 km, adopted a figure 8 structure (Fig. 5.6c), with
the centres of the eddies separated by 130 km (less than three times the mean radius
of the vortices, which was ∼ 55 km). From October 26 to November 8 there were
no clear AVHRR or SeaWiFS images available. Afterwards, a bigger, reinforced
anticyclonic eddy, with a diameter of ∼ 160 km, was observed, as the result of the
coalescence between the two eddies (Figs. 5.4h and 5.7a). This suggests that vortex
merging is a quick process, which can last less than 10 days. In spite of its short
duration, the process could be detected also in the weekly SLA maps, where the two
approaching eddies appear as an elongated feature (Fig. 5.4g).