12.4.3 Role of Topography: Conveyor and Obstacle
for the Connectivity
Coastline orientation, capes, islands, embayments, etc. are well-known topographic
features that interact with the flow and result in hydrodynamic processes with often
relevant consequences noticeably on the marine ecosystems. A pronounced cape on
an otherwise straight-like coastline represents a serious obstacle for a coastal jet
flowing along the shore, since it has the potential to disturb and destabilize the jet
downstream the cape. There are examples of perturbations resembling Von Karman
vortex streets generated by oceanic currents flowing through islands (Jimenez et al.
2008), which, moreover can trap patches of chlorophyll and become productive
biological environments. Or flows deflected by the shoreline orientation that evolves
in rather steady structures, etc.
The Gulf of Cadiz–Alboran Sea system holds a variety of such structures. One of
them already cited is Point Europa, which causes the AJ separation from the shore
and enables submesoscale instabilities with important consequences for hydrodynamic and biological connectivity. Cape Tres Forcas in the southern shore of the
basin is a pronounced cape in a rather straight coastline that turns into a barrier that
prevents east-to-west zonal connectivity in the south part of the Alboran Sea. It also
hampers the west-to-east connection for different reasons. The geographical morphology of the cape facilitates the formation and growth of the EAG by deflecting
the AJ in a cross-basin direction as it approaches the cape. Under these circumstances, the southern shore west of the cape would be more likely connected with the
northeast Alboran shore than with the south shore east of the cape, as suggested by
Muñoz et al. (2015). This connection relies, however, on the stability of the
mesoscale, the permanency of the EAG in this case, which is affected by
submesoscale variability.
Another relevant example of the influence and consequences of topography is
illustrated in Fig. 12.7. Its oceanographic scope is the very Strait of Gibraltar, and it
emphasizes the role played by the shoreline orientation to achieve connectivity
between opposite shores separated by the energetic AJ. Several oceanographic
processes are involved, among which tides stand out. The experiment presented in
Fig. 12.7 shows the time evolution of passive drifters (virtual particles) initially
released at the western approach of the Strait at two different locations. The strong
tidal currents (see inset) result in a back-and-forth motion of the particles with
prevailing net eastwards (along-AJ) displacements. No cross-jet (i.e., north-south)
displacement is observed for the cases of particles initially released in the central and
north part, which move following zonal trajectories. Things are different for particles
released in the south, as a considerable fraction are seen crossing the eastern part of
the Strait 20 hours after their release (Fig. 12.7f). The likely origin of such deflection
is the coastline orientation west of Point Cires, which would allow for limited, but
not negligible, south-to-north connectivity.
As a collateral result of Fig. 12.7, it is worth mentioning the fact that tidal currents
can lead to a net westward transport if the passive drifters are located to the west of
12 Biophysical Processes Determining the Connectivity of the Alboran Sea Fish. . .
471
for the Connectivity
Coastline orientation, capes, islands, embayments, etc. are well-known topographic
features that interact with the flow and result in hydrodynamic processes with often
relevant consequences noticeably on the marine ecosystems. A pronounced cape on
an otherwise straight-like coastline represents a serious obstacle for a coastal jet
flowing along the shore, since it has the potential to disturb and destabilize the jet
downstream the cape. There are examples of perturbations resembling Von Karman
vortex streets generated by oceanic currents flowing through islands (Jimenez et al.
2008), which, moreover can trap patches of chlorophyll and become productive
biological environments. Or flows deflected by the shoreline orientation that evolves
in rather steady structures, etc.
The Gulf of Cadiz–Alboran Sea system holds a variety of such structures. One of
them already cited is Point Europa, which causes the AJ separation from the shore
and enables submesoscale instabilities with important consequences for hydrodynamic and biological connectivity. Cape Tres Forcas in the southern shore of the
basin is a pronounced cape in a rather straight coastline that turns into a barrier that
prevents east-to-west zonal connectivity in the south part of the Alboran Sea. It also
hampers the west-to-east connection for different reasons. The geographical morphology of the cape facilitates the formation and growth of the EAG by deflecting
the AJ in a cross-basin direction as it approaches the cape. Under these circumstances, the southern shore west of the cape would be more likely connected with the
northeast Alboran shore than with the south shore east of the cape, as suggested by
Muñoz et al. (2015). This connection relies, however, on the stability of the
mesoscale, the permanency of the EAG in this case, which is affected by
submesoscale variability.
Another relevant example of the influence and consequences of topography is
illustrated in Fig. 12.7. Its oceanographic scope is the very Strait of Gibraltar, and it
emphasizes the role played by the shoreline orientation to achieve connectivity
between opposite shores separated by the energetic AJ. Several oceanographic
processes are involved, among which tides stand out. The experiment presented in
Fig. 12.7 shows the time evolution of passive drifters (virtual particles) initially
released at the western approach of the Strait at two different locations. The strong
tidal currents (see inset) result in a back-and-forth motion of the particles with
prevailing net eastwards (along-AJ) displacements. No cross-jet (i.e., north-south)
displacement is observed for the cases of particles initially released in the central and
north part, which move following zonal trajectories. Things are different for particles
released in the south, as a considerable fraction are seen crossing the eastern part of
the Strait 20 hours after their release (Fig. 12.7f). The likely origin of such deflection
is the coastline orientation west of Point Cires, which would allow for limited, but
not negligible, south-to-north connectivity.
As a collateral result of Fig. 12.7, it is worth mentioning the fact that tidal currents
can lead to a net westward transport if the passive drifters are located to the west of
12 Biophysical Processes Determining the Connectivity of the Alboran Sea Fish. . .
471