area built from hydrographic observations (green arrow in Fig. 12.4; see also Cano
1977; Tintore et al. 1991; Garcia-Lafuente et al. 1998).
The north-to-south drifts of the AJ in the northwestern part of the Alboran Sea as
the jet enters the basin, linked to the variability of the internal hydraulics of the
exchange through the Strait (Sarhan et al. 2000), is another mechanism able to
transport sardine and other neritic larvae offshore, as reported by Vargas-Yañez
and Sabates (2007). According to these authors, the same process could account for
inshore transport and upwelling of larvae of mesopelagic species such as Maurolicus
muelleri and Benthosema glaciale.
Instabilities associated with mesoscale and submesoscale fields are quite probably
the most efficient mechanism for north-south hydrodynamic connectivity. SanchezGarrido et al. (2013) show that these instabilities are not sporadic processes but
rather the consequence of the evolution of structures that are regularly fed by the
energetic AJ. One of them is a small cyclonic eddy located in the northwestern area
of the basin off Estepona (C1 in Fig. 12.5), whose origin is the separation of the AJ
from the Spanish shore when it flows past Point Europa (Gibraltar Rock). According
to Sanchez-Garrido et al. (2013), the lateral friction of the AJ with the north shore of
the Strait generates positive relative vorticity, which is advected by the AJ. Part of it
accumulates in the cyclonic eddy and makes it to increase in size (Fig. 12.5b and c)
until it cannot grow anymore. In these circumstances, it becomes unstable and gets
rid of the accumulated vorticity by releasing submesoscale cyclonic vortices that
wander around the basin. They can even trigger longer spatial-scale disturbances that
eventually lead to the WAG disappearance, as suggested by Fig. 12.5d. The wandering eddies last for days and have chances to end up on the southern shore of the
Alboran Sea. Any biological material in their interior will have the same fate, in
which case the vortices would establish an intermittent connection between both
shores. Obviously, this intermittency does not guarantee the survival of larvae unless
larval trophic resources are available during the vortex wandering.
Numerical simulations by Sanchez-Garrido et al. (2013) confirm the key role that
tidal currents in the Strait play in triggering and enhancing the process, as they are
first-class contributors to the generation of relative vorticity. If tides are removed in
the simulation, the mechanism is still at work, but at a much slower pace. Spring
tides are, therefore, preferred periods for this intermittent connection to happen.
Propitious meteorological conditions are important to enhance the process as well. It
is worth mentioning that such processes have been rarely documented by in situ
observations. One of them, analyzed in Garcia-Lafuente et al. (1998), was reported
to occur in the summer of the year 1993. The authors presented strong evidences of
north-to-south transport of biological material, as they identified a small cyclonic
eddy close to the southern shore of the Alboran Sea (see red arrow in Fig. 12.4) with
indisputable biological content and hydrological properties of waters from the
northern part of the basin (Fig. 12.6).
Wind stress is another external agent that propitiates migration of surface inshore
waters offshore-ward. This is the case often found in upwelling systems such as the
west Iberian Peninsula (Smyth et al. 2001; Alvarez-Salgado et al. 2001; Sanchez
et al. 2008) or northwest Africa (Rodriguez et al. 2006; Sangra 2015) upwelling
468
J. Garcia-Lafuente et al.
1977; Tintore et al. 1991; Garcia-Lafuente et al. 1998).
The north-to-south drifts of the AJ in the northwestern part of the Alboran Sea as
the jet enters the basin, linked to the variability of the internal hydraulics of the
exchange through the Strait (Sarhan et al. 2000), is another mechanism able to
transport sardine and other neritic larvae offshore, as reported by Vargas-Yañez
and Sabates (2007). According to these authors, the same process could account for
inshore transport and upwelling of larvae of mesopelagic species such as Maurolicus
muelleri and Benthosema glaciale.
Instabilities associated with mesoscale and submesoscale fields are quite probably
the most efficient mechanism for north-south hydrodynamic connectivity. SanchezGarrido et al. (2013) show that these instabilities are not sporadic processes but
rather the consequence of the evolution of structures that are regularly fed by the
energetic AJ. One of them is a small cyclonic eddy located in the northwestern area
of the basin off Estepona (C1 in Fig. 12.5), whose origin is the separation of the AJ
from the Spanish shore when it flows past Point Europa (Gibraltar Rock). According
to Sanchez-Garrido et al. (2013), the lateral friction of the AJ with the north shore of
the Strait generates positive relative vorticity, which is advected by the AJ. Part of it
accumulates in the cyclonic eddy and makes it to increase in size (Fig. 12.5b and c)
until it cannot grow anymore. In these circumstances, it becomes unstable and gets
rid of the accumulated vorticity by releasing submesoscale cyclonic vortices that
wander around the basin. They can even trigger longer spatial-scale disturbances that
eventually lead to the WAG disappearance, as suggested by Fig. 12.5d. The wandering eddies last for days and have chances to end up on the southern shore of the
Alboran Sea. Any biological material in their interior will have the same fate, in
which case the vortices would establish an intermittent connection between both
shores. Obviously, this intermittency does not guarantee the survival of larvae unless
larval trophic resources are available during the vortex wandering.
Numerical simulations by Sanchez-Garrido et al. (2013) confirm the key role that
tidal currents in the Strait play in triggering and enhancing the process, as they are
first-class contributors to the generation of relative vorticity. If tides are removed in
the simulation, the mechanism is still at work, but at a much slower pace. Spring
tides are, therefore, preferred periods for this intermittent connection to happen.
Propitious meteorological conditions are important to enhance the process as well. It
is worth mentioning that such processes have been rarely documented by in situ
observations. One of them, analyzed in Garcia-Lafuente et al. (1998), was reported
to occur in the summer of the year 1993. The authors presented strong evidences of
north-to-south transport of biological material, as they identified a small cyclonic
eddy close to the southern shore of the Alboran Sea (see red arrow in Fig. 12.4) with
indisputable biological content and hydrological properties of waters from the
northern part of the basin (Fig. 12.6).
Wind stress is another external agent that propitiates migration of surface inshore
waters offshore-ward. This is the case often found in upwelling systems such as the
west Iberian Peninsula (Smyth et al. 2001; Alvarez-Salgado et al. 2001; Sanchez
et al. 2008) or northwest Africa (Rodriguez et al. 2006; Sangra 2015) upwelling
468
J. Garcia-Lafuente et al.