the CT probes. Velocity records, on the other hand, exhibit a sharp pulse involving
the whole water column in the high-frequency series at CSS (labels (3) in Fig. 8)
with greater amplitude at middle depths. The pulse can be also identified in the total
current, which is waning now, by a sudden westward increase of the deep layer
velocity of up to 70 cm s
−1 and short duration (labels (5) in Fig. 6b). It almost
certainly corresponds with the passing of the bore over the sill. Therefore, the
velocity observations in CSS would be suitable not only to follow the formation and
subsequent evolution of the hydraulic control and associated west-of-CS hydraulic
jump but also to detect its further release and eastwards propagation. Intriguingly,
the velocity series at CSN are rather insensitive to these processes, the
high-frequency contribution appearing somewhat blurred even though it tends to
increase its energy during the flood tide too (Figs. 5a, 8a). It appears as if the
east-of-CS hydraulic processes are better seen in CSN while the west-of-CS ones
leave clearer signatures in CSS.
Discussion and Conclusions
The temperature, salinity and velocity data collected at CSN and CSS sites in the
sill of the Strait of Gibraltar have proven to be useful to observe and follow the
successive hydraulic states that take place in this particular environment along
the tidal cycle. The data illustrate the different periodicities in the occurrence and
strength of the internal features (Fig. 4), the most obvious one being the fortnightly
cycle, which is fairly intuitive, but also a diurnal inequality arising from the relatively important diurnal tidal currents [7]. Being interested in high-energy internal
processes, a specific spring-tide period has been chosen as representative in order to
carry out an in-depth analysis (Fig. 5) that can be extended to all others cycles.
In addition to the alikeness of the data recorded in either site, the analysis has
also highlighted differences in the observed patterns. In general terms, it could be
said that CSN is better suited to observe internal processes associated with
hydraulic transitions occurring over and on the eastern side of the main sill (i.e. the
east-of-CS hydraulic jumps) while CSS is more sensitive to the processes taking
place over and on the west side, which are the most representative and, consequently, the most addressed in the literature. The bottom topography tends to
support this differentiation in the sense that the configuration of the closed
depression east of CS (LB in Fig. 1b) seems to be more the erosion outcome of
eventual strong eastward deep flow through CSN rather than through CSS, whereas
the erosion of the western depression (PB in Fig. 1b) suggests a more efficient
contribution from the flow through CSS, even though it appears to be contributed
by both channels.
An obvious physical process that has influence on the cross-strait flow structure
is the Earth’s rotation. It gives rise to a semi-geostrophic flow (across-strait geostrophic balance) that holds even at tidal frequencies [5–7], with the interface being
shallower in the north side for the mean exchange. However, the interface slope
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J. García-Lafuente et al.
the whole water column in the high-frequency series at CSS (labels (3) in Fig. 8)
with greater amplitude at middle depths. The pulse can be also identified in the total
current, which is waning now, by a sudden westward increase of the deep layer
velocity of up to 70 cm s
−1 and short duration (labels (5) in Fig. 6b). It almost
certainly corresponds with the passing of the bore over the sill. Therefore, the
velocity observations in CSS would be suitable not only to follow the formation and
subsequent evolution of the hydraulic control and associated west-of-CS hydraulic
jump but also to detect its further release and eastwards propagation. Intriguingly,
the velocity series at CSN are rather insensitive to these processes, the
high-frequency contribution appearing somewhat blurred even though it tends to
increase its energy during the flood tide too (Figs. 5a, 8a). It appears as if the
east-of-CS hydraulic processes are better seen in CSN while the west-of-CS ones
leave clearer signatures in CSS.
Discussion and Conclusions
The temperature, salinity and velocity data collected at CSN and CSS sites in the
sill of the Strait of Gibraltar have proven to be useful to observe and follow the
successive hydraulic states that take place in this particular environment along
the tidal cycle. The data illustrate the different periodicities in the occurrence and
strength of the internal features (Fig. 4), the most obvious one being the fortnightly
cycle, which is fairly intuitive, but also a diurnal inequality arising from the relatively important diurnal tidal currents [7]. Being interested in high-energy internal
processes, a specific spring-tide period has been chosen as representative in order to
carry out an in-depth analysis (Fig. 5) that can be extended to all others cycles.
In addition to the alikeness of the data recorded in either site, the analysis has
also highlighted differences in the observed patterns. In general terms, it could be
said that CSN is better suited to observe internal processes associated with
hydraulic transitions occurring over and on the eastern side of the main sill (i.e. the
east-of-CS hydraulic jumps) while CSS is more sensitive to the processes taking
place over and on the west side, which are the most representative and, consequently, the most addressed in the literature. The bottom topography tends to
support this differentiation in the sense that the configuration of the closed
depression east of CS (LB in Fig. 1b) seems to be more the erosion outcome of
eventual strong eastward deep flow through CSN rather than through CSS, whereas
the erosion of the western depression (PB in Fig. 1b) suggests a more efficient
contribution from the flow through CSS, even though it appears to be contributed
by both channels.
An obvious physical process that has influence on the cross-strait flow structure
is the Earth’s rotation. It gives rise to a semi-geostrophic flow (across-strait geostrophic balance) that holds even at tidal frequencies [5–7], with the interface being
shallower in the north side for the mean exchange. However, the interface slope
206
J. García-Lafuente et al.