rising tide when the associated flood (westward) tidal currents enhance the velocity
in the Mediterranean layer.
These features linked to the ebb and flood tides are addressed separately in the
following sections. The discussion is just limited to spring tide situations, which is
when the energetic high-frequency motions we are interested in are observed. Data
within the spring tide period marked by the rectangle in Fig. 4 will be used in the
analysis, for it is representative of all other spring tide cycles.
Ebb Tide
Shortly before the slack tide corresponding to high water, the water column above
CS consists mainly of the Mediterranean water that has been flowing westwards
during the previous flood tide (Fig. 2c is a good reference for this situation).
Accordingly, the salinity registered by the CT probes at CSN and CSS is at their
maxima (labels (1) in Fig. 6), suggesting that it is at these moments when the less
mixed, purer Mediterranean waters are flowing out at those depths. A similar result
is inferred from temperature data, which are at their minima. At that moment the
observations illustrate the known cross-strait distribution of Mediterranean water
masses in CS, with colder WMDW flowing preferably through CSS and warmer
LIW doing it over CSN [25, 26]. The total current in the lower part of the water
column still points westwards, whereas it has already reversed (or it is about to) in
the upper column (labels (1), Fig. 6).
About 3 h after the high water, the ebb tide becomes maximum and so does the
eastward current in the upper layer (labels (2), Fig. 6). The Atlantic water transport
into the Mediterranean Sea also reaches a maximum, which in turn implies a
thickening of the Atlantic layer that deepens the interface [23, 27]. In CSN, the
deepening is perceived as a sudden increase of temperature and decrease of salinity,
which are not detected in CSS (labels (2), Fig. 6). The ebb current is strong enough
to reverse the flow in the lower Mediterranean layer, which now moves to the
Mediterranean Sea. Under these circumstances, the establishment of hydraulic
control and the eventual formation of a weak hydraulic jump east-of-CS, such as the
one sketched in Fig. 3f or illustrated in Fig. 2f, emerges as a realistic possibility, in
which case these peaks should be caused by processes related to the evolution and
fate of this jump. During neap tides, the reduced mixing along with the diminished
tidal transports prevent the mixed water to reach so deep and it is barely detected by
the CT probes, which measure rather constant values of temperature and salinity
(Fig. 4a, b).
During the following 2 h approximately (from labels (2) to (3) in Fig. 6) the
thermo-haline properties of water registered at CSN keep on differing from pure
Mediterranean water. After the mentioned sudden change, both variables display
irregular high frequency fluctuations and, occasionally, gradual changes towards
characteristics of more mixed-with-Atlantic water. The pattern has strong dependence
on the diurnal inequality, since it is only distinctly displayed every two tidal cycles.
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in the Mediterranean layer.
These features linked to the ebb and flood tides are addressed separately in the
following sections. The discussion is just limited to spring tide situations, which is
when the energetic high-frequency motions we are interested in are observed. Data
within the spring tide period marked by the rectangle in Fig. 4 will be used in the
analysis, for it is representative of all other spring tide cycles.
Ebb Tide
Shortly before the slack tide corresponding to high water, the water column above
CS consists mainly of the Mediterranean water that has been flowing westwards
during the previous flood tide (Fig. 2c is a good reference for this situation).
Accordingly, the salinity registered by the CT probes at CSN and CSS is at their
maxima (labels (1) in Fig. 6), suggesting that it is at these moments when the less
mixed, purer Mediterranean waters are flowing out at those depths. A similar result
is inferred from temperature data, which are at their minima. At that moment the
observations illustrate the known cross-strait distribution of Mediterranean water
masses in CS, with colder WMDW flowing preferably through CSS and warmer
LIW doing it over CSN [25, 26]. The total current in the lower part of the water
column still points westwards, whereas it has already reversed (or it is about to) in
the upper column (labels (1), Fig. 6).
About 3 h after the high water, the ebb tide becomes maximum and so does the
eastward current in the upper layer (labels (2), Fig. 6). The Atlantic water transport
into the Mediterranean Sea also reaches a maximum, which in turn implies a
thickening of the Atlantic layer that deepens the interface [23, 27]. In CSN, the
deepening is perceived as a sudden increase of temperature and decrease of salinity,
which are not detected in CSS (labels (2), Fig. 6). The ebb current is strong enough
to reverse the flow in the lower Mediterranean layer, which now moves to the
Mediterranean Sea. Under these circumstances, the establishment of hydraulic
control and the eventual formation of a weak hydraulic jump east-of-CS, such as the
one sketched in Fig. 3f or illustrated in Fig. 2f, emerges as a realistic possibility, in
which case these peaks should be caused by processes related to the evolution and
fate of this jump. During neap tides, the reduced mixing along with the diminished
tidal transports prevent the mixed water to reach so deep and it is barely detected by
the CT probes, which measure rather constant values of temperature and salinity
(Fig. 4a, b).
During the following 2 h approximately (from labels (2) to (3) in Fig. 6) the
thermo-haline properties of water registered at CSN keep on differing from pure
Mediterranean water. After the mentioned sudden change, both variables display
irregular high frequency fluctuations and, occasionally, gradual changes towards
characteristics of more mixed-with-Atlantic water. The pattern has strong dependence
on the diurnal inequality, since it is only distinctly displayed every two tidal cycles.
202
J. García-Lafuente et al.