387
constant winds during 50 hours and starting from a resting sea. Currpntly, research is
focussed to obtain more realistic numerical results, starting from a no wind stationary
state, imposing variable (intensity and direction) winds, seasonal input fluxes through
the Strait, taking into account the influence of tidal fluctuations over the Strait. etc ...
4.2 Strait of Gibraltar
Water mass exchange in the Strait can be sketched as follows: Atlantic water flows into
the Mediterranean; high evaporation causes water in the Mediterranean to become more
saline, and thus more dense than the incoming Atlantic water. Then this heavier water
sinks and come back to the Atlantic beneath the inflowing fresher water. We are then
in the situation mentioned above: two layers of different densities, flowing in opposite
directions.
Interesting phenomena arise on the interface of the two layers. At the shallowest
section of the Strait, known as the Camarinal Sill, the lower layer accelerates as water
falls in a deeper basin. Because of this acceleration, flow can become supercritical and
then an internal hydraulic jump appears, where the flow changes into a subcritical state.
To the east lies the narrowest distance across the Strait: the Tarifa narrows. The
inflow Atlantic water is constricted and therefore accelerated. Flow may become supercritical agajn. The internal waves created in this section reflect. ofl' the walls of the
Strait, creating cross-hatched structures easily recognized in satellite picturps.
Armi and Farmer [1] used a hydraulic model to understand the data obtained in
the Gibraltar experiment. They proposed the control region hypothesis, i.e. when
there are transitions from sub critical to supercritical flow in both Camarinal and Tarifa
sections, the water exchange is maximal. Nevertheless, this hypothesis is not sufficient
to explain the whole mechanism of water exchange across the Strait. Some other phenomena, as tides, play an important role. Tides are not important in the Strait, but
tidal currents are. Tides in the Atlantic present a much wider amplitude than in the
Mediterranean, causing strong currents to appear. During high tide in the Atlantic, the
speed of the out flowing current decreases and it may even reverse and flows back into
the Mediterranean. This phenomenon can make the internal hydraulic jump developed
in Camarinal Sill to become an internal bore, propagating to the east, followed by a
train of internal waves. Tidal currents from east to west cause also strong perturbations
in the interface. These perturbations can generate internal solitary waves propagating
eastward and westward from Camarinal Sill.
Other important features of the hydrodynamics in the Strait are due to several
causes, as Coriolis effects, instabilities of the interface, turbulent regimes. mixture of
different waters, etc ...
The two layers shallow water model described here can not simulate all of these
phenomena as a consequence of the simplifying hypothesis made. Nevertheless, such
a model can be useful to study the transitions from subcritical to supercritical states,
internal hydraulic jumps and bores, interaction between tidal currents and the bottom,
In figures 11 to 14 some preliminary results obtained for the Strait of Gibraltar
are presented. Figures 11 and 12 show the velocity field when no-wind is imposed.
The acceleration of the Atlantic and the Mediterranean water in Tarifa narrows and in
constant winds during 50 hours and starting from a resting sea. Currpntly, research is
focussed to obtain more realistic numerical results, starting from a no wind stationary
state, imposing variable (intensity and direction) winds, seasonal input fluxes through
the Strait, taking into account the influence of tidal fluctuations over the Strait. etc ...
4.2 Strait of Gibraltar
Water mass exchange in the Strait can be sketched as follows: Atlantic water flows into
the Mediterranean; high evaporation causes water in the Mediterranean to become more
saline, and thus more dense than the incoming Atlantic water. Then this heavier water
sinks and come back to the Atlantic beneath the inflowing fresher water. We are then
in the situation mentioned above: two layers of different densities, flowing in opposite
directions.
Interesting phenomena arise on the interface of the two layers. At the shallowest
section of the Strait, known as the Camarinal Sill, the lower layer accelerates as water
falls in a deeper basin. Because of this acceleration, flow can become supercritical and
then an internal hydraulic jump appears, where the flow changes into a subcritical state.
To the east lies the narrowest distance across the Strait: the Tarifa narrows. The
inflow Atlantic water is constricted and therefore accelerated. Flow may become supercritical agajn. The internal waves created in this section reflect. ofl' the walls of the
Strait, creating cross-hatched structures easily recognized in satellite picturps.
Armi and Farmer [1] used a hydraulic model to understand the data obtained in
the Gibraltar experiment. They proposed the control region hypothesis, i.e. when
there are transitions from sub critical to supercritical flow in both Camarinal and Tarifa
sections, the water exchange is maximal. Nevertheless, this hypothesis is not sufficient
to explain the whole mechanism of water exchange across the Strait. Some other phenomena, as tides, play an important role. Tides are not important in the Strait, but
tidal currents are. Tides in the Atlantic present a much wider amplitude than in the
Mediterranean, causing strong currents to appear. During high tide in the Atlantic, the
speed of the out flowing current decreases and it may even reverse and flows back into
the Mediterranean. This phenomenon can make the internal hydraulic jump developed
in Camarinal Sill to become an internal bore, propagating to the east, followed by a
train of internal waves. Tidal currents from east to west cause also strong perturbations
in the interface. These perturbations can generate internal solitary waves propagating
eastward and westward from Camarinal Sill.
Other important features of the hydrodynamics in the Strait are due to several
causes, as Coriolis effects, instabilities of the interface, turbulent regimes. mixture of
different waters, etc ...
The two layers shallow water model described here can not simulate all of these
phenomena as a consequence of the simplifying hypothesis made. Nevertheless, such
a model can be useful to study the transitions from subcritical to supercritical states,
internal hydraulic jumps and bores, interaction between tidal currents and the bottom,
In figures 11 to 14 some preliminary results obtained for the Strait of Gibraltar
are presented. Figures 11 and 12 show the velocity field when no-wind is imposed.
The acceleration of the Atlantic and the Mediterranean water in Tarifa narrows and in