4.5 Tidal Currents
The tides in the Mediterranean Sea are a consequence of the astronomical forces
directly acting on it, and the propagation of energy from the Atlantic Ocean through
the Strait of Gibraltar. In a very simple way, it can be said that the tide in the WMED
behaves as a standing wave with an amplitude node for sea surface elevation around
0–2
E and maximum amplitudes in the Strait of Gibraltar and at the eastern limit of
the Tyrrhenian Sea (Arabelos et al. 2011; Albérola et al. 1995). The Alboran Sea is
located to the west of the nodal line and currents have a phase of 90
with respect to
the sea surface, being zero for the high tide and increasing eastwards while sea level
decreases. A simple explanation for this behaviour by means of a linear analytical
model can be seen in García-Lafuente et al. (1994).
In the Alboran Sea, the main constituent is the M2 harmonic. Its amplitude
decreases from 30 cm in the Strait of Gibraltar to 10 cm at Cape Gata (Arabelos
et al. 2011, Albérola et al. 1995). The tidal currents associated to the M2 harmonic
are of the order of 2 cm/s, increasing with depth to 4 cm/s, very likely because of the
effect of topography on contracting streamlines (Albérola et al. 1995). More
recently, observations of Lagrangian currents from drifters have evidenced energy
peaks at the K1 (diurnal) and M2, S2 (semi-diurnal) frequency bands (Poulain et al.
2013).
Although the barotropic tide has a low amplitude in the Mediterranean Sea, it
produces large internal tides in the Strait of Gibraltar which enhance mixing between
the incoming AW and the outflowing Mediterranean Waters. As a result of these
mixing processes, the long-term exchange between the Atlantic Ocean and the
Mediterranean Sea is enhanced and the properties of the AW within the Alboran
Sea are modified (Naranjo et al. 2014).
4.6 The Upper Layer Circulation
As already described, the AJ is a fast current of AW flowing through the Strait of
Gibraltar into the Alboran Sea. Although the speed of the AJ can be modulated
within the Strait by the tidal currents, its average value is around 1 m/s (Perkins et al.
1990).
Figure 4.3 shows the most common features of the Alboran Sea upper layer
circulation. The circulation of the Alboran Sea is very complex and many different
situations can be observed. Nevertheless, from a very simplistic point of view, two
main different modes of circulation can be defined. The first one, probably the most
frequently described in the scientific literature, is the two-gyre circulation mode
(light grey lines in Fig. 4.3, Vargas-Yáñez et al. 2002). The second one is the coastal
circulation mode (dark grey lines in Fig. 4.3, Vargas-Yáñez et al. 2002). In the first
circulation mode, two anticyclonic gyres are well developed occupying both the
Western and Eastern Alboran sub-basins (Western Alboran Gyre, WAG, and
4 The Oceanographic and Climatic Context
93
The tides in the Mediterranean Sea are a consequence of the astronomical forces
directly acting on it, and the propagation of energy from the Atlantic Ocean through
the Strait of Gibraltar. In a very simple way, it can be said that the tide in the WMED
behaves as a standing wave with an amplitude node for sea surface elevation around
0–2
E and maximum amplitudes in the Strait of Gibraltar and at the eastern limit of
the Tyrrhenian Sea (Arabelos et al. 2011; Albérola et al. 1995). The Alboran Sea is
located to the west of the nodal line and currents have a phase of 90
with respect to
the sea surface, being zero for the high tide and increasing eastwards while sea level
decreases. A simple explanation for this behaviour by means of a linear analytical
model can be seen in García-Lafuente et al. (1994).
In the Alboran Sea, the main constituent is the M2 harmonic. Its amplitude
decreases from 30 cm in the Strait of Gibraltar to 10 cm at Cape Gata (Arabelos
et al. 2011, Albérola et al. 1995). The tidal currents associated to the M2 harmonic
are of the order of 2 cm/s, increasing with depth to 4 cm/s, very likely because of the
effect of topography on contracting streamlines (Albérola et al. 1995). More
recently, observations of Lagrangian currents from drifters have evidenced energy
peaks at the K1 (diurnal) and M2, S2 (semi-diurnal) frequency bands (Poulain et al.
2013).
Although the barotropic tide has a low amplitude in the Mediterranean Sea, it
produces large internal tides in the Strait of Gibraltar which enhance mixing between
the incoming AW and the outflowing Mediterranean Waters. As a result of these
mixing processes, the long-term exchange between the Atlantic Ocean and the
Mediterranean Sea is enhanced and the properties of the AW within the Alboran
Sea are modified (Naranjo et al. 2014).
4.6 The Upper Layer Circulation
As already described, the AJ is a fast current of AW flowing through the Strait of
Gibraltar into the Alboran Sea. Although the speed of the AJ can be modulated
within the Strait by the tidal currents, its average value is around 1 m/s (Perkins et al.
1990).
Figure 4.3 shows the most common features of the Alboran Sea upper layer
circulation. The circulation of the Alboran Sea is very complex and many different
situations can be observed. Nevertheless, from a very simplistic point of view, two
main different modes of circulation can be defined. The first one, probably the most
frequently described in the scientific literature, is the two-gyre circulation mode
(light grey lines in Fig. 4.3, Vargas-Yáñez et al. 2002). The second one is the coastal
circulation mode (dark grey lines in Fig. 4.3, Vargas-Yáñez et al. 2002). In the first
circulation mode, two anticyclonic gyres are well developed occupying both the
Western and Eastern Alboran sub-basins (Western Alboran Gyre, WAG, and
4 The Oceanographic and Climatic Context
93