any case, after their disappearance, the development of the WAG and EAG requires
several weeks or even months (Vargas-Yáñez et al. 2002).
Besides the WAG migration episodes and the transition to the coastal circulation
mode, other processes with shorter spatial scales can be observed at the sub-inertial
scale (with periods of several days). These processes would include the north-south
oscillations of the AJ (Sarhan et al. 2000) which can induce the upwelling of
Mediterranean waters at the northern sector of the Alboran Sea, and the advection
of mesoscale structures (~20 km diameter) around the anticyclonic gyres (GarcíaLafuente et al. 1998; La Violette 1984).
The circulation of the upper layer of the Alboran Sea seems to be also subject to a
seasonal variability, although the studies and information concerning this time scale
are scarce. García-Martínez et al. (2018) and Vargas-Yáñez et al. (2017) have
revealed the existence of a salinity seasonal cycle in the continental shelf and
slope waters of the northern coast of the Alboran Sea. This cycle exhibits lower
salinity values and a higher influence of the AW in autumn. Salinity would reach
maximum values during winter and spring. It has been hypothesized that this cycle
could be linked to the wind seasonal cycle in the Alboran Sea. Westerly winds are
the prevailing ones during winter and spring. This would favour wind-induced
upwelling at the northern coast of the Alboran Sea. During summer, the wind
intensity decreases considerably and the easterly winds are more frequent. This
could explain the observed decrease of salinity during summer. Nevertheless,
westerly winds recover during autumn when the salinity reaches its minimum
value in the northern waters. This phase difference between winds and salinity
could be linked to the different time resolution of wind and salinity time series
used, or to a recent extension of the summer season during the 1990s and beginning
of the twenty-first century, when TS data were collected in the mentioned literature
(García-Martínez et al. 2018; Vargas-Yáñez et al. 2017). Another possibility is that
the Alboran Sea anticyclonic gyres have a larger size and affect in a much more
direct way to the Spanish coast during summer and autumn.
The transport through the Strait of Gibraltar and the AJ speed is a factor that could
affect the development of the Alboran Sea gyres (Velez-Belchí et al. 2005). Therefore, it is important to consider the existence of a seasonal cycle for the exchange
through Gibraltar (García-Lafuente et al. 2007), with maximum outflow in April.
These authors relate this increase of the outflow to the formation and replenishment
of deep waters during winter. Although it is very speculative for the moment, we
suggest that the seasonal changes in the volumes exchanged through Gibraltar could
influence the circulation of the Alboran Sea.
4.7 Intermediate and Deep Water Circulation
The circulation of intermediate and deep waters in the Alboran Sea is not as well
known as that of the upper layer. Much of the information concerning the upper layer
circulation comes from geostrophic calculations. These estimations require the
4 The Oceanographic and Climatic Context
99
several weeks or even months (Vargas-Yáñez et al. 2002).
Besides the WAG migration episodes and the transition to the coastal circulation
mode, other processes with shorter spatial scales can be observed at the sub-inertial
scale (with periods of several days). These processes would include the north-south
oscillations of the AJ (Sarhan et al. 2000) which can induce the upwelling of
Mediterranean waters at the northern sector of the Alboran Sea, and the advection
of mesoscale structures (~20 km diameter) around the anticyclonic gyres (GarcíaLafuente et al. 1998; La Violette 1984).
The circulation of the upper layer of the Alboran Sea seems to be also subject to a
seasonal variability, although the studies and information concerning this time scale
are scarce. García-Martínez et al. (2018) and Vargas-Yáñez et al. (2017) have
revealed the existence of a salinity seasonal cycle in the continental shelf and
slope waters of the northern coast of the Alboran Sea. This cycle exhibits lower
salinity values and a higher influence of the AW in autumn. Salinity would reach
maximum values during winter and spring. It has been hypothesized that this cycle
could be linked to the wind seasonal cycle in the Alboran Sea. Westerly winds are
the prevailing ones during winter and spring. This would favour wind-induced
upwelling at the northern coast of the Alboran Sea. During summer, the wind
intensity decreases considerably and the easterly winds are more frequent. This
could explain the observed decrease of salinity during summer. Nevertheless,
westerly winds recover during autumn when the salinity reaches its minimum
value in the northern waters. This phase difference between winds and salinity
could be linked to the different time resolution of wind and salinity time series
used, or to a recent extension of the summer season during the 1990s and beginning
of the twenty-first century, when TS data were collected in the mentioned literature
(García-Martínez et al. 2018; Vargas-Yáñez et al. 2017). Another possibility is that
the Alboran Sea anticyclonic gyres have a larger size and affect in a much more
direct way to the Spanish coast during summer and autumn.
The transport through the Strait of Gibraltar and the AJ speed is a factor that could
affect the development of the Alboran Sea gyres (Velez-Belchí et al. 2005). Therefore, it is important to consider the existence of a seasonal cycle for the exchange
through Gibraltar (García-Lafuente et al. 2007), with maximum outflow in April.
These authors relate this increase of the outflow to the formation and replenishment
of deep waters during winter. Although it is very speculative for the moment, we
suggest that the seasonal changes in the volumes exchanged through Gibraltar could
influence the circulation of the Alboran Sea.
4.7 Intermediate and Deep Water Circulation
The circulation of intermediate and deep waters in the Alboran Sea is not as well
known as that of the upper layer. Much of the information concerning the upper layer
circulation comes from geostrophic calculations. These estimations require the
4 The Oceanographic and Climatic Context
99