17.3 North Atlantic Oscillation Effect on the Alboran
Sea Area
The North Atlantic Oscillation reflects fluctuations in atmospheric pressure at sea
level between the Icelandic Low and the High of Azores. The NAO is the most
important source of variability, associated with many meteorological variations in
the North Atlantic region, affecting wind speed and direction and differences in
temperature and rainfall, particularly in winter (Paeth et al. 2003; Báez, Gimeno,
et al. 2013; Báez, Macías, et al. 2013; Clark et al. 2017). The NAO index can be
positive or negative. It is widely known that the positive phases of NAO induce
higher than average westerly winds across northern midlatitudes with a dry climate
in the Mediterranean region, while the negative phases of NAO induce major
precipitation in southern Europe. The NAO is not the only climatic index correlated
with interannual climate variability in the Northern Hemisphere (Hurrell 1995).
The NAO index defined by Sir Gilbert Thomas Walker and Bliss (1932) was the
difference of surface pressure between two stations placed at a similar longitude but
different latitude, with one placed at high latitudes (Akureyri, Iceland) and the other
at middle latitudes (Ponta Delgada, Azores). However, instrumental data is currently
avoided, and an estimate based on remote sensing is preferred. However, to improve
the instrumental series, the meteorological station of the Rock of Gibraltar was
proposed as the southern limit (Jones et al. 1997). This fact helps us understand
the importance of the NAO in the climate and meteorological conditions of the
Alboran Sea and adjacent areas. Báez et al. (2013) proved that the NAO, in
combination with the Arctic Oscillation, a climate oscillation index which represents
the state of atmospheric circulation over the Arctic, could affect the oceanographic
conditions of the Alboran Sea, with great influence on the upwelling and sea surface
temperature (SST). This is due to a combination of factors, such as the orography of
the Alboran Sea. Thus, the Alboran Sea is shaped like a small funnel which gradually
narrows towards the Atlantic Ocean and widens towards the Mediterranean Sea. The
coast is very steep in the north of the Alboran Basin, dominated by mountain ranges
such as the Sierra Nevada with high peaks (i.e., Mulhacen and Veleta peaks over
3000 m high), and the Rif in the south, similarly with high peaks (i.e., Mount
Tidighin peak with 2455 m). Under favorable weather conditions, these peaks are
freshwater reservoirs, being melted in spring and pouring the freshwater into the sea,
thus altering the depth of the mixture layer and modifying the oceanography of the
Alboran Sea.
It is widely accepted that small pelagic fish, such as anchovy and sardine, and
migrating tuna, among other fish resources, respond to large-scale climate oscillations, such as the North Atlantic Oscillation (NAO). Thus, NAO could affect their
abundance, recruitment, capturability, and physical condition of fish (Báez et al.
2011; Báez and Real 2011; Báez et al. 2013; Rubio et al. 2016). Nevertheless, there
are multiple examples of the effect of the NAO on pelagic, coastal, and demersal
ecosystems in the Alboran Sea (Báez et al. 2014; Báez 2016; Fernández et al. 2020).
17 North Atlantic Oscillation Effect on the Biology and Fisheries of Tuna. . .
583
Sea Area
The North Atlantic Oscillation reflects fluctuations in atmospheric pressure at sea
level between the Icelandic Low and the High of Azores. The NAO is the most
important source of variability, associated with many meteorological variations in
the North Atlantic region, affecting wind speed and direction and differences in
temperature and rainfall, particularly in winter (Paeth et al. 2003; Báez, Gimeno,
et al. 2013; Báez, Macías, et al. 2013; Clark et al. 2017). The NAO index can be
positive or negative. It is widely known that the positive phases of NAO induce
higher than average westerly winds across northern midlatitudes with a dry climate
in the Mediterranean region, while the negative phases of NAO induce major
precipitation in southern Europe. The NAO is not the only climatic index correlated
with interannual climate variability in the Northern Hemisphere (Hurrell 1995).
The NAO index defined by Sir Gilbert Thomas Walker and Bliss (1932) was the
difference of surface pressure between two stations placed at a similar longitude but
different latitude, with one placed at high latitudes (Akureyri, Iceland) and the other
at middle latitudes (Ponta Delgada, Azores). However, instrumental data is currently
avoided, and an estimate based on remote sensing is preferred. However, to improve
the instrumental series, the meteorological station of the Rock of Gibraltar was
proposed as the southern limit (Jones et al. 1997). This fact helps us understand
the importance of the NAO in the climate and meteorological conditions of the
Alboran Sea and adjacent areas. Báez et al. (2013) proved that the NAO, in
combination with the Arctic Oscillation, a climate oscillation index which represents
the state of atmospheric circulation over the Arctic, could affect the oceanographic
conditions of the Alboran Sea, with great influence on the upwelling and sea surface
temperature (SST). This is due to a combination of factors, such as the orography of
the Alboran Sea. Thus, the Alboran Sea is shaped like a small funnel which gradually
narrows towards the Atlantic Ocean and widens towards the Mediterranean Sea. The
coast is very steep in the north of the Alboran Basin, dominated by mountain ranges
such as the Sierra Nevada with high peaks (i.e., Mulhacen and Veleta peaks over
3000 m high), and the Rif in the south, similarly with high peaks (i.e., Mount
Tidighin peak with 2455 m). Under favorable weather conditions, these peaks are
freshwater reservoirs, being melted in spring and pouring the freshwater into the sea,
thus altering the depth of the mixture layer and modifying the oceanography of the
Alboran Sea.
It is widely accepted that small pelagic fish, such as anchovy and sardine, and
migrating tuna, among other fish resources, respond to large-scale climate oscillations, such as the North Atlantic Oscillation (NAO). Thus, NAO could affect their
abundance, recruitment, capturability, and physical condition of fish (Báez et al.
2011; Báez and Real 2011; Báez et al. 2013; Rubio et al. 2016). Nevertheless, there
are multiple examples of the effect of the NAO on pelagic, coastal, and demersal
ecosystems in the Alboran Sea (Báez et al. 2014; Báez 2016; Fernández et al. 2020).
17 North Atlantic Oscillation Effect on the Biology and Fisheries of Tuna. . .
583