15.1.2 Oceanography
From an oceanographic point of view, the Canary Islands are a region of great
interest due to the interaction of different phenomena: the upwelling off the
African coast, the cold Canary Current, waves and trade winds. The combination
of these phenomena creates a complex mosaic of environments throughout the
archipelago, with peculiarities in the physical–chemical conditions of its waters.
These processes cool the waters of the archipelago that would otherwise have
higher temperatures in relation to the latitude (Brito et al. 1984; Aguilera et al.
1994; Fernández-Palacios and Martín-Esquivel 2001a; Fernández-Palacios et al.
2001b). The waters of the archipelago are situated on the eastern limit of the North
Atlantic subtropical gyre, occupying an intermediate position between the rich
cold upwelling waters of the northwest African coast and the warm and oligotrophic waters of the open ocean, giving rise to the so-called Transitional Coastal
Zone (Braun and Molina 1988; Barton et al. 1998; Barton and Arístegui 2004).
The surface seawater temperature ranges from minimum values of 17–19 °C in
April-March to maximum values of around 23–25 °C in September–October. This
surface temperature regime is slightly cooler than expected at the islands’ latitude
in the Atlantic Ocean. This is due to the cold Canary Current and the upwelling off
the African coast (Braun and Molina 1988; Molina et al. 1996; Hernández-Guerra
and Nykjaer 1997; Tejera et al. 2002; Marrero-Díaz et al. 2006).
The upwelling off the nearby African coast has its origin in the action of trade
winds from the northeast over the northwestern coasts of Africa. This causes, following Ekman’s theory (Ekman1905), the emergence of deep waters perpendicular
to the direction of the winds and towards the archipelago. Upwelled seawater is
colder, lower in salinity and richer in nutrients compared to surface water. The
incidence of this phenomenon over the archipelago has an important spatial and
temporal variation. The eastern islands, the closest to it up-welling, are more affected
by this process. Its influence on central and western islands is less and mainly due to
the incidence of the so-called upwelling filaments. Temporally, upwelling is more
regular in those months when the trade winds are more intense and constant (May–
August) (Braun and Molina 1988; Nykjaer and Van Camp 1994; Hernández-Guerra
and Nykjaer 1997; Tejera et al. 2002; Barton and Arístegui 2004).
The cold Canary Current, which flows NNE–SSW, is a branch of the Gulf
Current, a cyclonic system of currents that carries warm waters from the Caribbean
Sea to the northeast Atlantic Ocean. As it crosses the Bay of Biscay, this current
begins to cool, becoming a cold current when it reaches the Canaries. The average
speed of this current is 0.1–0.2 ms
-1 , but undergoes major seasonal changes
reaching minimum values of 0.025 ms
-1 in summer. Each island acts as a barrier
during the flow of the current throughout the archipelago. This causes changes in
speed and creates mesoscale phenomena such as cyclonic and anticyclonic eddies
that produce sinking and upwelling events of small amounts of waters, and downstream warm wakes in the south and southwest regions of some islands (Braun and
Molina 1988; Arístegui et al. 1997; Tejera et al. 2002; Barton and Arístegui 2004).
15 Echinoderms of the Canary Islands, Spain
475
From an oceanographic point of view, the Canary Islands are a region of great
interest due to the interaction of different phenomena: the upwelling off the
African coast, the cold Canary Current, waves and trade winds. The combination
of these phenomena creates a complex mosaic of environments throughout the
archipelago, with peculiarities in the physical–chemical conditions of its waters.
These processes cool the waters of the archipelago that would otherwise have
higher temperatures in relation to the latitude (Brito et al. 1984; Aguilera et al.
1994; Fernández-Palacios and Martín-Esquivel 2001a; Fernández-Palacios et al.
2001b). The waters of the archipelago are situated on the eastern limit of the North
Atlantic subtropical gyre, occupying an intermediate position between the rich
cold upwelling waters of the northwest African coast and the warm and oligotrophic waters of the open ocean, giving rise to the so-called Transitional Coastal
Zone (Braun and Molina 1988; Barton et al. 1998; Barton and Arístegui 2004).
The surface seawater temperature ranges from minimum values of 17–19 °C in
April-March to maximum values of around 23–25 °C in September–October. This
surface temperature regime is slightly cooler than expected at the islands’ latitude
in the Atlantic Ocean. This is due to the cold Canary Current and the upwelling off
the African coast (Braun and Molina 1988; Molina et al. 1996; Hernández-Guerra
and Nykjaer 1997; Tejera et al. 2002; Marrero-Díaz et al. 2006).
The upwelling off the nearby African coast has its origin in the action of trade
winds from the northeast over the northwestern coasts of Africa. This causes, following Ekman’s theory (Ekman1905), the emergence of deep waters perpendicular
to the direction of the winds and towards the archipelago. Upwelled seawater is
colder, lower in salinity and richer in nutrients compared to surface water. The
incidence of this phenomenon over the archipelago has an important spatial and
temporal variation. The eastern islands, the closest to it up-welling, are more affected
by this process. Its influence on central and western islands is less and mainly due to
the incidence of the so-called upwelling filaments. Temporally, upwelling is more
regular in those months when the trade winds are more intense and constant (May–
August) (Braun and Molina 1988; Nykjaer and Van Camp 1994; Hernández-Guerra
and Nykjaer 1997; Tejera et al. 2002; Barton and Arístegui 2004).
The cold Canary Current, which flows NNE–SSW, is a branch of the Gulf
Current, a cyclonic system of currents that carries warm waters from the Caribbean
Sea to the northeast Atlantic Ocean. As it crosses the Bay of Biscay, this current
begins to cool, becoming a cold current when it reaches the Canaries. The average
speed of this current is 0.1–0.2 ms
-1 , but undergoes major seasonal changes
reaching minimum values of 0.025 ms
-1 in summer. Each island acts as a barrier
during the flow of the current throughout the archipelago. This causes changes in
speed and creates mesoscale phenomena such as cyclonic and anticyclonic eddies
that produce sinking and upwelling events of small amounts of waters, and downstream warm wakes in the south and southwest regions of some islands (Braun and
Molina 1988; Arístegui et al. 1997; Tejera et al. 2002; Barton and Arístegui 2004).
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