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5 Bathymetry, Climate and Atmospheric Forcing

5 Bathymetry, Climate and Atmospheric Forcing

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210



6 The Canary/Iberia Current Upwelling System



and the northwest African shelf (e.g., Barton et al. 2004). A further feature of major

topographical importance is the Canaries archipelago at 28° N, which lies at the

transition zone between coastal and oceanic waters.

The northwest Iberian shelf is relatively narrow, ranging in width from 30 km in

the north to 50 km off the Douro Rover mouth in the south. The shelf break lies at a

depth of 160–180 m. The mid-shelf region, at depths of 100–140 m, hosts the

northwest Iberian mud belt—the major depocentre for fine-grained sediment from

the late Holocene to present (e.g., Oberle et al. 2014). The mudbelt is located

between plutonic and metamorphic outcrops on the inner shelf and transgressive to

modern well-sorted fine sands on the outer shelf.

The eastern Gulf of Cadiz continental margin is characterised by a crescent-shaped

continental shelf up to 45 km wide, with the shelf break at 130–140 m depth.

The continental slope displays an irregular bathymetry and is defined by rough ridge

and valley morphology, bordered by two wide terraces between 400 and 800 m water

depth (Baraza et al. 2003). The Gulf of Cadiz is an area where the Atlantic and

Mediterranean waters meet, resulting in a complex current pattern.

The continental shelf of northwest Africa is generally 40–60 km wide, but

ranges from a minimum of 13 km width at Cape Ghir (30.5° N) to 120 km width

between Cape Bojador and just to the south of Cape Blanc (Seibold 1982). The

shelf edge shoals from about 150 m offshore of Morocco to an average of about

110 m seaward of the Sahara, and ranges between 130 and 150 m near Senegal

(Tooms et al. 1971). Compared to other eastern boundary current upwelling

regions, the northwest African shelf is relatively shallow with average depths of

only 60–80 m. Numerous canyons and gullies cut the slope, except for the section

between 25° S and Cape Blanc.



6.5.2



Climate and Atmospheric Forcing



The Canary Current Coastal Upwelling System covers almost 30° of latitude, from

the Senegalese coast at about 15° N to the Cantabrian coast at 44° N (see Fig. 6.3).

The area of interest therefore encompasses a range of climatic conditions, from

tropical to the temperate mid-latitudes. As described by Ekman (1905) and later by

Bakun (1973), coastal upwelling of the region is the consequence of the alongshore

component of the trade winds; that is, coastal winds associated with the eastern

flanks of either the Bermuda High in boreal summer or the Azores High in boreal

winter (see Fig. 2.13). On longer decadal to century time scales, however, it appears

from core studies that upwelling-driven SSTs off Morocco vary out of phase with

northern hemisphere temperature anomalies (McGregor et al. 2007), so that warm

conditions existed during the Little Ice Age and cold conditions during the

Medieval Warm Period. In general, there has been consistent cooling and increased

upwelling throughout the 20th century, in agreement with increasing wind speeds at

Cape Ghir.



6.5 Bathymetry, Climate and Atmospheric Forcing



211



Fig. 6.5 Long-term average

seasonal cycle of the

offshore Ekman transport

(−Qx, in m2/s), calculated at

the different subregions of

the Canary current upwelling

system (from Arístegui et al.

2009)



The wind-driven offshore transport in the surface Ekman transport varies by one

order of magnitude between the southern reaches of the upwelling region (2.16 m2/

s, 12-month average at 17°30′ N) and the north (0.38 m2/s, 6-month average at 41°

30′ N) (Fig. 6.5). There is a pronounced seasonal variability in offshore Ekman

transports in the upwelling region associated with seasonal variations of the coastal

winds. In most of the region, the long-term mean upwelling index (offshore Ekman

transport) peaks in boreal summer (July–August) and attains minimum values in

winter (December). The Mauritanian–Senegalese region behaves differently and

shows an earlier peak in May–June and a minimum in offshore Ekman transports in

August–September. Note that in contrast to this long-term average, seasonal patterns of alongshore winds in the Mauritanian–Senegalese region vary significantly

from year to year (Fig. 6.6). Hence, the average seasonal pattern shown in Fig. 6.5

can be misleading.



Fig. 6.6 Upwelling index calculated from ship data in the Mauritania region, from 1964 to 1992

(from Demarcq 1998)



212



6 The Canary/Iberia Current Upwelling System



Fig. 6.7 a Seasonal

variability of the southern

limit of the Canary current

upwelling system. Full dots

indicate observed upwelling,

circles indicate observed

absence of upwelling.

b Frequency of occurrence of

winds favourable for

upwelling (wind direction is

in the quarter between

alongshore towards the south

and exactly offshore). Taken

from Tomczak and Godfrey

(2003), adapted from

Schemainda et al. (1975)



The coastal region between 20° and 30° N is characterized by upwellingfavourable wind stress all year-round (Bakun and Nelson 1991). Maximum magnitudes are found north of 21° N during boreal summer and south of 21° N during

boreal winter (Mittelstaedt 1991). South of 15° N the Inter-Tropical Convergence

Zone (ITCZ) becomes the main driver of the upwelling seasonality. As a consequence, the southern limit of the upwelling system shifts northward and southward

seasonally by more than 1000 km (Fig. 6.7). The Canary Current upwelling reaches

its southernmost extent in winter when the trade wind is strongest. It then extends

well past Cape Blanc—the separation point of the Canary Current from the African

coast.

Over the Iberian Peninsula (37°–43° N), upwelling-favourable equatorward

winds dominate during spring and summer months, whereas downwellingfavourable mean winds prevail in the other months. It is remarkable that due to

the different orientation of the western and northern Galician coasts, northerly

winds produce upwelling off the western coast whereas easterly winds do it off the

northern coast. The orientation of the coast changes abruptly north of Cape

Finisterre, in such a way that both northerly and easterly winds are upwelling

favourable there (McClain et al. 1986; Torres et al. 2003). Similar considerations

apply to Cape St Vincent (Fiúza 1983; Relvas and Barton 2002).



6.5 Bathymetry, Climate and Atmospheric Forcing



6.5.3



213



Atmospheric Nutrient Inputs



Atmospheric inputs are relatively minor along the coastal upwelling systems of the

west coast of the Americas and southern Africa, although they do occur in the

Benguela system (see Chap. 7). In the Canary Current upwelling system, however,

atmospheric inputs represent a major source of nutrients, particularly iron (Neuer et al.

2004). The Sahara desert is the major supplier of dust to the subtropical North Atlantic

and Eurafrican Mediterranean with an estimated annual deposition of 220 Â 109 kg

per annum (Duce and Tindale 1991). The dust plume can be traced year-round by

satellite and its area of maximum deposition shifts from around 5° N in winter to

around 20° N in summer, driven by the shift of the Inter-Tropical Convergence Zone

(Moulin et al. 1997). The Canary Island region (28°–29° N) is located on the northern

margin of the main dust cloud and thus receives episodic pulses with peaks reaching

the area mainly in winter and in summer/autumn (Torres-Padrón et al. 2002).



Fig. 6.8 Phytoplankton and zooplankton response north of Gran Canaria (Canary Islands) after a

major sand storm from the Sahara. Panel (a) SeaWiFs satellite image of the dust storm from 26

February 2000 [http://visibleearth.nasa.gov/cgibin/viewrecord?22352, (accessed 5 April 2016)].

Panel (b) Chlorophyll-a (mg/m3) at 5 m depth. Panel (c–f) Average zooplankton biomass (mg dry

weight/m2) and standard error separately represented for four zooplankton size fractions. The date

of the dust storm in late February is marked with arrows. Stars indicate chlorophyll and zooplankton responses to the dust storm one week later. Taken from Hernández-León et al. (2004)



214



6 The Canary/Iberia Current Upwelling System



Atmospheric dust has two different impacts on the ocean's biogeochemistry.

First it can provide nutrients for phytoplankton growth, both macronutrients and

trace metals (e.g., Gao et al. 2003). Second, it can accelerate or induce carbon

sedimentation by adsorption, ballasting and possibly aggregation of marine particles such as detritus or faecal pellets (e.g. Fischer et al. 2007). Neuer et al. (2004)

found a close correlation between the atmospheric dust concentration in winter and

the magnitude of the downward particle flux at a station near Gran Canaria (Canary

Islands). At the end of February 2000, an impressive cloud of dust originating in the

Sahara desert probably contributed the additional nutrients that led to an increase in

chlorophyll and small zooplankton in the Canary Island region a week later

(Fig. 6.8; Hernández-León et al. 2004).



6.6

6.6.1



Physical Oceanography

Circulation



The main large-scale currents associated with the eastern part of the anticyclonic

North Atlantic subtropical gyre are the North Atlantic Current, the Azores Current

and the Canary Current (Fig. 6.9). The North Atlantic Current branches into three

southward-flowing streams as it moves eastwards across the basin, forming the

broad, slow and generally southward-flowing Portugal Current and the Azores

Current on the eastern boundary. The Portugal Current connects the northern

branch of the North Atlantic Current with the Canary Current (Pérez et al. 2001),

flowing southwards year round from 45°–50° N to 10°–20° W (Krauss 1986),

although its contribution is small (Barton 2001). The remainder of the North

Atlantic Current heads northeastward, becoming the North Atlantic Drift located

between Iceland and the British Isles. The Canary Current is supplied also by an

eastward branch of the Azores Current, which passes north of and around Madeira.

The total transport of the Canary Current is seasonally constant, although its

geostrophic circulation varies; it is stronger in summer near the African coast, while

in winter it is stronger west of the Canary Islands (Stramma and Siedler 1988).

Three papers by Pelíz and Fiúza (1999) and Pelíz et al. (2002, 2005) characterize

the surface circulation off Iberia. The circulation pattern near the Iberian shelf is

complex (Fig. 6.10). A pronounced year-round feature is a poleward coastal flow

associated with a low-salinity water lens, which extends all along the coast as a

narrow band—the Western Iberia Buoyant Plume (WIBP). This plume is related to

buoyancy input from the many regional rivers (the Douro, Minho and Mondego

Rivers, other smaller rivers, and the Rías Baixas) (Pelíz et al. 2002). The WIBP

influences upwelling by increasing stratification over the shelf and by the creation

of an inshore frontal region that promotes northward baroclinic transport. Apart

from this year-round feature, currents on the west Iberian shelf experience a marked



6.6 Physical Oceanography



215



Fig. 6.9 General structure of the recirculation branches of the North Atlantic Current. NAD North

Atlantic Drift; PC Portugal Current; AC Azores Current; CC Canary Current; SAF Sub-Artic

Front; PF Polar Front; AF Arctic Front. Lines are SST contours. Taken from Eynaud et al. (2007)



seasonal variability defined by the coastal wind regime along the western and

northern coast.

During spring and summer (from March/April to September/October) northeasterly

winds predominate in the Iberian basin (Wooster et al. 1976; Bakun and Nelson 1991),

producing the southward flowing Portugal Coastal Current at the surface (<100 m)

(Fig. 6.10a) and the northward flowing Portugal Coastal Undercurrent on the slope.

The Portugal Coastal Current is associated with seasonal coastal upwelling at the

Iberian margin, and the export of coastal surface waters to the open ocean, especially at

the recurrent upwelling centres and filaments along the western Iberian coast

(Fig. 6.11; e.g., Haynes et al. 1993; Pelíz and Fiúza 1999).

southwesterly winds are predominant during the autumn and winter months,

provoking a reversal of the surface circulation to form the Iberia Polar Current

(also called Portugal Coastal Counter Current) (Fig. 6.10b). The existence of this

poleward surface flow was described first by Wooster et al. (1976) and subsequently by Frouin et al. (1990) and Haynes and Barton (1990) along the

western Iberian coast and by Pingree and Le Cann (1990) along the Cantabrian

coast. At about 39°–40° N, winter imagery of sea surface temperatures reveals a

recurrent meandering frontal system, named the Western Iberia Winter Front



216



6 The Canary/Iberia Current Upwelling System



Fig. 6.10 Schematic of the surface circulation (arrows) with a focus on the west Iberian shelf.

Dashed lines indicate frontal regions. WIBP Western Iberia Buoyant Plume. Small arrows near the

coast indicate the flow direction of river plumes. a Summer. b Winter

Fig. 6.11 Early

satellite-based evidence of

filaments forming along the

western Iberian coast for

September 2, 1982. Taken

from Haynes et al. (1993)



(see Pelíz et al. 2005). This front represents the transition to the southern area of the

Iberian Basin, where the Portugal Current is less influential, and supports the

eastward advection of relatively warm and salty water that feeds into the Iberia

Polar Current (Pelíz et al. 2005).



6.6 Physical Oceanography



217



The Iberia Poleward Current is a narrow (25–40 km) slope-trapped tongue-like

structure that flows northward for more than 1500 km off the coasts of the Iberian

Peninsula and southeast France. It normally arrives in the Cantabrian Sea at the

beginning of every winter (this is well documented, see e.g., Gil 2003). The January

warm-water extension of the Iberia Poleward Current along the Cantabrian coast

has been referred to as Navidad because it starts to be evident around Christmas and

New Year. Due to the inability of the poleward flow to follow abrupt changes of

topography, such as around Cape Ortegal, just east of Cape Finisterre, and the Cape

Ferret Canyon in the southeast Bay of Biscay, the Iberian Poleward Current exhibits

a turbulent nature that results in local instabilities and gives rise to eddies that

separate from the current and move offshore into the Bay of Biscay region (e.g.

Pingree and Le Cann 1990; García-Soto et al. 2002).

The coastal upwelling region from Gibraltar to Cape Blanc is maintained by the

presence of favourable northeasterly winds throughout the year, although winds and

upwelling are more intense during the summer months (see Fig. 6.5). In contrast

with the Iberian coast, the northwest African coast is largely influenced by the

general circulation of the North Atlantic subtropical gyre, particularly by its eastern

branch—the Canary Current. This current flows equatorward while interacting with

the coastal upwelling waters. It detaches from the coast near Cape Blanc (21° N),

flowing westward at the latitude of Cape Vert (15° N). South of Cape Blanc, a large

permanent cyclonic recirculation—the Mauritanian Gyre—develops as a consequence of the offshore displacement of the Canary Current. In winter, a near shore,

narrow equatorward flow develops over the shelf in response to coastal upwelling

forced by the southward migration of the trade wind band (Hughes and Barton

1974). Several studies (e.g., Stramma and Siedler 1988; Siedler and Onken 1996)

have described the seasonal variability of the Canary Current, confirming the

existence of the inflow of water from the open ocean into the coastal upwelling

region north of the Canary Islands.

The poleward undercurrent (Barton 1989) is a persistent feature off the northwest African coast (Fig. 6.12). It appears to be strongest at about 100–200 m depth,

but extends down to about 1000 m. The undercurrent has a width of <100 km and a

mean speed of 10 cm/s at the level of maximum flow. Generally the poleward

undercurrent follows the shelf break. South of Cape Blanc, the poleward undercurrent is indistinguishable from the surface Mauritanian Current. North of Cape

Blanc the upwelling front is found far offshore and creates a near-slope northward

path to the undercurrent (Peña-Izquierdo et al. 2012). Trajectories of subsurface

drifters indicate that a substantial volume fraction of the undercurrent recirculates

offshore just south of Cape Blanc.

A number of authors have provided estimates of the flow in the Canary Current,

but these are very variable depending on how they were obtained. Early work,

summarized by Stramma (1984), suggested that the eastward flow of the Azores

Current is up to *14 Sv. As shown above, however, the flow splits into a number

of separate streams between 35° W and the African coast, all of which are generally

weak (2–8 cm/s), and it is not always easy to determine which flow stream is being

discussed. Stramma (1984) suggested that the Portugal Current transports about



218



6 The Canary/Iberia Current Upwelling System



Fig. 6.12 Two sections of

alongshore flow measured by

current meters (solid dots)

near 20° N off northwest

Africa, showing the structure

of the poleward undercurrent.

Speeds are given in cm/s,

northward positive. Taken

from Barton (2001)



3 Sv southward, but only about 1 Sv of this actually feeds the Canary Current along

the coast of Morocco. Later work (e.g., Navarro-Pérez and Barton 2001; Knoll et al.

2002; Hernandez-Guerra et al. 2005; Mason et al. 2011) using both hydrography,

current meter measurements and fine-scale models, suggests that the Azores Current

transports only about 10 Sv, and that the mean southward flow along the African

coastline is less than 2 Sv, possibly as low as 0.2 Sv (Knoll et al. 2002). This flow

appears to be seasonal, and while offshore there may be net southward flow of up to

about 4.5 Sv, along the coast the flow can be northward, particularly in winter and

spring.



6.6.2



Bathymetric Features and Frontal Zones



The complex bathymetry of both the Iberian Peninsula and the North African coast

leads to persistent hydrological features in the Canary Current system. The Rías

Baixas, a series of large coastal inlets at the northern end of the Iberian coast, forms

an isolated ecosystem that amplifies upwelling-induced biogeochemical signals

(Álvarez et al. 2005). Nutrient supply to the rias is enhanced during the upwelling



6.6 Physical Oceanography



219



season, while the local topography gives rise to cool filaments during this time and

slope water eddies during the downwelling season (see Fig. 6.11). Both filaments

and eddies enhance the exchange of water, dissolved species, and suspended

material between the coastal region and the open ocean (Álvarez-Salgado et al.

2010). Further south, capes and promontories such as Cape Roca and Cape St.

Vincent in Portugal, and Cape Ghir and Cape Blanc in Africa, produce additional

filaments of upwelled coastal water that can stretch up to several hundred kilometres into the open ocean, transporting waters rich in organic matter into the

impoverished oligotrophic waters of the subtropical gyre.

Another singularity of this eastern boundary system is the presence of the

Canary Archipelago, close to the northwest African coast, which interrupts the main

flow of the Canary Current and introduces large mesoscale variability, mainly in the

form of vortex streets downstream of the islands that form a consistent pathway that

the eddies follow (Arístegui et al. 1994). The interaction between the island eddies

and upwelling filaments in this Canary-Coastal Transition Zone (also called the

Canary eddy corridor; see Sect. 6.7.3) is another mechanism for the exchange of

water properties, and thus acts as an efficient route for transporting organic matter to

the open ocean (e.g., Pelegrí et al. 2005). Research cruises have shown how the

water recirculates southward along the continental slope, where quasi-permanent

filaments stretch offshore and exchange water properties with island eddies

(Arístegui et al. 1997; Barton et al. 1998). Reversals in the main flow have been

observed close to the Canary-Coastal Transition Zone during late autumn and

winter (e.g. Navarro-Pérez and Barton 2001). These flow diversions, probably

caused by a weakening of the trade winds south of Cape Ghir (Pelegrí et al. 2005),

allow a northward flow to develop between Cape Blanc and Cape Juby, and consequently an offshore spread of organic matter produced in upwelling waters near

the Canary Islands region (Arístegui et al. 1997).

Pelegrí et al. (2005) suggested that the Canary Current region is characterised by

the presence of two cells transporting upwelled waters into the open ocean. The first

one would be the standard vertical cell, present in all upwelling systems, with

Ekman offshore transport responding to the wind variations. The second one would

be the horizontal circulation cell that starts with open ocean water reaching the coast

north of Cape Ghir and which is closed by the offshore export of water through

several upwelling filaments and the flow diversion at Cape Ghir. The joint action of

both cells would cause this upwelling region to be a key region for export of

organic matter and nutrients to the open ocean.

The Cape Verde Frontal Zone, located at around 15° N offshore, separates water

of tropical (southern) origin and water of subtropical (northern) origin. Water

carried southward by the Canary Current and water carried northward by the

Mauritania Current converge in this zone and feed into the North Equatorial

Current (Fig. 6.3). This frontal zone is found to intersect the slope north of Cape

Blanc, with its position varying between 21° and 23° N, although tropical influences can sometimes predominate over the slope as far north as 24° N

(Peña-Izquierdo et al. 2012).



220



6.6.3



6 The Canary/Iberia Current Upwelling System



Water Masses and Nutrient Concentrations



Most of the region, from Cape Finisterre to Cape Blanc, is dominated by ENACW,

which is responsible for the fertility of coastal waters during upwelling events. Two

branches of ENACW of either subtropical or subpolar origin occupy the depth

range from 50–100 to 450–750 m, depending on latitude. The subtropical portion

sits above subpolar waters off the western Iberian coast and north of the Canary

region, so nutrient-poorer (0–6 lM of nitrate) subtropical water upwells first, followed by nutrient-richer (6–10 lM of nitrate) subpolar water that enters the shelf

only during strong upwelling events. Bay of Biscay Central Water, characterized by

a temperature of 12 °C and 6 lM of nitrate, dominates the central waters off the

Cantabrian coast (e.g., Botas et al. 1989).

The nutrient regime in coastal waters of northwest Africa is conditioned by the

presence of two different water masses. A marked front at about 21° N (Cape

Blanc), which is the coastward extension of the Cape Verde Frontal Zone, separates

North Atlantic Central Water (NACW) from the slightly cooler, less saline, and

nutrient-richer South Atlantic Central Water (SACW) . SACW crosses the equator

in the North Brazil Current and then is carried eastwards in the North Equatorial

Countercurrent to feed into the northward flow off the coast of West Africa. The

boundary between these two water masses is convoluted, variable in location and

characterized by intense mixing and interleaving processes (e.g., Hagen and

Schemainda 1987). Since the SACW is richer in nutrients than the NACW, a

meridionally decreasing nutrient gradient is apparent in the northward flowing

waters. Compared to other eastern boundary upwelling regions, the source waters

off northwest Africa are somewhat poorer in nutrients, but richer in oxygen, largely

as a result of the basin-scale circulation and the fact that the upwelling ENACW

comes from the ventilated thermocline, unlike in other regions (Codispoti et al.

1982; Castro et al. 2000; Álvarez-Salgado et al. 2010).



6.6.4



Spatial Differences in Upwelling Dynamics



Arístegui et al. (2009) discuss differences in the upwelling dynamics in different

subregions of the Canary Current upwelling region in comparison to the classical

mechanism. The classical upwelling circulation for eastern boundary systems

(Fig. 6.13a) consists of an equatorward wind stress provoking offshore transport in

the surface Ekman layer which is replaced by onshore flow in and below the

upwelled pycnocline. The offshore surface Ekman drift creates an equatorward

geostrophic coastal upwelling jet, and a poleward undercurrent is found trapped on

the continental slope (see Chap. 2).



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