228
Chapter 9: The Atlantic Ocean
boreal winter, wherever their back trajectory lies over the desert (Hastenrath, 1985). These
Harmattan winds bear a heavy burden of mineral dust over the ocean from Morocco
to the Gulf of Guinea and may deposit as much as 25 g m
−2 y
−1 at the sea surface off
Senegal. This dust deposition is among the heaviest anywhere in the oceans and is similar
to deposition of loess clays in the northwestern Pacific; such aeolian deposits may be a
major source of turbidity in inshore waters and have, of course, been invoked in releasing
autotrophic cells from Fe limitation, as was discussed in Chapter 5.
Response of the Pelagic Ecosystems
Although the North Atlantic bloom of late winter or spring importantly affects the
shelf waters of Iberia south to the Gulf of Cadiz and is forced by stratification, growth
of phytoplankton in the CNRY province is overwhelmingly controlled by wind-driven
variations in vertical transport of nutrients into the euphotic zone. An intuitive and
simple negative relationship between chlorophyll and temperature was apparent in the
earliest satellite images for which both fields were available (van Camp et al., 1991),
similar to that established off South Africa (Shannon, 1985; Lutjeharms et al., 1985), and
is abundantly confirmed by relevant images available today. The dimension of upwelling
cells is smaller than the area over which appropriate wind stress is applied, because water
depth restricts upwelling to a band only 10–20 km wide (Barber and Smith, 1981) except
in the case of shelf-edge upwelling.
The continuous series of chlorophyll images available since 1997 demonstrate unequivocally how the consequences of upwelling dominate phytoplankton ecology in this
province. The 30-day SeaWiFS and MODIS composites cannot reveal the details of the
evolution of individual upwelling events and conceal the form of individual filaments,
but they unequivocally show how upwelling differs seasonally and in relative strength
along the coast between Iberia and Senegal. This series of images confirms quite clearly
the influence of the meridionally migrating belt of trade winds but also shows that even
in seasons when NE wind stress is expected to be minimal, upwelling may not be entirely
absent. The images strikingly confirm the far wider extent of the effect of upwelling
to the south of Cape Blanc and how, along the Iberian, Moroccan, and Saharan coast,
surface chlorophyll in excess of 25 mg m
−3 is restricted to a narrow coastal belt, of order
30–40 km wide. The filaments that extend seaward from the upwelling centers along these
northern coasts usually include much lower chlorophyll values (around 02 mg m
−3 )
than to the south of Cape Blanc where the biological consequences of upwelling are far
more extensive. In winter months, as in November 1999, it is not unusual for a discrete
high-chlorophyll feature (5–7 mg m
−3 ) to extend as much as 400 km offshore from the
Mauretanian bight. In September–November 1998, this feature was continuous with a
high-chlorophyll, eddylike enhancement in the NECC, as already discussed. In October
and November 2000, the divergence of the NECC between poleward flow and eastward
flow into the Guinea Current appears to have been unusually far to the south. In those
months, upwelling seems to have occurred as far south as Cape Roxo, and enhanced
surface chlorophyll extended over much of the Bissagos shelf off Guinea. This was an
unusual but not unique event during the 8-year series of images now available.
Perhaps one explanation of these observations lies in the different nutrient content of
water that is upwelled on either side of Cape Blanc. To the south, upwelled South Atlantic
Central water (SACW) is relatively nutrient rich (NO 3 = 14–20 M kg
−1 ), whereas, from
Cape Blanc to the Iberian Peninsula, relatively nutrient-poor North Atlantic Central water
is upwelled and may contain as little as 26 M kg
−1 (Alvarez-Salgado et al., 2001). A
complex front between the two regimes occurs off Cape Blanc so that SACW may at times
be upwelled as far north as Cape Barbas (Minas et al., 1982). Calculations by AlvarezSalgado et al. show that the low nutrient levels off Iberia have a direct effect on rates of
Chapter 9: The Atlantic Ocean
boreal winter, wherever their back trajectory lies over the desert (Hastenrath, 1985). These
Harmattan winds bear a heavy burden of mineral dust over the ocean from Morocco
to the Gulf of Guinea and may deposit as much as 25 g m
−2 y
−1 at the sea surface off
Senegal. This dust deposition is among the heaviest anywhere in the oceans and is similar
to deposition of loess clays in the northwestern Pacific; such aeolian deposits may be a
major source of turbidity in inshore waters and have, of course, been invoked in releasing
autotrophic cells from Fe limitation, as was discussed in Chapter 5.
Response of the Pelagic Ecosystems
Although the North Atlantic bloom of late winter or spring importantly affects the
shelf waters of Iberia south to the Gulf of Cadiz and is forced by stratification, growth
of phytoplankton in the CNRY province is overwhelmingly controlled by wind-driven
variations in vertical transport of nutrients into the euphotic zone. An intuitive and
simple negative relationship between chlorophyll and temperature was apparent in the
earliest satellite images for which both fields were available (van Camp et al., 1991),
similar to that established off South Africa (Shannon, 1985; Lutjeharms et al., 1985), and
is abundantly confirmed by relevant images available today. The dimension of upwelling
cells is smaller than the area over which appropriate wind stress is applied, because water
depth restricts upwelling to a band only 10–20 km wide (Barber and Smith, 1981) except
in the case of shelf-edge upwelling.
The continuous series of chlorophyll images available since 1997 demonstrate unequivocally how the consequences of upwelling dominate phytoplankton ecology in this
province. The 30-day SeaWiFS and MODIS composites cannot reveal the details of the
evolution of individual upwelling events and conceal the form of individual filaments,
but they unequivocally show how upwelling differs seasonally and in relative strength
along the coast between Iberia and Senegal. This series of images confirms quite clearly
the influence of the meridionally migrating belt of trade winds but also shows that even
in seasons when NE wind stress is expected to be minimal, upwelling may not be entirely
absent. The images strikingly confirm the far wider extent of the effect of upwelling
to the south of Cape Blanc and how, along the Iberian, Moroccan, and Saharan coast,
surface chlorophyll in excess of 25 mg m
−3 is restricted to a narrow coastal belt, of order
30–40 km wide. The filaments that extend seaward from the upwelling centers along these
northern coasts usually include much lower chlorophyll values (around 02 mg m
−3 )
than to the south of Cape Blanc where the biological consequences of upwelling are far
more extensive. In winter months, as in November 1999, it is not unusual for a discrete
high-chlorophyll feature (5–7 mg m
−3 ) to extend as much as 400 km offshore from the
Mauretanian bight. In September–November 1998, this feature was continuous with a
high-chlorophyll, eddylike enhancement in the NECC, as already discussed. In October
and November 2000, the divergence of the NECC between poleward flow and eastward
flow into the Guinea Current appears to have been unusually far to the south. In those
months, upwelling seems to have occurred as far south as Cape Roxo, and enhanced
surface chlorophyll extended over much of the Bissagos shelf off Guinea. This was an
unusual but not unique event during the 8-year series of images now available.
Perhaps one explanation of these observations lies in the different nutrient content of
water that is upwelled on either side of Cape Blanc. To the south, upwelled South Atlantic
Central water (SACW) is relatively nutrient rich (NO 3 = 14–20 M kg
−1 ), whereas, from
Cape Blanc to the Iberian Peninsula, relatively nutrient-poor North Atlantic Central water
is upwelled and may contain as little as 26 M kg
−1 (Alvarez-Salgado et al., 2001). A
complex front between the two regimes occurs off Cape Blanc so that SACW may at times
be upwelled as far north as Cape Barbas (Minas et al., 1982). Calculations by AlvarezSalgado et al. show that the low nutrient levels off Iberia have a direct effect on rates of