Atlantic Coastal Biome
243
Equatorward flow in the Benguela Current is carried in a series of jet currents that are
strongest in summer, and in eddying flow that responds to coastal and continental shelf
topography and incorporates a series of wind-forced upwelling cells (Shannon, 1985;
Lutjeharms and Meeuwis, 1987). The broad equatorward flow has two components that
diverge off Cape Columbine at 33
S: (i)—aligned with the shelf edge is a meandering,
offshore core of maximum velocity, that represents a shelf edge jet, and (ii)—closer to
the coast, the weaker Columbine jet is transformed progressively northward into variable
coastal flow. Flow of the shelf edge jet is topographically steered so that it lies consistently
over the continental slope from the Cape up to Luderitz (27
S), where some separation
from the slope first occurs.
Separation into the northern limb of the southern subtropical gyral circulation is
completed in the vicinity of Cabo Frio at 18
S, after the offshore flow has passed through a
gap in the deep topography of the Walvis Ridge at 20
S; here, warm water in the seasonally
shifting Angola Front is encountered (Shannon et al., 1987). This front represents the
convergence zone between the poleward surface flow of tropical surface water in the
Angola Current and the equatorward flow of cool Benguela water and is marked at
the surface by a very strong temperature gradient. Tonguelike mesoscale intrusions of
warm water, especially in austral summer, pass across the front that also marks the
transition between strong stratification in the tropical water mass to the north and weak
stratification to the south. Especially in late austral winter, the chlorophyll signature of
upwelled water that passes toward the west along the Benguela/Angola Front forms a
diffuse feature across the South Atlantic to at least 10
W. This region of high chlorophyll
biomass is separated from the chlorophyll signature of the equatorial current system in
satellite images by a zone of blue water.
As also occurs in the Canary Current, there is significant deposition at the sea surface
of aerosol particulates; off southwestern Africa, aeolian dust events are associated with
offshore katabatic “berg” winds from the deserts of southwestern Africa, which blow
strongly for periods of several days, often simultaneously over as much as 1500 km of the
coastline between 20 and 30
S. Such offshore wind events result in a succession of small
upwelling cells, restricted to <10 km from the coast.
Day-night alternation of the land-sea breeze system is strongly developed (as it is off
all coasts that are backed by major deserts) and the sea breeze has a fetch of 100–150 km.
As Shannon (1985) comments, this variability can be expected to modulate diel processes
in the upwelling system. At the secular scale, equatorward wind stress, and hence cool
conditions in the Benguela, was strong in the 1920s and 1930s but a warm period
intervened in the 1940s; upwelling wind stress then progressively increased again until the
1990s (Shannon and O’Toole, 2003). Such large-scale changes in wind field intensity and
direction significantly modify the intensity and distribution of upwelling, stratification
and surface temperature. The consequences of “Benguela Niños” in the equatorial Atlantic
have been discussed in Chapter 8: like the intrusions of Agulhas influence from the
south, these events transport warm, nutrient-poor tropical water across the borders of
the Benguela province.
Coastal upwelling in the Benguela Current, as in other eastern boundary currents
(Mooers et al., 1978), results in cross-shelf circulation and a set of fronts related to
rotary motion within the upwelling cells. Near the shelf break, there is frequently a
prominent front where upwelled water sinks along the shoreward side, to create a coastal
cell, while Atlantic water diverges on the seaward side. Where upwelling is especially
vigorous, the upwelled water may be circumscribed by a surface front closer inshore than
the shelf-break convergent/divergent front (20–30 km wide) that can be traced north to
Cape Frio, becoming more diffuse toward the north. The outer part of the divergence
zone is associated with surface slicks above internal waves (Shannon, 1985). Particularly
pronounced divergence zones, and jet currents, occur off Cape Columbine and the Cape
243
Equatorward flow in the Benguela Current is carried in a series of jet currents that are
strongest in summer, and in eddying flow that responds to coastal and continental shelf
topography and incorporates a series of wind-forced upwelling cells (Shannon, 1985;
Lutjeharms and Meeuwis, 1987). The broad equatorward flow has two components that
diverge off Cape Columbine at 33
S: (i)—aligned with the shelf edge is a meandering,
offshore core of maximum velocity, that represents a shelf edge jet, and (ii)—closer to
the coast, the weaker Columbine jet is transformed progressively northward into variable
coastal flow. Flow of the shelf edge jet is topographically steered so that it lies consistently
over the continental slope from the Cape up to Luderitz (27
S), where some separation
from the slope first occurs.
Separation into the northern limb of the southern subtropical gyral circulation is
completed in the vicinity of Cabo Frio at 18
S, after the offshore flow has passed through a
gap in the deep topography of the Walvis Ridge at 20
S; here, warm water in the seasonally
shifting Angola Front is encountered (Shannon et al., 1987). This front represents the
convergence zone between the poleward surface flow of tropical surface water in the
Angola Current and the equatorward flow of cool Benguela water and is marked at
the surface by a very strong temperature gradient. Tonguelike mesoscale intrusions of
warm water, especially in austral summer, pass across the front that also marks the
transition between strong stratification in the tropical water mass to the north and weak
stratification to the south. Especially in late austral winter, the chlorophyll signature of
upwelled water that passes toward the west along the Benguela/Angola Front forms a
diffuse feature across the South Atlantic to at least 10
W. This region of high chlorophyll
biomass is separated from the chlorophyll signature of the equatorial current system in
satellite images by a zone of blue water.
As also occurs in the Canary Current, there is significant deposition at the sea surface
of aerosol particulates; off southwestern Africa, aeolian dust events are associated with
offshore katabatic “berg” winds from the deserts of southwestern Africa, which blow
strongly for periods of several days, often simultaneously over as much as 1500 km of the
coastline between 20 and 30
S. Such offshore wind events result in a succession of small
upwelling cells, restricted to <10 km from the coast.
Day-night alternation of the land-sea breeze system is strongly developed (as it is off
all coasts that are backed by major deserts) and the sea breeze has a fetch of 100–150 km.
As Shannon (1985) comments, this variability can be expected to modulate diel processes
in the upwelling system. At the secular scale, equatorward wind stress, and hence cool
conditions in the Benguela, was strong in the 1920s and 1930s but a warm period
intervened in the 1940s; upwelling wind stress then progressively increased again until the
1990s (Shannon and O’Toole, 2003). Such large-scale changes in wind field intensity and
direction significantly modify the intensity and distribution of upwelling, stratification
and surface temperature. The consequences of “Benguela Niños” in the equatorial Atlantic
have been discussed in Chapter 8: like the intrusions of Agulhas influence from the
south, these events transport warm, nutrient-poor tropical water across the borders of
the Benguela province.
Coastal upwelling in the Benguela Current, as in other eastern boundary currents
(Mooers et al., 1978), results in cross-shelf circulation and a set of fronts related to
rotary motion within the upwelling cells. Near the shelf break, there is frequently a
prominent front where upwelled water sinks along the shoreward side, to create a coastal
cell, while Atlantic water diverges on the seaward side. Where upwelling is especially
vigorous, the upwelled water may be circumscribed by a surface front closer inshore than
the shelf-break convergent/divergent front (20–30 km wide) that can be traced north to
Cape Frio, becoming more diffuse toward the north. The outer part of the divergence
zone is associated with surface slicks above internal waves (Shannon, 1985). Particularly
pronounced divergence zones, and jet currents, occur off Cape Columbine and the Cape