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Chapter 9: The Atlantic Ocean
Benguela Current Coastal Province (BENG)
Extent of the Province
BENG is the shelf of southwestern Africa from the Cape of Good Hope north to the
region of Cape Frio (18
S). Excluded is the retroflection region of the Agulhas Current
south and southwest of the Cape of Good Hope itself that is included in the EAFR coastal
province. Included is the upwelling eddy field of the Benguela Current.
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
This continental margin is a trailing edge (recollect the mid-Atlantic ridge sea-floor
spreading center) so we may expect wider shelf areas than off the western coast of South
America: and so they are. The shelf of southwestern Africa is about 150–180 km wide,
and deep at that, with the break of slope at about 400 m. It is only at salient capes
that the shelf becomes narrower, although at Cape Frio, at the northern end of the
province, the isobaths deeper than 500 m become incorporated into the topography of
the Walvis Ridge that extends deep into the Atlantic in a southwesterly direction. Muddy
deposits lie inshore and the outer shelf carries relic quartz sands and rock. The shelf also
includes anomalously deep zones; double shelf breaks are common at about two depths:
150–200 m and 300–500 m. Shelf topography plays a significant role in determining the
locations of anomalies in the shallow circulation. Maximum range of spring tides is about
2 m, but I have found no references to tidal fronts between mixed and stratified areas of
shelf; a shelf-edge frontal feature is discussed later as part of the cross-shelf circulation
that results from upwelling at the coast.
Defining Characteristics of Regional Oceanography
The Benguela Current forms the eastern limb of the anticyclonic gyre of the South
Atlantic and originates in the flow of the South Atlantic Current around the southern
limb of the gyre, but also receives intermittent intrusions of Indian Ocean water from
retroflection of the Agulhas Current.
This system is complex, but is well explored and has been the subject of several
recent reviews (Chapman and Shannon, 1985; Shillington, 1998). Because the continent
of Africa does not extend so far southward as the American continent, the Benguela is
unique among the four eastern boundary currents because it interacts, poleward, with
the western boundary current of the adjacent ocean. Thus, the Agulhas Current passes
partly around South Africa prior to retroflection back into the Indian Ocean. Because
this mechanism is unstable, some Agulhas retroflection rings are lost into the southwest
Atlantic, later to interact with the outer part of the Benguela Current off Namibia; the
frequency of such intrusions tends to be variable at the decadal scale.
The wind stress field over the Benguela Current is typical of those overlying other
eastern boundary currents and represents the interaction between the atmospheric anticyclone of the southeast Atlantic and the deserts behind the coast of southwestern Africa.
Winds are perennially favorable to upwelling south of about 15
S, and cyclonic wind
stress curl lies inshore of the wind velocity maximum. A coastal zone of cyclonic wind
stress originates between Cape Point and Cape Columbine (33–35
S), where it is narrow,
but widens to the north until at about 20
S the cyclonicity becomes very patchy, especially in austral winter and spring. Cyclonic curl maxima and hence exceptionally strong
upwelling-favorable winds occur near Luderitz and Cape Frio; in the latter region, these
conditions are persistent both seasonally, and also at shorter time scales. This persistence
distinguishes the northern from the southern Benguela region, where upwelling is modulated at periods of about 1 week by relaxation or reversal of coastal winds, in response
to the passage of cyclones poleward of the Cape peninsula.
Chapter 9: The Atlantic Ocean
Benguela Current Coastal Province (BENG)
Extent of the Province
BENG is the shelf of southwestern Africa from the Cape of Good Hope north to the
region of Cape Frio (18
S). Excluded is the retroflection region of the Agulhas Current
south and southwest of the Cape of Good Hope itself that is included in the EAFR coastal
province. Included is the upwelling eddy field of the Benguela Current.
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
This continental margin is a trailing edge (recollect the mid-Atlantic ridge sea-floor
spreading center) so we may expect wider shelf areas than off the western coast of South
America: and so they are. The shelf of southwestern Africa is about 150–180 km wide,
and deep at that, with the break of slope at about 400 m. It is only at salient capes
that the shelf becomes narrower, although at Cape Frio, at the northern end of the
province, the isobaths deeper than 500 m become incorporated into the topography of
the Walvis Ridge that extends deep into the Atlantic in a southwesterly direction. Muddy
deposits lie inshore and the outer shelf carries relic quartz sands and rock. The shelf also
includes anomalously deep zones; double shelf breaks are common at about two depths:
150–200 m and 300–500 m. Shelf topography plays a significant role in determining the
locations of anomalies in the shallow circulation. Maximum range of spring tides is about
2 m, but I have found no references to tidal fronts between mixed and stratified areas of
shelf; a shelf-edge frontal feature is discussed later as part of the cross-shelf circulation
that results from upwelling at the coast.
Defining Characteristics of Regional Oceanography
The Benguela Current forms the eastern limb of the anticyclonic gyre of the South
Atlantic and originates in the flow of the South Atlantic Current around the southern
limb of the gyre, but also receives intermittent intrusions of Indian Ocean water from
retroflection of the Agulhas Current.
This system is complex, but is well explored and has been the subject of several
recent reviews (Chapman and Shannon, 1985; Shillington, 1998). Because the continent
of Africa does not extend so far southward as the American continent, the Benguela is
unique among the four eastern boundary currents because it interacts, poleward, with
the western boundary current of the adjacent ocean. Thus, the Agulhas Current passes
partly around South Africa prior to retroflection back into the Indian Ocean. Because
this mechanism is unstable, some Agulhas retroflection rings are lost into the southwest
Atlantic, later to interact with the outer part of the Benguela Current off Namibia; the
frequency of such intrusions tends to be variable at the decadal scale.
The wind stress field over the Benguela Current is typical of those overlying other
eastern boundary currents and represents the interaction between the atmospheric anticyclone of the southeast Atlantic and the deserts behind the coast of southwestern Africa.
Winds are perennially favorable to upwelling south of about 15
S, and cyclonic wind
stress curl lies inshore of the wind velocity maximum. A coastal zone of cyclonic wind
stress originates between Cape Point and Cape Columbine (33–35
S), where it is narrow,
but widens to the north until at about 20
S the cyclonicity becomes very patchy, especially in austral winter and spring. Cyclonic curl maxima and hence exceptionally strong
upwelling-favorable winds occur near Luderitz and Cape Frio; in the latter region, these
conditions are persistent both seasonally, and also at shorter time scales. This persistence
distinguishes the northern from the southern Benguela region, where upwelling is modulated at periods of about 1 week by relaxation or reversal of coastal winds, in response
to the passage of cyclones poleward of the Cape peninsula.