244
Chapter 9: The Atlantic Ocean
peninsula and cold-core eddies appear as isolated areas of chlorophyll enhancement along
this frontal zone; beyond the shelf-break front, an eddy field marks the edge of the clear
waters of the subtropical gyre of the South Atlantic.
Shillington (1998) points out that strong interaction occurs between coastal upwelling
cells and Agulhas rings that have defied retroflection, and subsequently pass northward
well beyond the shelf break; these rings may entrain cold water so as to extend a filament
of upwelled water anomalously far out into the South Atlantic. These, ending in a pair of
contrary-rotating gyres, may extend even as far west as the dateline from central regions
of the Benguela upwelling area (see Color plate 9).
The upwelling centers identified by Shannon (1985) and Lutjeharms and Meeuwis
(1987) are not always observable as entities in the satellite images, although coincidentally,
one of the earliest SeaWiFS images was of an upwelling center below cloudless skies
that represented an atmospheric high-pressure cell. For practical purposes, the upwelling
centers can be grouped as follows:
Cape Peninsula (34
S) and Cape Columbine (32
S): Upwelling is largely restricted to
the austral summer trimester (December–February), and the average temperature of
upwelled water in both cells is about 17
C. The upwelling cells off Cape Columbine
may generate cold filaments that extend 500–700 km offshore and have a longshore
spacing of 200–300 km, whereas those off the Cape peninsula are about half that
dimension. Each of these upwelling cells is frequently bounded seaward by a jet
current, and off Cape Columbine this jet is isolated from the shelf edge jet by a
clear divergence zone (Shannon, 1985). In summer, upwelling may extend around
the Cape peninsula, as far to the east as Cape Agulhas (35
S 20
E).
Hondeklip Bay, Namaqualand (29
S): A regional maximum of cyclonic wind stress curl
and upwelling occurs persistently in the bight south of the Orange River; the shelf
here is narrow and deep, and the source of water brought to the surface in coastal
upwelling is also deep. A persistent mesoscale cold filament or tongue originates
at this upwelling center. Upwelling response to wind stress is slower than that in
the regions to the south; maximum upwelling occurs in October–December, and
minimum upwelling occurs in May–July.
Luderitz to Walvis Bay (24–22
S): This is the most intense upwelling area, especially
off Luderitz itself, and forms a major environmental barrier in the Benguela Current
system. Upwelling occurs throughout the year, being seen in >80% of images
scanned by Lutjeharms and Meeuwis (1987) over a 3-year period; nevertheless,
there is a clear seasonal cycle with minimal upwelling in December–January and
maximal in July–September. SST at 200 m accordingly varies between 15 and 19
C.
Upwelled water is quite cool (mean temperature is 165
C at Luderitz and 17
C
in the Namaqua cell) because the shelf break is exceptionally deep and the source
of upwelled water lies at 200–300 m. The seaward extent of upwelling is greatest
here, reaching almost 300 km off Luderitz, and the upwelling event scale is long
compared with that of regions to the south. Shannon (1985) comments that here
the event scale resembles Peru and northwest Africa rather than Oregon or the
southern Benguela.
Central (21–23
) and northern Namibia, especially Cape Frio (19–17
S): Upwelling is
continuous throughout the year but again is strongest in the austral winter trimester
(June–August), with some extension into spring (September–November). During
summer the water column is sufficiently stratified so as somewhat to constrain
upwelling. Upwelled water is >16
C (mean of upwelling cells is 195
C at Cunene
and 185
C in the Namibia cell). The continental shelf off central Namibia is
narrower and shoaler than farther south, having a shelf break at 140 m about 100 km
offshore. For this reason, very cool water is unavailable as the source of upwelling.
The upwelling cells in this sector extend about 150 km offshore. There may be a
Chapter 9: The Atlantic Ocean
peninsula and cold-core eddies appear as isolated areas of chlorophyll enhancement along
this frontal zone; beyond the shelf-break front, an eddy field marks the edge of the clear
waters of the subtropical gyre of the South Atlantic.
Shillington (1998) points out that strong interaction occurs between coastal upwelling
cells and Agulhas rings that have defied retroflection, and subsequently pass northward
well beyond the shelf break; these rings may entrain cold water so as to extend a filament
of upwelled water anomalously far out into the South Atlantic. These, ending in a pair of
contrary-rotating gyres, may extend even as far west as the dateline from central regions
of the Benguela upwelling area (see Color plate 9).
The upwelling centers identified by Shannon (1985) and Lutjeharms and Meeuwis
(1987) are not always observable as entities in the satellite images, although coincidentally,
one of the earliest SeaWiFS images was of an upwelling center below cloudless skies
that represented an atmospheric high-pressure cell. For practical purposes, the upwelling
centers can be grouped as follows:
Cape Peninsula (34
S) and Cape Columbine (32
S): Upwelling is largely restricted to
the austral summer trimester (December–February), and the average temperature of
upwelled water in both cells is about 17
C. The upwelling cells off Cape Columbine
may generate cold filaments that extend 500–700 km offshore and have a longshore
spacing of 200–300 km, whereas those off the Cape peninsula are about half that
dimension. Each of these upwelling cells is frequently bounded seaward by a jet
current, and off Cape Columbine this jet is isolated from the shelf edge jet by a
clear divergence zone (Shannon, 1985). In summer, upwelling may extend around
the Cape peninsula, as far to the east as Cape Agulhas (35
S 20
E).
Hondeklip Bay, Namaqualand (29
S): A regional maximum of cyclonic wind stress curl
and upwelling occurs persistently in the bight south of the Orange River; the shelf
here is narrow and deep, and the source of water brought to the surface in coastal
upwelling is also deep. A persistent mesoscale cold filament or tongue originates
at this upwelling center. Upwelling response to wind stress is slower than that in
the regions to the south; maximum upwelling occurs in October–December, and
minimum upwelling occurs in May–July.
Luderitz to Walvis Bay (24–22
S): This is the most intense upwelling area, especially
off Luderitz itself, and forms a major environmental barrier in the Benguela Current
system. Upwelling occurs throughout the year, being seen in >80% of images
scanned by Lutjeharms and Meeuwis (1987) over a 3-year period; nevertheless,
there is a clear seasonal cycle with minimal upwelling in December–January and
maximal in July–September. SST at 200 m accordingly varies between 15 and 19
C.
Upwelled water is quite cool (mean temperature is 165
C at Luderitz and 17
C
in the Namaqua cell) because the shelf break is exceptionally deep and the source
of upwelled water lies at 200–300 m. The seaward extent of upwelling is greatest
here, reaching almost 300 km off Luderitz, and the upwelling event scale is long
compared with that of regions to the south. Shannon (1985) comments that here
the event scale resembles Peru and northwest Africa rather than Oregon or the
southern Benguela.
Central (21–23
) and northern Namibia, especially Cape Frio (19–17
S): Upwelling is
continuous throughout the year but again is strongest in the austral winter trimester
(June–August), with some extension into spring (September–November). During
summer the water column is sufficiently stratified so as somewhat to constrain
upwelling. Upwelled water is >16
C (mean of upwelling cells is 195
C at Cunene
and 185
C in the Namibia cell). The continental shelf off central Namibia is
narrower and shoaler than farther south, having a shelf break at 140 m about 100 km
offshore. For this reason, very cool water is unavailable as the source of upwelling.
The upwelling cells in this sector extend about 150 km offshore. There may be a