210
Chapter 9: The Atlantic Ocean
the Falklands plateau to just south of Cape Town near 35–45
S. Although the subtropical
gyre of the South Atlantic is more persistently under the influence of the trade winds
than that of the North Atlantic a significant seasonal variation in its mixed-layer depth is
nevertheless forced by seasonal variation in wind stress and surface heat flux. This effect
is zonally asymmetric, reaching deeper in the western part of the ocean than in the east.
Winter heat loss at the sea surface is remarkably uniform (ranging between only −100
and −125 W m
−2 ) in July over the whole region from 10
S in the SEC right down to
45
S. It is not surprising, then, that even at relatively low latitudes a deepening of the
mixed layer should occur during the austral winter (Levitus, 1982), when the ITCZ is at
its most northerly position and highest wind speeds and wave heights occur from the
equator to 25
S. The greatest mixed-layer depths (<100 m) occur along a zonal trough
at about 20
S in July, across the west-central part of the gyre.
Flow of the Benguela Current along the African coast begins to turn to the northwest
(thus leaving the region designated as the BENG Province) and passes across the equatorward side of the gyre as a broad stream at about 15–30
S. The axis of this flow lies across
the ocean in a southeast/northwest direction to join the intensified western boundary
current that then returns the flow southward as the Brazil Current. As occurs in the
northern hemisphere, prominent tropical instability waves, associated with the equatorial
divergence, may be observed in the surface chlorophyll field along the northern border
of SATL, these being most prominent when trade wind stress across the equatorial zone
is maximal in late austral winter (August–October).
The confluence between the Falkland and Brazil Currents at about 38–40
S, which
forces each to turn eastward across the ocean within the Subtropical Convergence zone, is
a highly energetic region that dominates the circulation pattern of the southern Atlantic
Ocean and is rich in mesoscale features that are prominent in the SLA field (Fig. 9.19).
This distracts our attention from the fact that the South Atlantic subtropical gyre actually
exists as a double-cell circulation (Tsuchiya, 1985), the foci of the two “pinched” (to
use Tsuchiya’s word) gyres lying close to the western coasts. A zone of eddying, also
visible in the SLA field, lies southwest across the ocean: is this a surface indication of a
subtropical countercurrent that, as Qiu (1999) suggests (see SPSG), is to be anticipated at
this latitude in each subtropical gyre? Although Tsuchiya does not mention it, we should
note that his two subgyres lie alongside the Rio Grande Ridge that rises to within 2000 m
of the sea surface at around 29
S. Is this topography involved in locating the circulation
features?
Despite this possibility, it is the Confluence region itself and the region of heavy
meandering and SST anomalies at the boundary of the Subtropical Convergence zone of
the South Atlantic Current (SAC) across the southern part of this province that takes
our attention, even though the area of meanders itself is attributed here to the SSTC
Province. Significant changes in the latitude at which the Confluence occurs, and flow
separates from the continent, have been observed almost throughout the last century
(Olson et al., 1988); this variability appears to be forced by variability in the Brazil
Current and so by the relative strength of the seasonal trade winds associated with
the Southern Oscillation. SST anomalies here are weaker than those associated with
eastern Pacific ENSO variability, even if associated with drought conditions over South
America.
Because the retroflection loop of the Agulhas Current southwest of the Cape of Good
Hope (see EAFR) carries more eddy kinetic energy than anywhere else in the southern
hemisphere, large warm-core eddies are shed when an Agulhas intrusion into the Atlantic
occurs, usually 5–10 times each year. These unusually large (300-km) eddies have very
long lifetimes, and some survive to reach the Brazilian coastal boundary where they
have been resolved in altimeter data. They are therefore of potential significance in the
structure of the whole subtropical gyre (Peterson and Stramma, 1991; Shannon, 1985).
Chapter 9: The Atlantic Ocean
the Falklands plateau to just south of Cape Town near 35–45
S. Although the subtropical
gyre of the South Atlantic is more persistently under the influence of the trade winds
than that of the North Atlantic a significant seasonal variation in its mixed-layer depth is
nevertheless forced by seasonal variation in wind stress and surface heat flux. This effect
is zonally asymmetric, reaching deeper in the western part of the ocean than in the east.
Winter heat loss at the sea surface is remarkably uniform (ranging between only −100
and −125 W m
−2 ) in July over the whole region from 10
S in the SEC right down to
45
S. It is not surprising, then, that even at relatively low latitudes a deepening of the
mixed layer should occur during the austral winter (Levitus, 1982), when the ITCZ is at
its most northerly position and highest wind speeds and wave heights occur from the
equator to 25
S. The greatest mixed-layer depths (<100 m) occur along a zonal trough
at about 20
S in July, across the west-central part of the gyre.
Flow of the Benguela Current along the African coast begins to turn to the northwest
(thus leaving the region designated as the BENG Province) and passes across the equatorward side of the gyre as a broad stream at about 15–30
S. The axis of this flow lies across
the ocean in a southeast/northwest direction to join the intensified western boundary
current that then returns the flow southward as the Brazil Current. As occurs in the
northern hemisphere, prominent tropical instability waves, associated with the equatorial
divergence, may be observed in the surface chlorophyll field along the northern border
of SATL, these being most prominent when trade wind stress across the equatorial zone
is maximal in late austral winter (August–October).
The confluence between the Falkland and Brazil Currents at about 38–40
S, which
forces each to turn eastward across the ocean within the Subtropical Convergence zone, is
a highly energetic region that dominates the circulation pattern of the southern Atlantic
Ocean and is rich in mesoscale features that are prominent in the SLA field (Fig. 9.19).
This distracts our attention from the fact that the South Atlantic subtropical gyre actually
exists as a double-cell circulation (Tsuchiya, 1985), the foci of the two “pinched” (to
use Tsuchiya’s word) gyres lying close to the western coasts. A zone of eddying, also
visible in the SLA field, lies southwest across the ocean: is this a surface indication of a
subtropical countercurrent that, as Qiu (1999) suggests (see SPSG), is to be anticipated at
this latitude in each subtropical gyre? Although Tsuchiya does not mention it, we should
note that his two subgyres lie alongside the Rio Grande Ridge that rises to within 2000 m
of the sea surface at around 29
S. Is this topography involved in locating the circulation
features?
Despite this possibility, it is the Confluence region itself and the region of heavy
meandering and SST anomalies at the boundary of the Subtropical Convergence zone of
the South Atlantic Current (SAC) across the southern part of this province that takes
our attention, even though the area of meanders itself is attributed here to the SSTC
Province. Significant changes in the latitude at which the Confluence occurs, and flow
separates from the continent, have been observed almost throughout the last century
(Olson et al., 1988); this variability appears to be forced by variability in the Brazil
Current and so by the relative strength of the seasonal trade winds associated with
the Southern Oscillation. SST anomalies here are weaker than those associated with
eastern Pacific ENSO variability, even if associated with drought conditions over South
America.
Because the retroflection loop of the Agulhas Current southwest of the Cape of Good
Hope (see EAFR) carries more eddy kinetic energy than anywhere else in the southern
hemisphere, large warm-core eddies are shed when an Agulhas intrusion into the Atlantic
occurs, usually 5–10 times each year. These unusually large (300-km) eddies have very
long lifetimes, and some survive to reach the Brazilian coastal boundary where they
have been resolved in altimeter data. They are therefore of potential significance in the
structure of the whole subtropical gyre (Peterson and Stramma, 1991; Shannon, 1985).