Atlantic Trade Wind Biome
203
by variable eastward flow, mainly subsurface. Symmetrically below the equator is the
eastward flow of the Atlantic Equatorial Undercurrent (EUC), originating north of Brazil;
this is 250–300 km wide, has a vertical thickness of 150–250 m, and slopes progressively
upward to the east. Depth of the EUC core is 50–75 m, and inversion with westward
flow of the SEC lying above it occurs at only 15–35 m (Neumann, 1969; McCreary et al.,
1984; Voituriez, 1981). Note that the water carried in the EUC has high salinity and is
of nutrient-depleted origin.
Within the flow of the SEC is embedded the eastward flow of an SECC at about 5
S.
This has been described as weak, narrow, and rather variable. It is in no way comparable
to the NECC in the WTRA and is captured neither in 90-d surface velocity analyses,
nor in drifter-derived climatology, nor yet in ship-drift archives: the NECC is, of course,
prominent in each of these. The topography of the thermocline below the SECC flow
should, theoretically, carry a ridge parallel with the equator and close to it, but this is
hardly observable, perhaps because of divergence at the equator that shoals the isotherms
for >20
C.
Thus, except for the eastward NECC–Guinea Current, surface flow over the entire
eastern Atlantic is predominantly, and year-round, toward the northwest and west: this
applies even across the subsurface Angola Dome, although some meandering is evident there. The dominance of this vector in the regional surface flow has important
consequences for the motion of shallow features, such as the Congo plume of light,
low-salinity water.
This situation is modified in anomalous years of “Atlantic El Niño,” when weakened
trade winds may come to have an eastward component; in such episodes, the SEC
is weakened while NECC-GC, the EUC, and (in exceptional events, termed “Benguela
Niños”) the SECC strengthen. This anomalous flow results in the accumulation of warm,
near-surface water in the eastern part of ETRA that “discharges” (to use the term of Binet
et al., 2001) southward along the Congo-Angola coast. Such events occur during positive
anomalies of the Southern Oscillation Index and hence the Pacific cold phase, as in 1934,
1963, 1984, and 1995; during the 1984 event, eastward flow dominated from 0
to 5
S.
Such anomalous SECC flow passes south, over water upwelled in the Benguela Current,
as well as equatorward along the Congo coast (Shannon et al., 1986).
As in the other tropical oceans, the poleward turn of eastward flow on encountering
the continent creates a cyclonic dome in the thermocline topography: here, this is the
subsurface Angola Dome, which lies west of the Angola Bight, centered at about 13
S 5
E.
Although the details of circulation in this undersampled region are unclear, this large
cyclonic gyre is forced by the poleward turn of both EUC (as the Angola Current on the
eastward side of the gyre) as well as by the SECC that feeds directly into its northern
slope (Shannon, 1985; Mercier et al., 2003). At the axis of the tropical gyre, at the Angola
Dome, the pycnocline reaches closer to the surface than it does in the Guinea Dome,
perhaps with significance for algal growth processes (Voituriez, 1981). This feature is
associated with Ekman upwelling, but here this is significantly weaker than in the NECC
and Guinea Dome of the WTRA province: seasonal climatological upwelling velocity
in the SECC/Angola gyre field is only 25% of that in the comparable North Atlantic
situation. Below this tropical gyre lies the most strongly developed subsurface oxygen
minimum in the entire Atlantic Ocean (Bubnov, 1972; Chapman and Shannon, 1985).
It may be useful to point out that the gyre is not a circular feature lying in the Angola
Bight, as represented in the usual diagram of this feature, but the area over which Ekman
upwelling is important (and that is what is interesting in the present context) is a much
larger, somewhat triangular region based in the Angola Bight but extending NW beyond
the Greenwich meridian (McClain and Firestone, 1993).
A further complication in the interpretation of this province is the fact that the water
of the Congo River, with a rate of discharge second only to the Amazon, enters the
203
by variable eastward flow, mainly subsurface. Symmetrically below the equator is the
eastward flow of the Atlantic Equatorial Undercurrent (EUC), originating north of Brazil;
this is 250–300 km wide, has a vertical thickness of 150–250 m, and slopes progressively
upward to the east. Depth of the EUC core is 50–75 m, and inversion with westward
flow of the SEC lying above it occurs at only 15–35 m (Neumann, 1969; McCreary et al.,
1984; Voituriez, 1981). Note that the water carried in the EUC has high salinity and is
of nutrient-depleted origin.
Within the flow of the SEC is embedded the eastward flow of an SECC at about 5
S.
This has been described as weak, narrow, and rather variable. It is in no way comparable
to the NECC in the WTRA and is captured neither in 90-d surface velocity analyses,
nor in drifter-derived climatology, nor yet in ship-drift archives: the NECC is, of course,
prominent in each of these. The topography of the thermocline below the SECC flow
should, theoretically, carry a ridge parallel with the equator and close to it, but this is
hardly observable, perhaps because of divergence at the equator that shoals the isotherms
for >20
C.
Thus, except for the eastward NECC–Guinea Current, surface flow over the entire
eastern Atlantic is predominantly, and year-round, toward the northwest and west: this
applies even across the subsurface Angola Dome, although some meandering is evident there. The dominance of this vector in the regional surface flow has important
consequences for the motion of shallow features, such as the Congo plume of light,
low-salinity water.
This situation is modified in anomalous years of “Atlantic El Niño,” when weakened
trade winds may come to have an eastward component; in such episodes, the SEC
is weakened while NECC-GC, the EUC, and (in exceptional events, termed “Benguela
Niños”) the SECC strengthen. This anomalous flow results in the accumulation of warm,
near-surface water in the eastern part of ETRA that “discharges” (to use the term of Binet
et al., 2001) southward along the Congo-Angola coast. Such events occur during positive
anomalies of the Southern Oscillation Index and hence the Pacific cold phase, as in 1934,
1963, 1984, and 1995; during the 1984 event, eastward flow dominated from 0
to 5
S.
Such anomalous SECC flow passes south, over water upwelled in the Benguela Current,
as well as equatorward along the Congo coast (Shannon et al., 1986).
As in the other tropical oceans, the poleward turn of eastward flow on encountering
the continent creates a cyclonic dome in the thermocline topography: here, this is the
subsurface Angola Dome, which lies west of the Angola Bight, centered at about 13
S 5
E.
Although the details of circulation in this undersampled region are unclear, this large
cyclonic gyre is forced by the poleward turn of both EUC (as the Angola Current on the
eastward side of the gyre) as well as by the SECC that feeds directly into its northern
slope (Shannon, 1985; Mercier et al., 2003). At the axis of the tropical gyre, at the Angola
Dome, the pycnocline reaches closer to the surface than it does in the Guinea Dome,
perhaps with significance for algal growth processes (Voituriez, 1981). This feature is
associated with Ekman upwelling, but here this is significantly weaker than in the NECC
and Guinea Dome of the WTRA province: seasonal climatological upwelling velocity
in the SECC/Angola gyre field is only 25% of that in the comparable North Atlantic
situation. Below this tropical gyre lies the most strongly developed subsurface oxygen
minimum in the entire Atlantic Ocean (Bubnov, 1972; Chapman and Shannon, 1985).
It may be useful to point out that the gyre is not a circular feature lying in the Angola
Bight, as represented in the usual diagram of this feature, but the area over which Ekman
upwelling is important (and that is what is interesting in the present context) is a much
larger, somewhat triangular region based in the Angola Bight but extending NW beyond
the Greenwich meridian (McClain and Firestone, 1993).
A further complication in the interpretation of this province is the fact that the water
of the Congo River, with a rate of discharge second only to the Amazon, enters the