236
Chapter 9: The Atlantic Ocean
the height of internal waves occurs by the shoaling of the continental slope and may be
a persistent feature, as it is elsewhere.
Finally, we should remember that Cape Palmas, in western Ivory Coast, lies at only
about 4
N, or just about 250 km north of the equator. We may therefore expect some
interaction between upwelling dynamics here and the strong meandering of the equatorial
divergence related to Kelvin waves passing east along the equatorial wave guide (see
ETRA). These meanders are of an appropriate scale to interact with coastal processes,
and I shall present some evidence later to suggest that such interaction can, in fact, be
observed in the ecological response to vertical fluxes.
Central African Coast Here, the coastal oceanography has not been so fully investigated, and the current streams are rather variable, especially from Fernando Po down
to Cape Lopez. Here, the now very weak Guinea Current meets variable northward
wind-forced surface flow from equatorial regions.
Between Cape Lopez and Cape Frio (18
S) the circulation is theoretically equivalent
to that along the Guinea Coast, except that here the coastline is oriented north-south. A
weak equatorward surface current lies above a strong, poleward subsurface countercurrent
originating, as does the Forcados Current, in the terminal bifurcation of the EUC. This
is the Angola Current that, on meeting the strong equatorward flow of the Benguela
Current at Cape Frio, turns offshore and passes around the southern flank of the Angola
Dome (see ETRA) to form the Angola-Benguela Front.
The response of the Atlantic Ocean to the ENSO cycle was discussed earlier, but it
is in this region that the consequences of Atlantic Niño-like episodes are perhaps most
important. The strengthening of the SECC during “Benguela Niños,” as in 1995, drives
an influx of warm water to the coast of Angola, with ecological consequences to be
discussed below. When it is the NECC that is strengthened, during a “Guinea Niño,” it
is Bight of Biafra water that floods southward along the central African coastline, again
with important ecological consequences (Binet et al., 2001).
Surface wind drift over the whole of the Congo-Angola Bight is toward the northwest,
especially during the boreal summer and between Cape Lopez and Luanda. At this season,
the low-salinity plume of the Congo River, originating at 6
S, turns seaward and may
be traced as far out into the South Atlantic as 5
E. In boreal winter, this plume passes
northward along the coast toward Cape Lopez. South of this cape, coastal upwelling cells
occur down nearly to 11
S. These upwelling events are not as well documented as those
along the northern coast, but they occur during the same season and have been presumed
to be similarly forced, perhaps by Rossby waves propagating southward along the shelf
edge, although, in fact, the mechanism that forces this upwelling remains—as Ajao and
Houghton remark—“baffling,” especially because the timing of coastal upwelling events
precedes events observed along the equator; the explanations concerning an origin in
equatorial waves that have been offered for the Guinea upwelling lack conviction here.
The Congo effluent dominates the features that influence the ecology of the Central
African coast; this is the largest discharge of any river after the Amazon, from which
it differs in significant ways. The Congo carries a relatively small load of suspended
material (<120 t 10
−6 y
−1 or more than an order of magnitude less than the Amazon)
because of the relatively low gradients of the drainage basin, so that chemical rather than
physical erosion dominates upriver; organic material comprises 32% of the suspensoids,
and phosphate content is relatively high compared with other tropical rivers. The estuary
of the Congo is deep compared with that of the Amazon, which has extensive shoals,
so that the nutrient-rich Congo plume is less dynamically mixed with nutrient-poor
Atlantic water and retains its high nutrient status much farther seaward than the Amazon
(Cadee, 1979). Like the Amazon, the discharge rate of the Congo is weakly and negatively
correlated with tropical Pacific SST anomalies and the ENSO cycle; tropical rainfall is
exceptionally low during the warm ENSO phase.
Chapter 9: The Atlantic Ocean
the height of internal waves occurs by the shoaling of the continental slope and may be
a persistent feature, as it is elsewhere.
Finally, we should remember that Cape Palmas, in western Ivory Coast, lies at only
about 4
N, or just about 250 km north of the equator. We may therefore expect some
interaction between upwelling dynamics here and the strong meandering of the equatorial
divergence related to Kelvin waves passing east along the equatorial wave guide (see
ETRA). These meanders are of an appropriate scale to interact with coastal processes,
and I shall present some evidence later to suggest that such interaction can, in fact, be
observed in the ecological response to vertical fluxes.
Central African Coast Here, the coastal oceanography has not been so fully investigated, and the current streams are rather variable, especially from Fernando Po down
to Cape Lopez. Here, the now very weak Guinea Current meets variable northward
wind-forced surface flow from equatorial regions.
Between Cape Lopez and Cape Frio (18
S) the circulation is theoretically equivalent
to that along the Guinea Coast, except that here the coastline is oriented north-south. A
weak equatorward surface current lies above a strong, poleward subsurface countercurrent
originating, as does the Forcados Current, in the terminal bifurcation of the EUC. This
is the Angola Current that, on meeting the strong equatorward flow of the Benguela
Current at Cape Frio, turns offshore and passes around the southern flank of the Angola
Dome (see ETRA) to form the Angola-Benguela Front.
The response of the Atlantic Ocean to the ENSO cycle was discussed earlier, but it
is in this region that the consequences of Atlantic Niño-like episodes are perhaps most
important. The strengthening of the SECC during “Benguela Niños,” as in 1995, drives
an influx of warm water to the coast of Angola, with ecological consequences to be
discussed below. When it is the NECC that is strengthened, during a “Guinea Niño,” it
is Bight of Biafra water that floods southward along the central African coastline, again
with important ecological consequences (Binet et al., 2001).
Surface wind drift over the whole of the Congo-Angola Bight is toward the northwest,
especially during the boreal summer and between Cape Lopez and Luanda. At this season,
the low-salinity plume of the Congo River, originating at 6
S, turns seaward and may
be traced as far out into the South Atlantic as 5
E. In boreal winter, this plume passes
northward along the coast toward Cape Lopez. South of this cape, coastal upwelling cells
occur down nearly to 11
S. These upwelling events are not as well documented as those
along the northern coast, but they occur during the same season and have been presumed
to be similarly forced, perhaps by Rossby waves propagating southward along the shelf
edge, although, in fact, the mechanism that forces this upwelling remains—as Ajao and
Houghton remark—“baffling,” especially because the timing of coastal upwelling events
precedes events observed along the equator; the explanations concerning an origin in
equatorial waves that have been offered for the Guinea upwelling lack conviction here.
The Congo effluent dominates the features that influence the ecology of the Central
African coast; this is the largest discharge of any river after the Amazon, from which
it differs in significant ways. The Congo carries a relatively small load of suspended
material (<120 t 10
−6 y
−1 or more than an order of magnitude less than the Amazon)
because of the relatively low gradients of the drainage basin, so that chemical rather than
physical erosion dominates upriver; organic material comprises 32% of the suspensoids,
and phosphate content is relatively high compared with other tropical rivers. The estuary
of the Congo is deep compared with that of the Amazon, which has extensive shoals,
so that the nutrient-rich Congo plume is less dynamically mixed with nutrient-poor
Atlantic water and retains its high nutrient status much farther seaward than the Amazon
(Cadee, 1979). Like the Amazon, the discharge rate of the Congo is weakly and negatively
correlated with tropical Pacific SST anomalies and the ENSO cycle; tropical rainfall is
exceptionally low during the warm ENSO phase.