234
Chapter 9: The Atlantic Ocean
Reef corals exist only on Annobon, the most seaward of the Biafran islands; otherwise,
the coastline is fringed with sandy beaches, and with mangrove vegetation wherever there
is protection from surf.
Defining Characteristics of Regional Oceanography
The regional oceanography of this province is dominated by two factors: (i) the relatively
shallow mixed layer above the sharp and permanent thermocline that is characteristic of
the eastern Atlantic and (ii) the strongly seasonal reversal of the wind systems associated
with the migration of the ITCZ so that along the Guinea Coast dry NE trades dominate
in boreal winter, and wet SE trades in boreal summer. The Central African coastline is
dominated by SE winds the entire year.
Most reviews of its oceanography have partitioned the Gulf of Guinea, between the
subtropical cold fronts at Cape Roxo and Cape Lopez, into a central upwelling area off
Ghana and Ivory Coast and two nonupwelling regions to the east and west. This is an
oversimplification of the real environment of the Gulf of Guinea, and for reasons that
will become apparent, I have rather followed Ajao and Houghton (1998) in partitioning
this region into only two sections outlined earlier: (i) the Guinea Coast, from Cape Roxo
east into the corner of the Bight of Biafra, and (ii) the Central African coast, down to
Cape Frio at the Namibian border. We have a great deal more information about the
Guinea coast than the Central African coast, much of it from the colonial period and
immediately afterward.
Guinea Coast The continental shelf of the 2000 km east-west coastline is occupied by
the eastward flow of the coastal component of the NECC-GC (see NATR), here usually
termed simply the Guinea Current. This receives a minor contribution in boreal winter
from the Canary Current, close inshore around Cape Roxo, and, after intensifying its
flow relative to the NECC between 8
W and the Greenwich meridian, it then weakens
progressively toward the east. The Guinea Current carries warm, light water and occupies
a shallow (30 m), warm (27
C) mixed layer above a very sharp thermocline (<1
C m
−1 ).
Below this eastward flow there is a contrary, westward flow of cold, highly saline water
originating at the termination of the EUC; this undercurrent, which I shall call the
Forcados Current following the usage of canoe fishermen on the Nigerian coast, weakens
toward the west. Recall that during boreal summer the thermocline of the whole eastern
tropical Atlantic shoals (see ETRA) and slopes upward toward the coast; it is at this season
that the Forcados Current flow is likely to surface intermittently, so that the Guinea
Current apparently moves offshore (Verstraete, 1992). I have monitored this apparent
reversal of flow at the coast off Nigeria, finding that each episode lasts for periods from
one day to several weeks.
Although a shelf-break front is often identifiable, the Guinea Current above the shelf
is continuous with the eastward flow of the NECC-GC during most of the year; however,
satellite-tracked RAFOS floats now suggest that reversal of flow at the shelf edge in the
western part of the Guinea Coast occurs in October with the onset of the NE Trades; at
this time, several eastbound, surface-drogued, satellite-tracked floats reversed direction
and moved closely along the shelf-break from Liberia to Guinea over a period of several
months (Dave Hebert, pers. comm., 2005). This is a region of very complex circulation,
and it is no surprise that long-term changes do occur although they are only rather dimly
observed in the presently available data: it appears that coastal SST was anomalously
high during the 1980s, for example, both at the surface and deep on the shelf after an
unusually cold period during the 1970s.
Surface water masses are under the influence of the alternation of dry and wet seasons
along the Guinea Coast. Seasonal cloud cover modifies the cycle of solar irradiance, from
Chapter 9: The Atlantic Ocean
Reef corals exist only on Annobon, the most seaward of the Biafran islands; otherwise,
the coastline is fringed with sandy beaches, and with mangrove vegetation wherever there
is protection from surf.
Defining Characteristics of Regional Oceanography
The regional oceanography of this province is dominated by two factors: (i) the relatively
shallow mixed layer above the sharp and permanent thermocline that is characteristic of
the eastern Atlantic and (ii) the strongly seasonal reversal of the wind systems associated
with the migration of the ITCZ so that along the Guinea Coast dry NE trades dominate
in boreal winter, and wet SE trades in boreal summer. The Central African coastline is
dominated by SE winds the entire year.
Most reviews of its oceanography have partitioned the Gulf of Guinea, between the
subtropical cold fronts at Cape Roxo and Cape Lopez, into a central upwelling area off
Ghana and Ivory Coast and two nonupwelling regions to the east and west. This is an
oversimplification of the real environment of the Gulf of Guinea, and for reasons that
will become apparent, I have rather followed Ajao and Houghton (1998) in partitioning
this region into only two sections outlined earlier: (i) the Guinea Coast, from Cape Roxo
east into the corner of the Bight of Biafra, and (ii) the Central African coast, down to
Cape Frio at the Namibian border. We have a great deal more information about the
Guinea coast than the Central African coast, much of it from the colonial period and
immediately afterward.
Guinea Coast The continental shelf of the 2000 km east-west coastline is occupied by
the eastward flow of the coastal component of the NECC-GC (see NATR), here usually
termed simply the Guinea Current. This receives a minor contribution in boreal winter
from the Canary Current, close inshore around Cape Roxo, and, after intensifying its
flow relative to the NECC between 8
W and the Greenwich meridian, it then weakens
progressively toward the east. The Guinea Current carries warm, light water and occupies
a shallow (30 m), warm (27
C) mixed layer above a very sharp thermocline (<1
C m
−1 ).
Below this eastward flow there is a contrary, westward flow of cold, highly saline water
originating at the termination of the EUC; this undercurrent, which I shall call the
Forcados Current following the usage of canoe fishermen on the Nigerian coast, weakens
toward the west. Recall that during boreal summer the thermocline of the whole eastern
tropical Atlantic shoals (see ETRA) and slopes upward toward the coast; it is at this season
that the Forcados Current flow is likely to surface intermittently, so that the Guinea
Current apparently moves offshore (Verstraete, 1992). I have monitored this apparent
reversal of flow at the coast off Nigeria, finding that each episode lasts for periods from
one day to several weeks.
Although a shelf-break front is often identifiable, the Guinea Current above the shelf
is continuous with the eastward flow of the NECC-GC during most of the year; however,
satellite-tracked RAFOS floats now suggest that reversal of flow at the shelf edge in the
western part of the Guinea Coast occurs in October with the onset of the NE Trades; at
this time, several eastbound, surface-drogued, satellite-tracked floats reversed direction
and moved closely along the shelf-break from Liberia to Guinea over a period of several
months (Dave Hebert, pers. comm., 2005). This is a region of very complex circulation,
and it is no surprise that long-term changes do occur although they are only rather dimly
observed in the presently available data: it appears that coastal SST was anomalously
high during the 1980s, for example, both at the surface and deep on the shelf after an
unusually cold period during the 1970s.
Surface water masses are under the influence of the alternation of dry and wet seasons
along the Guinea Coast. Seasonal cloud cover modifies the cycle of solar irradiance, from