281
MODEL OF THE GULF OF GUINEA UPWELLING.
INFLUENCE OF THE COAST'S IRREGULARITIES.
2351 1
BAH
Jacques C.J. NIHOUL and A.
INTRODUCTION
1.
upwelling is
Every summer, for a period from early July through September,
an important feature along the Gulf of Guinea coast (fig. 1). According to
Houghton (1976), the cold water found along the northern boundary of the Gulf
is local in origin and not advected into the area by the Benguela current
coldest water is always found east of Cape Palmas and Cape Three Points. There
is no correlation between local winds and nearshore temperatures or changes in
.
the local ocean circulation.
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Fig.
1.
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100 E
Schematic map of the Gulf of Guinea, showing the region of upwelling
(shaded area) and the presumed surface (
(after Houghton, 1976).
) and subsurface (--) currents
The
282
Observations, during Gate, by the Soviet Ship RV. Parsat seemed to suggest
a forcing of oceanic origin and O'Brien (e.g. O'Brien et al., 1978; Adamec and
O'Brien, 1978) hypothesized that a baroclinic Kelvin wave is excited in the
western Atlantic by the onset of the southeast trades in May-June. This long
wave
/
strong upwelling impulse, propagates eastward across the Atlantic.
It then
propagates poleward as a coastal Kelvin wave and produces the upwelling event
along the Gulf of Guinea coast.
With a non-linear numerical model, using the g -plane approximation in an
ocean initially at rest, Adamec and O'Brien (1978) calculated the first baroclinic modal response of the ocean to a sudden increase
of the wind stress and
succeeded in reproducing the main features of the observations with a very good
agreement of both amplitude and time scales.
In all the applications of their model, however, the coast was assumed rectilinear and the possible effect
of capes on the upwelling's intensity could not
be studied.
The effect of capes is often discussed in terms of local conditions such as
the orientation of the coast with respect to the local wind or the deflection
of the coastal circulation.
An approximate calculation made by Bah (1980)
*
/
using current results of a
numerical model similar to that of Adamec and O'Brien (1978), showed a possible
amplification of the upwelling east of Cape Three Points, without being strongly
conclusive.
In the following, the influence of Cape Palmas and Cape Three Points is in-
vestigated, using an improved version of Bah's model, with a numerical grid
which takes into account the coastal
irregularities. It is shown that a Kelvin
wave generated upwelling, of the type hypothesized by O'Brien et al. (1978),
can explain, along a non-linear coast such as the northern coast of the Gulf of
Guinea, the kind of amplification of upwelling's intensity which is observed
east of the capes.
2.
DESCRIPTION OF THE MATHEMATICAL MODEL
Fig. 2 shows the model geometry with the irregular coastline.
The governing equations are derived from the Boussinesq equations (e.g.
Nihoul, 1982). Assuming two layers of uniform densities
Pl
(upper layer) and
(lower layer) and zero pressure gradient in the bottom layer, one can write,
u in the upper
in the g-plane approximation, the equations for the transport
P2
layer, in the form
3H
3t
*.
+7.U =
0
Some mispnnts_in the equations used for the calculation make this part of
the paper difficult to understand. The reader is advised to enquire into the
corrigenda.
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283
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Fig. 2. The model geometry with the irregular coastline
3U
+
St
V. (H
-1
UU) + B X2 (e 3
A
U) = -
g H 7h +
T
+
Pl
.
2
(2)
A v u
where
H = Ho
+
3)
h
(HO /^ 50 m) ,
is the undisturbed thickness of the upper layer
Ho
turbation of the upper layer's thickness and
of the upper layer.
A
h
the per-
the total (disturbed) thickness
is a horizontal diffusion coefficient taking into account
T
turbulence and shear effect,
g' =
H
is the wind stress,
g(p2 - PI>
(4)
P2
The details of the numerical model (discretization scheme, stability criteria,
boundary conditions, ...) are given in (Bah,
3.
1980) .
APPLICATION
O'Brien et al. (1978), Adamec and O'Brien (1978) and Bah (1980) have investigated several cases of oceanic responses to impulsive changes of the wind
stress, over part of the area
One of these cases assumes
*
a
ward wind stress over the western
sudden increase of
1500 km
-2
0.0125 Nm
of the basin.
in the west-
284
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Fig.
3.
Elevation of the interface 10 days after
the onset of the wind.
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20
Fig. 4. Elevation of the interface
20
days after the onset of the wind.
285
30
Fig. 5. Elevation of the interface
30
days after the onset of the wind.
iO
Fig. 6. Elevation of the interface 40 da.s afte. the onset of the
wind.
286
30
Fig. 7. Elevation of the interface 5
0
days after the onset of the wind.
<(
60
Fig. 8. Elevation of the interface
60
days after the onset of the wind.
287
^
Fig.
Fig.
9.
Elevation of the interface
70
10.
Elevation of the interface
80
days after the onset of the wind.
days after the onset of the wind.
288
90
Elevation of the interface
Fig. 11.
90
days after the onset of the wind.
This case has been simulated using the model described in section 2.
Figs. 3 to 11 show the propagation of the Kelvin wave generated upwelling
10, 20,
.
.
., 90 days after the onset of the wind. An amplification of the"
upwelling's intensity east of Cape Three Points is apparent.
On
fig. 12, the elevation of the interface in the Gulf of Guinea after
90
days is compared with the interface elevation which is found when one assumes
a linear coast line. The effect of the shape of the coast on the distribution
of the upwelling's intensities is indicated by the position of the
10 m
elevation-curve.
The Kelvin wave model appears thus to be able to reproduce the main features
of the upwelling in the Gulf of Guinea including the amplification of its
intensity east of the capes on the northern boundary.
4.
REFERENCES
Adamec, D. and O'Brien, J.J., 1978. The seasonal upwelling in the Gulf of Guinea
due to remote forcing. J. Phys. Oceanogr., 8: 1050.
Bah, A., 1980. Upwelling in the Gulf of Guinea. In: J.C.J. Nihoul (Editor), Ecohydrodynamics, Elsevier Publ., Amsterdam, pp. 99-140.
Houghton, R.w., 1976. Circulation and hydrographic structure of the Ghana conti-
nental shelf during the 1974 upwelling. J. Phys. Oceanogr., 6: 909-924.
Nihoul, J.C.J., 1982. Hydrodynamic models of shallow continental seas, Riga
Publ., Liege, 198 pp.
O'Brien, J.J., Adamec, D. and Moore, D., 1978. A simple model of upwelling in
the Gulf of Guinea. Geophys. Res., 5: 641-644.
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289
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Fig. 12.
Elevation of the interface calculated by the model for a rectilinear
coastline (above) and a realistic coastline taking the capes into account (below).
The results for a linear coastline are similar to those of O'Brien et al. (1978) ,
Adamee and O'Brien (1978) and Bah (1980).