Atlantic Trade Wind Biome
201
Eastern Tropical Atlantic Province (ETRA)
Extent of the Province
This province represents the oceanic Gulf of Guinea (sens. lat.) to the east of the boundary
with WTRA that is placed at 15
W. Unlike the WTRA, this province is almost symmetrical
about the equator, lying between the shelf-edge front of the east-west coast of West
Africa (Guinea to Nigeria) along about 5–10
N and a line drawn diagonally (NW-SE)
across the ocean between 5
S and 10
S. The eastern boundary is, of course the shelfbreak front along the north-south coast from Cameroon to Congo. As discussed later,
this arrangement is intended to include in one province the tropical gyral circulation
(SECC and Angola Dome) of the southern hemisphere to match the same elements of
the northern hemisphere included in WTRA. It is entirely oceanic, save for the chain of
small islands (Annobon to Fernando Po) in the Bight of Biafra.
Defining Characteristics of Regional Oceanography
The circulation of this province, within equatorial currents and the tropical gyre, is
largely a mirror image of that in the WTRA Province. Geostrophic flow, and hence the
topography of the thermocline, is rendered the more complex by the change of sign of
the Coriolis force at the equator, which the province straddles. As in other equatorial
provinces, seasonal variation in the circulation and topography of the thermocline is
forced by the overriding influence of the variable trade winds.
The ITCZ remains almost year-round over WTRA, lying above the northwest corner of
ETRA only in winter when in its most southerly position; southeasterly trade-wind stress
is thus more consistent in its direction than in WTRA, although wind stress at the equator
in ETRA at 4
W varies from about 02 dyn cm
−2 in February-March to 08 dyn cm
−2 in
July–September when the ITCZ moves rapidly northward so that by August reaches 15
N.
This causes the rapid extension of the strong (5–8 m sec
−1 ) southeasterly trade winds
across the equator that surge into the western basin of the tropical Atlantic, strongly
intensifying the zonal wind component there. Meanwhile, wind stress in the eastern
Atlantic strengthens principally in its meridional component (Hastenrath and Lamb,
1977). Almost the entire province lies under negative wind curl during this season, with
the southern limit of the SEC approximating to the change of sign in vertical motion
(Gordon and Bosley, 1991). It is to these seasonal changes in wind-stress and to the effect
of divergence in the wind field at the equator that we look for explanations of seasonal
changes in circulation and regional ecology.
From May to October, as a direct consequence of the impulsive intensification of
zonal wind stress, the equatorial thermocline shoals in the ETRA and deepens in the
western Atlantic, as already noted in the account of the WTRA province (Fig. 9.16). To
the east of a pivot line at 15–20
W, the base of the thermocline (defined as the depth of
the 15
C isotherm) shoals from 145 to 90 m, whereas the upper thermocline (the 20
C
isotherm) shoals by about 20 m. Thus, both the thermocline and the mixed layer above
it are vertically compressed in the eastern ocean.
This process is accompanied by a cooling of the mixed layer in the ETRA province
from 29
C down to about 23–24
C (e.g., Pérez et al., 2005). The seasonal cooling, now
that we can inspect its progress in 9-km, 8-day MODIS SST images, lies less strictly
along the equator than in the Pacific. Rather, it is a broad zone that narrows westward
and lies diagonally across the eastern Atlantic from the northern Angola Bight to the
equator at about 2–3
W, and thence along the equator; initial appearance occurs in May
or June, close to the coast at about 2–4
S. Within this somewhat diffuse cool zone may be
discerned some indication of a separate cool feature associated with the equator itself. The
201
Eastern Tropical Atlantic Province (ETRA)
Extent of the Province
This province represents the oceanic Gulf of Guinea (sens. lat.) to the east of the boundary
with WTRA that is placed at 15
W. Unlike the WTRA, this province is almost symmetrical
about the equator, lying between the shelf-edge front of the east-west coast of West
Africa (Guinea to Nigeria) along about 5–10
N and a line drawn diagonally (NW-SE)
across the ocean between 5
S and 10
S. The eastern boundary is, of course the shelfbreak front along the north-south coast from Cameroon to Congo. As discussed later,
this arrangement is intended to include in one province the tropical gyral circulation
(SECC and Angola Dome) of the southern hemisphere to match the same elements of
the northern hemisphere included in WTRA. It is entirely oceanic, save for the chain of
small islands (Annobon to Fernando Po) in the Bight of Biafra.
Defining Characteristics of Regional Oceanography
The circulation of this province, within equatorial currents and the tropical gyre, is
largely a mirror image of that in the WTRA Province. Geostrophic flow, and hence the
topography of the thermocline, is rendered the more complex by the change of sign of
the Coriolis force at the equator, which the province straddles. As in other equatorial
provinces, seasonal variation in the circulation and topography of the thermocline is
forced by the overriding influence of the variable trade winds.
The ITCZ remains almost year-round over WTRA, lying above the northwest corner of
ETRA only in winter when in its most southerly position; southeasterly trade-wind stress
is thus more consistent in its direction than in WTRA, although wind stress at the equator
in ETRA at 4
W varies from about 02 dyn cm
−2 in February-March to 08 dyn cm
−2 in
July–September when the ITCZ moves rapidly northward so that by August reaches 15
N.
This causes the rapid extension of the strong (5–8 m sec
−1 ) southeasterly trade winds
across the equator that surge into the western basin of the tropical Atlantic, strongly
intensifying the zonal wind component there. Meanwhile, wind stress in the eastern
Atlantic strengthens principally in its meridional component (Hastenrath and Lamb,
1977). Almost the entire province lies under negative wind curl during this season, with
the southern limit of the SEC approximating to the change of sign in vertical motion
(Gordon and Bosley, 1991). It is to these seasonal changes in wind-stress and to the effect
of divergence in the wind field at the equator that we look for explanations of seasonal
changes in circulation and regional ecology.
From May to October, as a direct consequence of the impulsive intensification of
zonal wind stress, the equatorial thermocline shoals in the ETRA and deepens in the
western Atlantic, as already noted in the account of the WTRA province (Fig. 9.16). To
the east of a pivot line at 15–20
W, the base of the thermocline (defined as the depth of
the 15
C isotherm) shoals from 145 to 90 m, whereas the upper thermocline (the 20
C
isotherm) shoals by about 20 m. Thus, both the thermocline and the mixed layer above
it are vertically compressed in the eastern ocean.
This process is accompanied by a cooling of the mixed layer in the ETRA province
from 29
C down to about 23–24
C (e.g., Pérez et al., 2005). The seasonal cooling, now
that we can inspect its progress in 9-km, 8-day MODIS SST images, lies less strictly
along the equator than in the Pacific. Rather, it is a broad zone that narrows westward
and lies diagonally across the eastern Atlantic from the northern Angola Bight to the
equator at about 2–3
W, and thence along the equator; initial appearance occurs in May
or June, close to the coast at about 2–4
S. Within this somewhat diffuse cool zone may be
discerned some indication of a separate cool feature associated with the equator itself. The