Atlantic Trade Wind Biome
195
of the North Equatorial Countercurrent (NECC) from northern Brazil to Senegal creates
a zone of shear with the westward flow of the SEC to the south.
The seasonal ecology of the WTRA clearly reflects how these circulation features
respond to the seasonal strength of the trade winds, and the meridional shift of the
intertropical convergence zone (ITCZ) that lies between them. There are two major
responses to this seasonality with which we shall be concerned: (i) the thermocline of
the equatorial Atlantic tilts seasonally about its zonal and meridional axes, and (ii) the
eastward flow in the NECC is seasonally variable. Although, as noted earlier, there is a
seasonal signal in regional surface salinity attributable to the Amazon effluent, this is a
trivial effect compared with the low-salinity plume from the Congo that spreads westward
across the eastern tropical Atlantic (see ETRA later).
Seasonal changes in thermocline depth in WTRA are related to the migration of the
trade-wind belts in a complex manner. In boreal summer, June to August, when the ITCZ
is in its most northerly location, westward wind stress over the equatorial Atlantic is at
its seasonal maximum and induces a double seasonal tilt of the equatorial thermocline:
one toward the west, the other northward (Merle, 1983; Houghton, 1983; Hastenrath
and Merle, 1987). These motions induce a major deepening of the thermocline from
August to October to the north of Brazil and force the mixed-layer climatology of the
entire WTRA province. Because this phenomenon has greater ecological importance in
the eastern part of the ocean, I shall return to it in the section devoted to the ETRA.
In boreal summer, the contribution of the equator-crossing SEC to the coastal North
Brazil Current (NBC) is maximal (Boisvert, 1967; Müller-Karger et al., 1988), so that
the strengthened NBC retroflects eastward at about 5
N, topographically locked to the
Demerara Rise. This process is associated with the propagation along the South American
shelf of mesoscale, anticyclonic eddies with a periodicity of about 50 days (see GUIA
Province); the similarity of these eddies to the “Great Whorl” of the Somali Current
(see ARAB Province) with respect to latitude, season, and formation is striking (Bruce
et al., 1985). The consequences of the retroflection for the ecology of the western oceanic
regions of WTRA are important and are discussed below.
Important seasonal change in ocean physics occur in a zone that lies transversely right
across the ocean at 10–12
N, where the NECC flows eastward along the southern flank
of a geostrophic ridge (Garzoli and Katz, 1983, Hastenrath and Merle, 1987). Sverdrup
(1947) showed that it was the curl of wind stress rather than wind stress itself that
drives generic ECCs (Philander, 1985), a fact that has important ecological consequences.
Ekman suction (Isemer and Hasse, 1987) creates divergence along the axis of the NECC,
and in the Atlantic this may attain a vertical velocity at the base of the Ekman layer of
∼20 × 10
−5 cm sec
−1 .
The rather complex changes in the NECC that are occasioned by seasonal changes
in wind forcing have recently been modeled by Elmoussaoui et al. (2005) and may
be summarized briefly as follows. In August and September, when the ITCZ is at its
most northerly position, NECC flow is continuous, eastward across the ocean at about
10
N; when it nears the African coast, part continues to the east as the oceanic Guinea
Current, while part flows around the tropical cyclonic gyre of the North Atlantic, the
Guinea Dome. In boreal winter, when the ITCZ is at its most southerly position, the
NECC lies under the northeast trades so that in October, a discontinuity in eastward
flow appears at about 35
W, and the two now-separated areas of eastward flow regress
respectively toward Africa and America. By March, the origin of eastward flow in the
NECC is at about 18
W not far from the African coast, just outside the coastal Guinea
Current; from here it continues right into the Bight of Biafra (Boisvert, 1967; Bruce et al.,
1985; Garzoli and Richardson, 1989). Surface flow in the entire WTRA is now westward,
is indistinguishable from the SEC, and is in approximately the same direction as the
trade winds. A small portion of this flow passes around the cyclonic gyre just west of
195
of the North Equatorial Countercurrent (NECC) from northern Brazil to Senegal creates
a zone of shear with the westward flow of the SEC to the south.
The seasonal ecology of the WTRA clearly reflects how these circulation features
respond to the seasonal strength of the trade winds, and the meridional shift of the
intertropical convergence zone (ITCZ) that lies between them. There are two major
responses to this seasonality with which we shall be concerned: (i) the thermocline of
the equatorial Atlantic tilts seasonally about its zonal and meridional axes, and (ii) the
eastward flow in the NECC is seasonally variable. Although, as noted earlier, there is a
seasonal signal in regional surface salinity attributable to the Amazon effluent, this is a
trivial effect compared with the low-salinity plume from the Congo that spreads westward
across the eastern tropical Atlantic (see ETRA later).
Seasonal changes in thermocline depth in WTRA are related to the migration of the
trade-wind belts in a complex manner. In boreal summer, June to August, when the ITCZ
is in its most northerly location, westward wind stress over the equatorial Atlantic is at
its seasonal maximum and induces a double seasonal tilt of the equatorial thermocline:
one toward the west, the other northward (Merle, 1983; Houghton, 1983; Hastenrath
and Merle, 1987). These motions induce a major deepening of the thermocline from
August to October to the north of Brazil and force the mixed-layer climatology of the
entire WTRA province. Because this phenomenon has greater ecological importance in
the eastern part of the ocean, I shall return to it in the section devoted to the ETRA.
In boreal summer, the contribution of the equator-crossing SEC to the coastal North
Brazil Current (NBC) is maximal (Boisvert, 1967; Müller-Karger et al., 1988), so that
the strengthened NBC retroflects eastward at about 5
N, topographically locked to the
Demerara Rise. This process is associated with the propagation along the South American
shelf of mesoscale, anticyclonic eddies with a periodicity of about 50 days (see GUIA
Province); the similarity of these eddies to the “Great Whorl” of the Somali Current
(see ARAB Province) with respect to latitude, season, and formation is striking (Bruce
et al., 1985). The consequences of the retroflection for the ecology of the western oceanic
regions of WTRA are important and are discussed below.
Important seasonal change in ocean physics occur in a zone that lies transversely right
across the ocean at 10–12
N, where the NECC flows eastward along the southern flank
of a geostrophic ridge (Garzoli and Katz, 1983, Hastenrath and Merle, 1987). Sverdrup
(1947) showed that it was the curl of wind stress rather than wind stress itself that
drives generic ECCs (Philander, 1985), a fact that has important ecological consequences.
Ekman suction (Isemer and Hasse, 1987) creates divergence along the axis of the NECC,
and in the Atlantic this may attain a vertical velocity at the base of the Ekman layer of
∼20 × 10
−5 cm sec
−1 .
The rather complex changes in the NECC that are occasioned by seasonal changes
in wind forcing have recently been modeled by Elmoussaoui et al. (2005) and may
be summarized briefly as follows. In August and September, when the ITCZ is at its
most northerly position, NECC flow is continuous, eastward across the ocean at about
10
N; when it nears the African coast, part continues to the east as the oceanic Guinea
Current, while part flows around the tropical cyclonic gyre of the North Atlantic, the
Guinea Dome. In boreal winter, when the ITCZ is at its most southerly position, the
NECC lies under the northeast trades so that in October, a discontinuity in eastward
flow appears at about 35
W, and the two now-separated areas of eastward flow regress
respectively toward Africa and America. By March, the origin of eastward flow in the
NECC is at about 18
W not far from the African coast, just outside the coastal Guinea
Current; from here it continues right into the Bight of Biafra (Boisvert, 1967; Bruce et al.,
1985; Garzoli and Richardson, 1989). Surface flow in the entire WTRA is now westward,
is indistinguishable from the SEC, and is in approximately the same direction as the
trade winds. A small portion of this flow passes around the cyclonic gyre just west of