206
Chapter 9: The Atlantic Ocean
topography between the NECC-GC and the northern part of the SEC. This pattern persists
and the blue hole increases in size until the equatorial bloom disperses in January and
February. From the evidence of surface chlorophyll during the SeaWiFS period, the 1984
warm event in the Gulf of Guinea (see earlier discussion) was exceptional as suggested
by the 1964–1993 monthly SST archives (Delacluse et al., 1994). This cycle was followed
and illustrated by Pérez et al. (2004), who were able to obtain a satisfactory empirical
relationship between SeaWiFS chlorophyll and in situ observations in the ETRA.
As already noted, the equatorial chlorophyll enhancement associated is—at first
glance—asymmetric and extends much farther south than would be anticipated. It now
seems probable that this represents, at least in part, the southern hemisphere equivalent
of the chlorophyll enrichment that lies along the NECC and Guinea Dome in the WTRA
province. Not that it is a simple mirror image of that feature, because of the asymmetry
of the African continent in the two hemispheres, and because rather than the broad
SW-NE feature of the NECC, here we find a variety of features. Most typically, it is a
broadly triangular region that narrows to the northwest and is based within the Angola
Bight; there are many permutations of this feature, the most extreme being a rather
narrow, zonal band of enhanced chlorophyll parallel to the equator, but at about 7–8
S.
Its eastern termination betrays it for what it is, for here it merges imperceptibly around
the northern edge of the Angola Dome, subsequently becoming increasingly wide and
increasingly strong. It may, then, represent the result of the same processes in the SECC
that were discussed earlier in relation to the zonal bloom in the NECC. It is not easy
to use the chlorophyll images to support the earlier assumptions concerning the ecology
of this region, based on investigations near the axis of the Angola Dome (Voituriez and
Herbland, 1982).
Rather, a relatively simple model has been invoked to explain the observations (Monger
et al., 1997). Phytoplankton growth is enhanced in May by distant uplift of the regional
thermocline, forced by winds in the western Atlantic, and the consequent entrainment
of nutrients into the photic zone, together with some mixing by wind stress although
this is minimal at very low latitudes. Residual nitrate along the equatorial divergence
during boreal summer represents the normal situation in which the rate of supply
temporarily exceeds the utilization rate. By August, the thermocline has returned to
depth but, because the EUC is now deeper than prior to uplift, such vertical entrainment
as occurs delivers water richer in nutrients than previously. Thus the active growth of
phytoplankton continues for some months until, in December, the EUC returns to its
boreal winter location above the thermocline. Any divergence after this time brings only
nutrient-depleted water to the surface.
The model proposed by Monger et al. was intended to apply only between 3
N and
3
S, being tested against survey and satellite data from that zone; thus, it was intended as
a model for the equatorial divergence cell, rather than for the regions to south and
north discussed earlier. Nevertheless, it requires that the nutricline should bear a shoal
ridge at some distance (say 5
latitude) to north and south of the equatorial divergence,
and that some deeper water should be drawn into the photic zone there. None of
which is inconsistent with the suggestion made above that the pigment enhancement
apparently associated with the elongated tropical gyre is delivered by a process distinct
from equatorial divergence.
Now, the third component of pigment enhancement to take our attention is
attributable to the Congo or Zaire plume. Wherever the indicated pigment biomass in
relevant satellite images exceeds, say, 2 mg chl m
−3 at the surface, is linear, and is based
at the Congo mouth, then we may say with some confidence that it is indeed the Congo
plume we are looking at. Such a feature is present in a high proportion of both 8- and
30-day images but has a seasonality that is not easy to generalize; thus, in the 3 years
2000–2003, it was especially strong and extended far across the tropical gyre in three
Chapter 9: The Atlantic Ocean
topography between the NECC-GC and the northern part of the SEC. This pattern persists
and the blue hole increases in size until the equatorial bloom disperses in January and
February. From the evidence of surface chlorophyll during the SeaWiFS period, the 1984
warm event in the Gulf of Guinea (see earlier discussion) was exceptional as suggested
by the 1964–1993 monthly SST archives (Delacluse et al., 1994). This cycle was followed
and illustrated by Pérez et al. (2004), who were able to obtain a satisfactory empirical
relationship between SeaWiFS chlorophyll and in situ observations in the ETRA.
As already noted, the equatorial chlorophyll enhancement associated is—at first
glance—asymmetric and extends much farther south than would be anticipated. It now
seems probable that this represents, at least in part, the southern hemisphere equivalent
of the chlorophyll enrichment that lies along the NECC and Guinea Dome in the WTRA
province. Not that it is a simple mirror image of that feature, because of the asymmetry
of the African continent in the two hemispheres, and because rather than the broad
SW-NE feature of the NECC, here we find a variety of features. Most typically, it is a
broadly triangular region that narrows to the northwest and is based within the Angola
Bight; there are many permutations of this feature, the most extreme being a rather
narrow, zonal band of enhanced chlorophyll parallel to the equator, but at about 7–8
S.
Its eastern termination betrays it for what it is, for here it merges imperceptibly around
the northern edge of the Angola Dome, subsequently becoming increasingly wide and
increasingly strong. It may, then, represent the result of the same processes in the SECC
that were discussed earlier in relation to the zonal bloom in the NECC. It is not easy
to use the chlorophyll images to support the earlier assumptions concerning the ecology
of this region, based on investigations near the axis of the Angola Dome (Voituriez and
Herbland, 1982).
Rather, a relatively simple model has been invoked to explain the observations (Monger
et al., 1997). Phytoplankton growth is enhanced in May by distant uplift of the regional
thermocline, forced by winds in the western Atlantic, and the consequent entrainment
of nutrients into the photic zone, together with some mixing by wind stress although
this is minimal at very low latitudes. Residual nitrate along the equatorial divergence
during boreal summer represents the normal situation in which the rate of supply
temporarily exceeds the utilization rate. By August, the thermocline has returned to
depth but, because the EUC is now deeper than prior to uplift, such vertical entrainment
as occurs delivers water richer in nutrients than previously. Thus the active growth of
phytoplankton continues for some months until, in December, the EUC returns to its
boreal winter location above the thermocline. Any divergence after this time brings only
nutrient-depleted water to the surface.
The model proposed by Monger et al. was intended to apply only between 3
N and
3
S, being tested against survey and satellite data from that zone; thus, it was intended as
a model for the equatorial divergence cell, rather than for the regions to south and
north discussed earlier. Nevertheless, it requires that the nutricline should bear a shoal
ridge at some distance (say 5
latitude) to north and south of the equatorial divergence,
and that some deeper water should be drawn into the photic zone there. None of
which is inconsistent with the suggestion made above that the pigment enhancement
apparently associated with the elongated tropical gyre is delivered by a process distinct
from equatorial divergence.
Now, the third component of pigment enhancement to take our attention is
attributable to the Congo or Zaire plume. Wherever the indicated pigment biomass in
relevant satellite images exceeds, say, 2 mg chl m
−3 at the surface, is linear, and is based
at the Congo mouth, then we may say with some confidence that it is indeed the Congo
plume we are looking at. Such a feature is present in a high proportion of both 8- and
30-day images but has a seasonality that is not easy to generalize; thus, in the 3 years
2000–2003, it was especially strong and extended far across the tropical gyre in three