Atlantic Trade Wind Biome
205
CZCS instrument demonstrated that significant seasonality does occur, a fact abundantly
confirmed by more recent satellite data. I can suggest no explanation for the contrary
indications from the CIPREA data, but EQUALANT data were obtained during a year of a
Benguela Niño. The CZCS images were read as suggesting that relatively high chlorophyll
(03–05 mg m
−3 ) “occurs almost uniformly over the eastern tropical Atlantic in June–
August, east of a meridional boundary at about 20–25
W” (Longhurst, 1993). However,
the superior quality and data flow of the images from later sensors shows that the real situation is more complex than suggested by my initial, and quite erroneous, interpretation;
the first satisfactory interpretation of the main features of phytoplankton response is that
of Signorini et al. (1999). This pattern has recently been confirmed from both satellite
observations and measurements made at sea by Pérez et al. (2005); these authors show
empirical relationships between AVHRR SST and SeaWiFS chlorophyll that differ in ETRA
(10
W) and WTRA (25
W) such that, for ETRA, chl = 350–012 SST r
2
= 067 n =
48 P = <00001, and for WTRA, chl = 287–010 SST r
2
= 051 n = 48 P = <00001.
This inverse correlation, the authors noted, confirmed interpretations made earlier (e.g.,
Longhurst, 1993).
This knowledge, and examination of many images, shows that there are several different
processes whose effects must be disentangled before each can be understood, and the
form of the basin of the eastern equatorial Atlantic makes this no simple task. There is,
indeed, a rather diffuse seasonal enhancement of surface chlorophyll almost everywhere
in ETRA, caused by a variety of responses to:
(i) Wind-driven linear equatorial divergence, with which is associated a chlorophyll
enhancement field that is clearly centered upon linear, equatorial upwelling that is,
compared with the Pacific, rather episodic
(ii) Ekman upwelling above and around the Angola dome, with which is associated a
second, broader zone of chlorophyll enhancement that narrows westward and may
be continuous in the early months of the cool season with enhancement at the
equatorial divergence
(iii) The highly pigmented, shallow plume of the Congo River that lies diagonally across
the tropical gyre, rooted at about 5
S, and is readily confused with both of the above
(iv) Offshore effects of upwelling processes within the NECC-GC that occur south of the
east-west coast of tropical West Africa (see GUIN) and thus along the zonal flow
of the Guinea Current
It will be necessary to deal with these individually, while recognizing that the effects of
(i), (ii), and (iii) are superimposed spatially and hence difficult to understand individually.
Chlorophyll enhancement associated with equatorial divergence is weak or even absent
in boreal winter and appears as a threadlike feature, usually in June but sometimes, as
in 2001, in May. It usually takes maximum zonal dimension in July and August when
mixed-layer depths are minimal, and weakens and diffuses in the following months.
The evolution of the canonical seasonal equatorial bloom, and some of its betweenyear variance, could be traced only incompletely in the CZCS images, 1978–1986, but,
in the continuous imagery available from SeaWiFS and MODIS sensors since 1997, the
seasonal and between-year changes can be followed more clearly. In each year, the surface
chlorophyll field is weakest in March and progressively strengthens from April to June,
the latter being the first month when it is fully developed. The onset of the regional
bloom was a little earlier and stronger in 1998 and 2000 than in the other years. In each
year, the maximum extent of the bloom is reached during July and August, with the
maxima of 2000 and 2001 being somewhat stronger and more dispersed meridionally
than other years. Finally, in September of each year the northern border of the equatorial
bloom retreats southward, leaving a zone of low-pigment biomass between it and the
enhancement associated with the NECC-GC. This zone is continuous with a “blue hole”
in the outer Bight of Biafra (see earlier discussion) and lies above the trough in pycnocline
205
CZCS instrument demonstrated that significant seasonality does occur, a fact abundantly
confirmed by more recent satellite data. I can suggest no explanation for the contrary
indications from the CIPREA data, but EQUALANT data were obtained during a year of a
Benguela Niño. The CZCS images were read as suggesting that relatively high chlorophyll
(03–05 mg m
−3 ) “occurs almost uniformly over the eastern tropical Atlantic in June–
August, east of a meridional boundary at about 20–25
W” (Longhurst, 1993). However,
the superior quality and data flow of the images from later sensors shows that the real situation is more complex than suggested by my initial, and quite erroneous, interpretation;
the first satisfactory interpretation of the main features of phytoplankton response is that
of Signorini et al. (1999). This pattern has recently been confirmed from both satellite
observations and measurements made at sea by Pérez et al. (2005); these authors show
empirical relationships between AVHRR SST and SeaWiFS chlorophyll that differ in ETRA
(10
W) and WTRA (25
W) such that, for ETRA, chl = 350–012 SST r
2
= 067 n =
48 P = <00001, and for WTRA, chl = 287–010 SST r
2
= 051 n = 48 P = <00001.
This inverse correlation, the authors noted, confirmed interpretations made earlier (e.g.,
Longhurst, 1993).
This knowledge, and examination of many images, shows that there are several different
processes whose effects must be disentangled before each can be understood, and the
form of the basin of the eastern equatorial Atlantic makes this no simple task. There is,
indeed, a rather diffuse seasonal enhancement of surface chlorophyll almost everywhere
in ETRA, caused by a variety of responses to:
(i) Wind-driven linear equatorial divergence, with which is associated a chlorophyll
enhancement field that is clearly centered upon linear, equatorial upwelling that is,
compared with the Pacific, rather episodic
(ii) Ekman upwelling above and around the Angola dome, with which is associated a
second, broader zone of chlorophyll enhancement that narrows westward and may
be continuous in the early months of the cool season with enhancement at the
equatorial divergence
(iii) The highly pigmented, shallow plume of the Congo River that lies diagonally across
the tropical gyre, rooted at about 5
S, and is readily confused with both of the above
(iv) Offshore effects of upwelling processes within the NECC-GC that occur south of the
east-west coast of tropical West Africa (see GUIN) and thus along the zonal flow
of the Guinea Current
It will be necessary to deal with these individually, while recognizing that the effects of
(i), (ii), and (iii) are superimposed spatially and hence difficult to understand individually.
Chlorophyll enhancement associated with equatorial divergence is weak or even absent
in boreal winter and appears as a threadlike feature, usually in June but sometimes, as
in 2001, in May. It usually takes maximum zonal dimension in July and August when
mixed-layer depths are minimal, and weakens and diffuses in the following months.
The evolution of the canonical seasonal equatorial bloom, and some of its betweenyear variance, could be traced only incompletely in the CZCS images, 1978–1986, but,
in the continuous imagery available from SeaWiFS and MODIS sensors since 1997, the
seasonal and between-year changes can be followed more clearly. In each year, the surface
chlorophyll field is weakest in March and progressively strengthens from April to June,
the latter being the first month when it is fully developed. The onset of the regional
bloom was a little earlier and stronger in 1998 and 2000 than in the other years. In each
year, the maximum extent of the bloom is reached during July and August, with the
maxima of 2000 and 2001 being somewhat stronger and more dispersed meridionally
than other years. Finally, in September of each year the northern border of the equatorial
bloom retreats southward, leaving a zone of low-pigment biomass between it and the
enhancement associated with the NECC-GC. This zone is continuous with a “blue hole”
in the outer Bight of Biafra (see earlier discussion) and lies above the trough in pycnocline