198
Chapter 9: The Atlantic Ocean
undertaken near 3
N 19
W in June 1997; this location is in the central Atlantic, so that
what was observed had nothing to do with the eventual encounter of this feature with
the NBC retroflection far to the west and what was observed may be therefore taken as
representative of processes within the TIWs discussed earlier. Water from the equatorial divergence (cold, nutrient-rich, and of high chlorophyll content) was found to be
advected northward and downward around the western side of the feature; as the circulation is completed back to the south, the pycnocline, nutricline and DCM were found
to move progressively surfaceward to enter again the equatorial water mass. So, as the
authors emphasize, “a fully three-dimensional circulation … dominates the distribution
of physical and biological tracers in the presence of tropical instabilities and maintains
the cusp-like shapes of temperature and chlorophyll observed from space.” The relative
vertical and horizontal distribution of SST and velocity, nitrate, primary production,
and in situ chlorophyll, together with zooplankton and micronekton biomass, were all
consistent; upwelling at depth within the vortex may not imply cross-isopycnal flux that
would supply new nutrients that may rather be supplied in the poleward surface flow
from the equatorial upwelling.
These observations suggest what must be the fundamental mechanism within TIWs
that produces their characteristic signature in the surface chlorophyll field: the fact that,
in the western part of the ocean, after retroflection of the NBC is initiated, water rich
in CDOM is entrained around the northern cusps does not require that this mechanism
should not function. We await examination of such a TIW with the double algorithm
technique used by Hu et al. in the NBC retroflection itself.
The Guinea Dome and the NECC The entire region occupied by the NECC exhibits
enhanced chlorophyll, both patchy and diffuse, that is dissociated from the lunate features and the retroflection of the NBC discussed earlier, although enhanced chlorophyll
within the NECC reaches its maximum westward extension in the same months that the
retroflection eddies are strongest, and remains so even as they diffuse toward the end of
the year. It is only in February or March that the expression of the NECC in the surface
chlorophyll field retreats to midocean. In exceptional years, as in 2001, this may not
occur until May.
Perhaps a sufficient explanation for this zone of high chlorophyll values lies in strong
vertical Ekman flux that occurs from 20
W to 40
W, particularly from June to October,
causing divergence along the crest of the thermal ridge between NECC and NEC (Isemer
and Hasse, 1987). Furthermore, as pointed out by Yentsch (1990), underlying the NECC is
a baroclinic ridge in the subsurface nitrate field taking values of 160 M at 150 m, similar
to concentrations at the same depth south of Greenland. Such high concentrations will
render any physical mechanism that tends to draw subsurface water toward the surface
unusually effective in supplying nutrients to the photic zone. In addition, the strongly
meandering flow of the NECC itself induces vertical motion within cyclonic eddies and
due to eddy/eddy interactions, and this motion is thought to be a nonnegligible source
of nutrient flux to surface waters (Dadou et al., 1996).
Seasonal changes in vertical Ekman velocity support this model: the greatest vertical
flux occurs when chlorophyll values are highest. In January, vertical Ekman velocity along
the NECC is weak but variable in sign, and by April, a broad band of zero vertical
transport corresponds with the area to be occupied later by the NECC. When the ITCZ
is in its northernmost position (July–September) and the southeast trades dominate the
wind field, a zone of exceptionally strong positive curl of the wind stress lies along
10–12
N from 20
W to 40
W (Isemer and Hasse, 1987; Hastenrath and Lamb, 1977).
This translates into Ekman suction (upwelling) rates of roughly 2 m d
−1 along the whole
length of the NECC during this season. Toward the end of this trimester, the locus of
highest values comes to lie nearer the African coast. Averaged over the year, the band of
Chapter 9: The Atlantic Ocean
undertaken near 3
N 19
W in June 1997; this location is in the central Atlantic, so that
what was observed had nothing to do with the eventual encounter of this feature with
the NBC retroflection far to the west and what was observed may be therefore taken as
representative of processes within the TIWs discussed earlier. Water from the equatorial divergence (cold, nutrient-rich, and of high chlorophyll content) was found to be
advected northward and downward around the western side of the feature; as the circulation is completed back to the south, the pycnocline, nutricline and DCM were found
to move progressively surfaceward to enter again the equatorial water mass. So, as the
authors emphasize, “a fully three-dimensional circulation … dominates the distribution
of physical and biological tracers in the presence of tropical instabilities and maintains
the cusp-like shapes of temperature and chlorophyll observed from space.” The relative
vertical and horizontal distribution of SST and velocity, nitrate, primary production,
and in situ chlorophyll, together with zooplankton and micronekton biomass, were all
consistent; upwelling at depth within the vortex may not imply cross-isopycnal flux that
would supply new nutrients that may rather be supplied in the poleward surface flow
from the equatorial upwelling.
These observations suggest what must be the fundamental mechanism within TIWs
that produces their characteristic signature in the surface chlorophyll field: the fact that,
in the western part of the ocean, after retroflection of the NBC is initiated, water rich
in CDOM is entrained around the northern cusps does not require that this mechanism
should not function. We await examination of such a TIW with the double algorithm
technique used by Hu et al. in the NBC retroflection itself.
The Guinea Dome and the NECC The entire region occupied by the NECC exhibits
enhanced chlorophyll, both patchy and diffuse, that is dissociated from the lunate features and the retroflection of the NBC discussed earlier, although enhanced chlorophyll
within the NECC reaches its maximum westward extension in the same months that the
retroflection eddies are strongest, and remains so even as they diffuse toward the end of
the year. It is only in February or March that the expression of the NECC in the surface
chlorophyll field retreats to midocean. In exceptional years, as in 2001, this may not
occur until May.
Perhaps a sufficient explanation for this zone of high chlorophyll values lies in strong
vertical Ekman flux that occurs from 20
W to 40
W, particularly from June to October,
causing divergence along the crest of the thermal ridge between NECC and NEC (Isemer
and Hasse, 1987). Furthermore, as pointed out by Yentsch (1990), underlying the NECC is
a baroclinic ridge in the subsurface nitrate field taking values of 160 M at 150 m, similar
to concentrations at the same depth south of Greenland. Such high concentrations will
render any physical mechanism that tends to draw subsurface water toward the surface
unusually effective in supplying nutrients to the photic zone. In addition, the strongly
meandering flow of the NECC itself induces vertical motion within cyclonic eddies and
due to eddy/eddy interactions, and this motion is thought to be a nonnegligible source
of nutrient flux to surface waters (Dadou et al., 1996).
Seasonal changes in vertical Ekman velocity support this model: the greatest vertical
flux occurs when chlorophyll values are highest. In January, vertical Ekman velocity along
the NECC is weak but variable in sign, and by April, a broad band of zero vertical
transport corresponds with the area to be occupied later by the NECC. When the ITCZ
is in its northernmost position (July–September) and the southeast trades dominate the
wind field, a zone of exceptionally strong positive curl of the wind stress lies along
10–12
N from 20
W to 40
W (Isemer and Hasse, 1987; Hastenrath and Lamb, 1977).
This translates into Ekman suction (upwelling) rates of roughly 2 m d
−1 along the whole
length of the NECC during this season. Toward the end of this trimester, the locus of
highest values comes to lie nearer the African coast. Averaged over the year, the band of