Atlantic Trade Wind Biome
197
course, this feature strengthens eastward following the general eastward shoaling of the
thermocline.
During periods when it is fully developed, the western termination of the divergence
leaves the equator to curve toward the northwest, parallel to the coast; this represents,
I believe, the effect of horizontal current shear along the northeast flank of the flow of
the equator-crossing SEC. This occurs in the same period when part of the flow of the
NBC is retroflected into the open ocean. The fate of the Amazon and Orinoco plumes
in the NBC and the retroflection into the NECC of some of this water are discussed in
more detail in the section devoted to the BRAZ province; here, it will be appropriate
to discuss only the subsequent fate of water that is retroflected, usually from August to
October. Between-year differences in the intensity of the retroflection are due to changes
in trade-wind intensity (Müller-Karger et al., 1988).
It is frequently suggested that the retroflection of the NBC sheds large anticyclonic
eddies, apparently generated within the retroflection. In reality, eddies that are shed into
the NBC are actually TIWs that have been entrained into the retroflection after their
long passage across the tropical Atlantic. This encounter creates the now-familiar cusp
of high-pigment water around the northern arc of the retroflection feature and is readily
observed in serial images.
Motion, and hence transport, around the northern arcs of these anticyclonic features
is eastward; thus, on encountering the offshore pigment field of the NBC, water having an
Amazon signature in salinity and dissolved organic matter (DOM) is entrained offshore
into the open ocean (e.g., Hu et al., 2004). Chlorophyll images in late summer may show
two to four contiguous lunate arcs, each representing an individual TIW, between the
NBC retroflection and the Guinea Dome. The entrainment, or retroflection, sequence
is initiated in June or July with the appearance of a curved tongue of high pigment
that passes northeast across the Demerara abyssal plain around an anticyclonic eddy that
has recently encountered the topography of South America. Toward the end of each
year, the lunate arcs become increasingly diffuse until they are indistinguishable from
the general chlorophyll enhancement characteristic of the NECC. Observe that these
mesoscale features move progressively westward across the ocean contrary, that is, to the
apparent flux of Amazon discharge water toward the east.
There has been some debate over how to interpret these pigment features. It has been
suggested that high-nutrient, high-turbidity water discharged from the Amazon accounts
for the observed pigment enhancement in the retroflected eddies (e.g., Müller-Karger
et al., 1988; Johns et al., 1990; Signorini et al., 1999). However, because it seemed unclear
how unutilized nutrients could be conserved so far from the river mouth over oceanic
depths, it was suggested (Longhurst, 1995) that what was observed was the result of
eddy upwelling (e.g., Woods, 1988). The matter is now more satisfactorily resolved by
the use of algorithms for SeaWiFS data suitable for both Case 1 and Case 2 water (Hu
et al., 2004); this analysis enables the separation in the data of colored DOM (CDOM
or “gelbstoffe”) from chlorophyll. In low-pigment, oligotrophic water in the center of a
TIW, which was being entrained into the retroflection, chlorophyll dominated absorption
at 443 nm, whereas in the high-pigment retroflection plume, CDOM dominated and
here there was no apparent correlation between the two pigment types. Observation (by
PALACE floats) of the eastward transport of low-salinity water in the NECC appears
to clinch the argument: the multiple lunate features observed by SeaWiFS across the
tropical western Atlantic are at least in part the consequence of CDOM from the Amazon
forests.
But to some unknown extent they must also represent enhanced phytoplankton
growth. Fortunately, we have an excellent description of physical and ecological processes
within a tropical instability wave, that suggested that these features should be characterized as “whirling ecosystems” (Menkes, 2002). An impeccable study at sea of a TIW was
197
course, this feature strengthens eastward following the general eastward shoaling of the
thermocline.
During periods when it is fully developed, the western termination of the divergence
leaves the equator to curve toward the northwest, parallel to the coast; this represents,
I believe, the effect of horizontal current shear along the northeast flank of the flow of
the equator-crossing SEC. This occurs in the same period when part of the flow of the
NBC is retroflected into the open ocean. The fate of the Amazon and Orinoco plumes
in the NBC and the retroflection into the NECC of some of this water are discussed in
more detail in the section devoted to the BRAZ province; here, it will be appropriate
to discuss only the subsequent fate of water that is retroflected, usually from August to
October. Between-year differences in the intensity of the retroflection are due to changes
in trade-wind intensity (Müller-Karger et al., 1988).
It is frequently suggested that the retroflection of the NBC sheds large anticyclonic
eddies, apparently generated within the retroflection. In reality, eddies that are shed into
the NBC are actually TIWs that have been entrained into the retroflection after their
long passage across the tropical Atlantic. This encounter creates the now-familiar cusp
of high-pigment water around the northern arc of the retroflection feature and is readily
observed in serial images.
Motion, and hence transport, around the northern arcs of these anticyclonic features
is eastward; thus, on encountering the offshore pigment field of the NBC, water having an
Amazon signature in salinity and dissolved organic matter (DOM) is entrained offshore
into the open ocean (e.g., Hu et al., 2004). Chlorophyll images in late summer may show
two to four contiguous lunate arcs, each representing an individual TIW, between the
NBC retroflection and the Guinea Dome. The entrainment, or retroflection, sequence
is initiated in June or July with the appearance of a curved tongue of high pigment
that passes northeast across the Demerara abyssal plain around an anticyclonic eddy that
has recently encountered the topography of South America. Toward the end of each
year, the lunate arcs become increasingly diffuse until they are indistinguishable from
the general chlorophyll enhancement characteristic of the NECC. Observe that these
mesoscale features move progressively westward across the ocean contrary, that is, to the
apparent flux of Amazon discharge water toward the east.
There has been some debate over how to interpret these pigment features. It has been
suggested that high-nutrient, high-turbidity water discharged from the Amazon accounts
for the observed pigment enhancement in the retroflected eddies (e.g., Müller-Karger
et al., 1988; Johns et al., 1990; Signorini et al., 1999). However, because it seemed unclear
how unutilized nutrients could be conserved so far from the river mouth over oceanic
depths, it was suggested (Longhurst, 1995) that what was observed was the result of
eddy upwelling (e.g., Woods, 1988). The matter is now more satisfactorily resolved by
the use of algorithms for SeaWiFS data suitable for both Case 1 and Case 2 water (Hu
et al., 2004); this analysis enables the separation in the data of colored DOM (CDOM
or “gelbstoffe”) from chlorophyll. In low-pigment, oligotrophic water in the center of a
TIW, which was being entrained into the retroflection, chlorophyll dominated absorption
at 443 nm, whereas in the high-pigment retroflection plume, CDOM dominated and
here there was no apparent correlation between the two pigment types. Observation (by
PALACE floats) of the eastward transport of low-salinity water in the NECC appears
to clinch the argument: the multiple lunate features observed by SeaWiFS across the
tropical western Atlantic are at least in part the consequence of CDOM from the Amazon
forests.
But to some unknown extent they must also represent enhanced phytoplankton
growth. Fortunately, we have an excellent description of physical and ecological processes
within a tropical instability wave, that suggested that these features should be characterized as “whirling ecosystems” (Menkes, 2002). An impeccable study at sea of a TIW was