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Chapter 9: The Atlantic Ocean
annual variability than seasonality: an unusually strong regional maximum occurred in
late 1998, whereas in subsequent years the seasonal maxima were much weaker. It is only
during the second half of the year, when the mixed layer is shoaler than the depth of
1% surface irradiance by about 20 m, that higher vertically integrated production rates
are computed. The satellite data support this result because there is some evidence of
a progressive accumulation of chlorophyll over most of the area of the province during
this period. This conclusion is not invalidated by the distant-field effect of the high
chlorophyll in the NECC that occurs in winter along the southern edge of NATR, and
dominates the integrated NATR chlorophyll field.
Perhaps clearer is the seasonal analysis of McClain et al., who determined that the
areal extent of chlorophyll values >007 mg m
−3 changes seasonally, being minimal in
December–January and maximal in June–September, following the irradiance cycle.
McClain et al. enquire how nutrients can be supplied to the euphotic zone to support
the observed productivity in such regions as this, where downwelling is characteristic
and nutriclines are deep. They suggest that the probable mechanism is surface Ekman
lateral flux and they compute for this region (and the four other similar subtropical gyral
regions) that nitrate and phosphate horizontal fluxes in the upper 50 m are net positive
into the gyre interiors. Here, in the North Atlantic, 85% of this lateral flux originates
across the southern boundary from the NECC. Despite this supply, the regional pattern of
phytoplankton biomass appears to be dominated by the local effects of nutricline depth;
as McClain et al. point out, the region of minimal surface chlorophyll corresponds not
with the focus of the gyre at the point of maximum sea level elevation but >1000 km
distant, where pycnocline depth is maximal.
Bulk analysis of chlorophyll pigments at the 1991–92 French “Eumeli” mesotrophic
station in NAST and the oligotrophic station in NATR show clearly the distinct characteristics of tropical phytoplankton. In NATR, low bulk chlorophyll values had a relatively
large zeaxanthin (cyanobacterial) component and a relatively small fucoxanthin (diatom)
component. But, the peridinin (dinoflagellate) component had a similar range of values
in the two provinces. Microzooplankton biomass and abundance (∼10 mg C m
−3 ) are
as small as anywhere in the oceans and 5–10 times smaller than in the neighboring
subtropical provinces.
In case we should become too confident that the NATR (and similar trade-wind
provinces in other oceans) are highly predictable, relatively invariant oceanic environments, the now-routine repetitive observations of the sea surface chlorophyll field by
satellite imagery may have some surprises for us. Consider the case of surface chlorophyll
images of the NATR in the fall of 2001 (see color plate 6). Here, we are surprised to
see a very large dendritic region of chlorophyll enhancement (< 05 mg chl m
−3 ) about
the size of Spain, lying far offshore in the oligotrophic region, centered about 32
W,
and at the latitude of Cap Blanc. The like occurs in no other monthly image during the
period 1997–2003. One might suppose it to have been a large parcel of upwelled water
that had taken an unusual offshore trajectory except for the fact that in the previous
month one can just discern the first indications of chlorophyll enhancement at the same
location. The following month, it is entirely absent. This feature superficially resembles
the midocean bloom that I find in the South Indian Ocean (see ISSG province), which
exhibits the same mesoscale swirls and rings, translates consistently eastward, and can be
observed over a period of several months. But the October 2001 feature was a far briefer
phenomenon than the other, which has a largely predictable return period each year,
in greater or lesser intensity. Further, as can be seen from the images, any correlation
between surface chlorophyll swirls and underlying mesoscale eddy features in the NATR
bloom is much weaker than that of the Indian Ocean.
During boreal summer, the initiation of retroflection of the NECC from the coast of
northern Brazil (see GUIA, later) occupies along a greater length of that coast than when
Chapter 9: The Atlantic Ocean
annual variability than seasonality: an unusually strong regional maximum occurred in
late 1998, whereas in subsequent years the seasonal maxima were much weaker. It is only
during the second half of the year, when the mixed layer is shoaler than the depth of
1% surface irradiance by about 20 m, that higher vertically integrated production rates
are computed. The satellite data support this result because there is some evidence of
a progressive accumulation of chlorophyll over most of the area of the province during
this period. This conclusion is not invalidated by the distant-field effect of the high
chlorophyll in the NECC that occurs in winter along the southern edge of NATR, and
dominates the integrated NATR chlorophyll field.
Perhaps clearer is the seasonal analysis of McClain et al., who determined that the
areal extent of chlorophyll values >007 mg m
−3 changes seasonally, being minimal in
December–January and maximal in June–September, following the irradiance cycle.
McClain et al. enquire how nutrients can be supplied to the euphotic zone to support
the observed productivity in such regions as this, where downwelling is characteristic
and nutriclines are deep. They suggest that the probable mechanism is surface Ekman
lateral flux and they compute for this region (and the four other similar subtropical gyral
regions) that nitrate and phosphate horizontal fluxes in the upper 50 m are net positive
into the gyre interiors. Here, in the North Atlantic, 85% of this lateral flux originates
across the southern boundary from the NECC. Despite this supply, the regional pattern of
phytoplankton biomass appears to be dominated by the local effects of nutricline depth;
as McClain et al. point out, the region of minimal surface chlorophyll corresponds not
with the focus of the gyre at the point of maximum sea level elevation but >1000 km
distant, where pycnocline depth is maximal.
Bulk analysis of chlorophyll pigments at the 1991–92 French “Eumeli” mesotrophic
station in NAST and the oligotrophic station in NATR show clearly the distinct characteristics of tropical phytoplankton. In NATR, low bulk chlorophyll values had a relatively
large zeaxanthin (cyanobacterial) component and a relatively small fucoxanthin (diatom)
component. But, the peridinin (dinoflagellate) component had a similar range of values
in the two provinces. Microzooplankton biomass and abundance (∼10 mg C m
−3 ) are
as small as anywhere in the oceans and 5–10 times smaller than in the neighboring
subtropical provinces.
In case we should become too confident that the NATR (and similar trade-wind
provinces in other oceans) are highly predictable, relatively invariant oceanic environments, the now-routine repetitive observations of the sea surface chlorophyll field by
satellite imagery may have some surprises for us. Consider the case of surface chlorophyll
images of the NATR in the fall of 2001 (see color plate 6). Here, we are surprised to
see a very large dendritic region of chlorophyll enhancement (< 05 mg chl m
−3 ) about
the size of Spain, lying far offshore in the oligotrophic region, centered about 32
W,
and at the latitude of Cap Blanc. The like occurs in no other monthly image during the
period 1997–2003. One might suppose it to have been a large parcel of upwelled water
that had taken an unusual offshore trajectory except for the fact that in the previous
month one can just discern the first indications of chlorophyll enhancement at the same
location. The following month, it is entirely absent. This feature superficially resembles
the midocean bloom that I find in the South Indian Ocean (see ISSG province), which
exhibits the same mesoscale swirls and rings, translates consistently eastward, and can be
observed over a period of several months. But the October 2001 feature was a far briefer
phenomenon than the other, which has a largely predictable return period each year,
in greater or lesser intensity. Further, as can be seen from the images, any correlation
between surface chlorophyll swirls and underlying mesoscale eddy features in the NATR
bloom is much weaker than that of the Indian Ocean.
During boreal summer, the initiation of retroflection of the NECC from the coast of
northern Brazil (see GUIA, later) occupies along a greater length of that coast than when