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Chapter 9: The Atlantic Ocean
Although the evidence is somewhat circumstantial, this appears to occur patchily and as
a response to local meteorological conditions that change from year to year, rather than
as the smooth poleward progression that might be inferred from mean conditions.
The general nutrient regime of the Atlantic has been discussed in Chapter 5 and, as
noted there, comparison with that of the North Pacific is profitable; the younger deep
water of the Atlantic and the very deep winter mixing cause nutrient concentrations at
standard depths to be lower than in the North Pacific. Moreover, it does not appear
necessary to invoke the smaller size of the North Atlantic basin, and hence a supposed
greater supply of continental Fe-bearing dust (as has been done) to explain its much
stronger spring bloom: the strikingly different stratification regimes of the two northern
oceans is a sufficient explanation.
Response of the Pelagic Ecosystems
The general seasonal pattern of phytoplankton growth responds to the succession of
deep winter mixing followed by vernal stratification, while the details of the sea-surface
chlorophyll field are determined by the distribution of eddy kinetic energy and local
short-term changes in irradiance and wind stress. It has been assumed in the past that
there is a simple northward progression of the spring bloom in the NADR, and that
sufficient stratification is also induced progressively northward. However (as is so often
the case), we find that reality is rather more complex, now that we have the tools for
satisfactory observation of the evolution of the chlorophyll field.
Formal analysis of oceanwide SeaWiFS images has been used to evaluate the date when
events in the chlorophyll record occur (Siegel et al., 2002). The date of the initiation of
the bloom at each pixel is taken to be when chlorophyll biomass increases 5% above
median conditions; this occurs in early February south of about 40
N and propagates
northward, so that at 50
N it occurs in early June. Further north still, initiation occurs
almost simultaneously right across the northern part of NADR and in SARC Province.
Computed critical depths (Z cr ) on the day of bloom initiation derived from this analysis
shoal slightly northward from ∼30 m at 40–45
N to ∼25 m at 55–60
N.
In any case, the seasonal development of phytoplankton biomass can now readily
be followed in SeaWiFS or MODIS 7-day and monthly sea surface chlorophyll images,
especially with the reprocessing No. 4 set, in which cloud masking is significantly reduced.
The seasonal evolution of MODIS surface chlorophyll in 2004 (see Color plate 5) is typical.
All January images show that surface chlorophyll is lower in NADR than to the south near
the Azores Front. February images suggest that chlorophyll enhancement in that month
is initiated by the increase of surface irradiance right across the ocean in the eddy field
associated with the subtropical frontal zone of the NAST province, at 35–40
N. In March,
the entire region up to 50–52
N shows evidence of accumulating chlorophyll that is
significantly enhanced—but not very much extended—during April. From May onward,
the bloom in NADR becomes continuous with those in the adjacent provinces, SARC to
the northeast and ARCT to the north. In late summer and autumn, the pattern continues
to evolve so that high surface chlorophyll values are increasingly biased northward with
southern areas becoming increasingly oligotrophic until October, when a minor increase
in surface chlorophyll once more appears across the ocean at 40–50
N, progressively to
weaken as winter approaches, wind-mixing deepens, and surface irradiance weakens.
The images confirm that patchiness does occur at all scales of chlorophyll enhancement,
although comparison of all available images for each month does suggest some recurrent
pattern. Two major linear regions of enhancement are seen in April 1999, one aligned with
the eddy field of the subtropical font in the NAST province, the other apparently along
the line of the meandering jet of the North Atlantic Current. This pattern of two regions
of high chlorophyll, one zonal, the other diagonal across the North Atlantic, is repeated
Chapter 9: The Atlantic Ocean
Although the evidence is somewhat circumstantial, this appears to occur patchily and as
a response to local meteorological conditions that change from year to year, rather than
as the smooth poleward progression that might be inferred from mean conditions.
The general nutrient regime of the Atlantic has been discussed in Chapter 5 and, as
noted there, comparison with that of the North Pacific is profitable; the younger deep
water of the Atlantic and the very deep winter mixing cause nutrient concentrations at
standard depths to be lower than in the North Pacific. Moreover, it does not appear
necessary to invoke the smaller size of the North Atlantic basin, and hence a supposed
greater supply of continental Fe-bearing dust (as has been done) to explain its much
stronger spring bloom: the strikingly different stratification regimes of the two northern
oceans is a sufficient explanation.
Response of the Pelagic Ecosystems
The general seasonal pattern of phytoplankton growth responds to the succession of
deep winter mixing followed by vernal stratification, while the details of the sea-surface
chlorophyll field are determined by the distribution of eddy kinetic energy and local
short-term changes in irradiance and wind stress. It has been assumed in the past that
there is a simple northward progression of the spring bloom in the NADR, and that
sufficient stratification is also induced progressively northward. However (as is so often
the case), we find that reality is rather more complex, now that we have the tools for
satisfactory observation of the evolution of the chlorophyll field.
Formal analysis of oceanwide SeaWiFS images has been used to evaluate the date when
events in the chlorophyll record occur (Siegel et al., 2002). The date of the initiation of
the bloom at each pixel is taken to be when chlorophyll biomass increases 5% above
median conditions; this occurs in early February south of about 40
N and propagates
northward, so that at 50
N it occurs in early June. Further north still, initiation occurs
almost simultaneously right across the northern part of NADR and in SARC Province.
Computed critical depths (Z cr ) on the day of bloom initiation derived from this analysis
shoal slightly northward from ∼30 m at 40–45
N to ∼25 m at 55–60
N.
In any case, the seasonal development of phytoplankton biomass can now readily
be followed in SeaWiFS or MODIS 7-day and monthly sea surface chlorophyll images,
especially with the reprocessing No. 4 set, in which cloud masking is significantly reduced.
The seasonal evolution of MODIS surface chlorophyll in 2004 (see Color plate 5) is typical.
All January images show that surface chlorophyll is lower in NADR than to the south near
the Azores Front. February images suggest that chlorophyll enhancement in that month
is initiated by the increase of surface irradiance right across the ocean in the eddy field
associated with the subtropical frontal zone of the NAST province, at 35–40
N. In March,
the entire region up to 50–52
N shows evidence of accumulating chlorophyll that is
significantly enhanced—but not very much extended—during April. From May onward,
the bloom in NADR becomes continuous with those in the adjacent provinces, SARC to
the northeast and ARCT to the north. In late summer and autumn, the pattern continues
to evolve so that high surface chlorophyll values are increasingly biased northward with
southern areas becoming increasingly oligotrophic until October, when a minor increase
in surface chlorophyll once more appears across the ocean at 40–50
N, progressively to
weaken as winter approaches, wind-mixing deepens, and surface irradiance weakens.
The images confirm that patchiness does occur at all scales of chlorophyll enhancement,
although comparison of all available images for each month does suggest some recurrent
pattern. Two major linear regions of enhancement are seen in April 1999, one aligned with
the eddy field of the subtropical font in the NAST province, the other apparently along
the line of the meandering jet of the North Atlantic Current. This pattern of two regions
of high chlorophyll, one zonal, the other diagonal across the North Atlantic, is repeated