Atlantic Trade Wind Biome
185
Atlantic; analysis of the surface wind field of the North Atlantic demonstrates that this
province corresponds to a zone of negative Ekman upwelling flux, of order −10 to
−20 × 10
−7 m sec
−1 . Both to the north and south of NATR, Ekman vertical flux takes
positive sign (McClain and Firestone, 1993) and there also the seasonal variability of
surface dynamic topography is greater. Finally, changes of sea surface temperature between
Niño and Niña conditions are significantly lower in the quiet NATR than in the more
active regions to north and south.
The critical characteristics of the province of interest to us are simple: in the western
part of the province, both sea-surface elevation and pycnocline depth take their maximum
values and, from here, the pycnocline slopes upward toward the edges of the province;
to the north, a relatively steep slope occurs at the STC, discussed for the NAST province,
whereas from May through December, a thermocline ridge lies along 10
N, trending
somewhat NE-SW and deepening to the west (Hastenrath and Merle, 1987). During
boreal winter, when the NECC become discontinuous west of 20
W, separation is difficult
between westward flow of the NEC and of the equator-crossing South Equatorial Current
(SEC), though the conjunction still occurs at about 10
N.
In NATR, mixed-layer depth varies seasonally, but not as a result of wind mixing
as in higher latitudes beyond the subtropical convergence; it is rather a response to
changes in the regional wind field, and hence to baroclinic adjustment and changes in
Ekman pumping. Essentially, here, pycnocline topography reflects transport so that a
deep thermocline trough lies southwest-northeast across the province at about 20–25
N
in most months, with greatest mixed-layer depths at 70–80 m in winter (December–May)
and rather shoaler (25–40 m) during the rest of the year.
The vertical displacement of the pycnocline carries with it the nutricline and, as
McClain et al. emphasize, there is a concurrent horizontal transport of nutrients peripherally around the subtropical gyre. It is to be noted that the location of the greatest
pycnocline depth, and hence the deepest nutricline, and the highest sea-level anomaly
differ by a significant distance, although both occur in the western part of the province.
The province is distinguished from the WTRA to the south by a zonal band of maximum
salinity (>37 ppt) that lies across the entire ocean, clearly differentiated across a salinity
gradient at 10–15
S from the less saline (<3625 ppt) equatorial water of the WTRA that
carries a seasonal Amazon signal (Dessier and Donguy, 1994).
The Cape Verde Islands (17–19
N) lie within the eastern limb of the anticyclonic gyre.
Their downstream eddies therefore modify the thermocline topography of the offshore
Canary Current as it passes to the southeast, away from the African continent (see NAST
Province for a discussion of this process at the Canaries).
Response of the Pelagic Ecosystems
There is little comprehensive information on the mixed-layer nitrate field, but values
appear to be uniformly low (02 M) at all seasons (Wroblewski, 1989; Glover and Brewer,
1988, ICITA Atlas). Higher values in wintertime (< 20 M) occur along the southern
boundary of the province at about 10
N, probably attributable to distant-field effects of
the zone of Ekman suction in the NECC of the adjacent WTRA (see later discussion).
Unfortunately, the repeated Atlantic Meridional Transect passes through the province at
its extreme eastern end and the “seasonal” signal for chlorophyll observed in AMT data,
and specifically attributed to this province, is probably induced by intermittent transit of
the survey ships across parcels of eutrophic surface water upwelled in the Canary Current.
The NATR has a consistently low and rather featureless surface chlorophyll field with
a seasonal cycle of very small magnitude and much irregularity, despite the significant
seasonal change in solar irradiance (500–800 W m
−2 in February and >1000 W m
−2 in
June). In fact, satellite chlorophyll, integrated over the entire province, shows greater
185
Atlantic; analysis of the surface wind field of the North Atlantic demonstrates that this
province corresponds to a zone of negative Ekman upwelling flux, of order −10 to
−20 × 10
−7 m sec
−1 . Both to the north and south of NATR, Ekman vertical flux takes
positive sign (McClain and Firestone, 1993) and there also the seasonal variability of
surface dynamic topography is greater. Finally, changes of sea surface temperature between
Niño and Niña conditions are significantly lower in the quiet NATR than in the more
active regions to north and south.
The critical characteristics of the province of interest to us are simple: in the western
part of the province, both sea-surface elevation and pycnocline depth take their maximum
values and, from here, the pycnocline slopes upward toward the edges of the province;
to the north, a relatively steep slope occurs at the STC, discussed for the NAST province,
whereas from May through December, a thermocline ridge lies along 10
N, trending
somewhat NE-SW and deepening to the west (Hastenrath and Merle, 1987). During
boreal winter, when the NECC become discontinuous west of 20
W, separation is difficult
between westward flow of the NEC and of the equator-crossing South Equatorial Current
(SEC), though the conjunction still occurs at about 10
N.
In NATR, mixed-layer depth varies seasonally, but not as a result of wind mixing
as in higher latitudes beyond the subtropical convergence; it is rather a response to
changes in the regional wind field, and hence to baroclinic adjustment and changes in
Ekman pumping. Essentially, here, pycnocline topography reflects transport so that a
deep thermocline trough lies southwest-northeast across the province at about 20–25
N
in most months, with greatest mixed-layer depths at 70–80 m in winter (December–May)
and rather shoaler (25–40 m) during the rest of the year.
The vertical displacement of the pycnocline carries with it the nutricline and, as
McClain et al. emphasize, there is a concurrent horizontal transport of nutrients peripherally around the subtropical gyre. It is to be noted that the location of the greatest
pycnocline depth, and hence the deepest nutricline, and the highest sea-level anomaly
differ by a significant distance, although both occur in the western part of the province.
The province is distinguished from the WTRA to the south by a zonal band of maximum
salinity (>37 ppt) that lies across the entire ocean, clearly differentiated across a salinity
gradient at 10–15
S from the less saline (<3625 ppt) equatorial water of the WTRA that
carries a seasonal Amazon signal (Dessier and Donguy, 1994).
The Cape Verde Islands (17–19
N) lie within the eastern limb of the anticyclonic gyre.
Their downstream eddies therefore modify the thermocline topography of the offshore
Canary Current as it passes to the southeast, away from the African continent (see NAST
Province for a discussion of this process at the Canaries).
Response of the Pelagic Ecosystems
There is little comprehensive information on the mixed-layer nitrate field, but values
appear to be uniformly low (02 M) at all seasons (Wroblewski, 1989; Glover and Brewer,
1988, ICITA Atlas). Higher values in wintertime (< 20 M) occur along the southern
boundary of the province at about 10
N, probably attributable to distant-field effects of
the zone of Ekman suction in the NECC of the adjacent WTRA (see later discussion).
Unfortunately, the repeated Atlantic Meridional Transect passes through the province at
its extreme eastern end and the “seasonal” signal for chlorophyll observed in AMT data,
and specifically attributed to this province, is probably induced by intermittent transit of
the survey ships across parcels of eutrophic surface water upwelled in the Canary Current.
The NATR has a consistently low and rather featureless surface chlorophyll field with
a seasonal cycle of very small magnitude and much irregularity, despite the significant
seasonal change in solar irradiance (500–800 W m
−2 in February and >1000 W m
−2 in
June). In fact, satellite chlorophyll, integrated over the entire province, shows greater