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Chapter 9: The Atlantic Ocean
The Falkland Current carries subantarctic water northward along the continental slope
from the latitude of Cape Horn, first around Birdwood Bank and then the Falkland
Plateau along the 1000-m isobath. Flow continues north along the continental slope to
the confluence with the Brazil Current at about 40
S (Peterson and Whitworth, 1989).
Cold-core eddies are shed, particularly across the Falkland plateau, and the more saline
shelf water originating in the Brazil Current is thereby modified. Eddy generation is
also especially active at the confluence zone, in the general latitude of Bahia Blanca,
because here the Falkland Current makes an abrupt cyclonic loop and returns toward
the southeast, alongside the seaward flow from the Brazil Current. This confluence then
continues eastward across the ocean as the Subtropical Convergence and stands out in
satellite imagery as a region rich in eddies.
The same interaction occurs here between tidal mixing and the seasonal cycle of
mixing and stratification that has been investigated principally in the NECS province.
The neritic area is mixed year-round by tidal stress, whereas deeper water over the shelf
is mixed in winter and stratified rapidly in spring as buoyancy is accumulated within
the upper 30 m of the water column. Because the shelf has so little relief, the tidal front
separating the mixed from stratified water in summer runs parallel to the coast along the
40-m isobath and clearly separates coastal from outer shelf water. The water column is
almost everywhere mixed down to the sediments by seasonal wind stress during winter
when the warmest and least dense water occurs near the coast, and a moderately steep
shelf-break front forms between the 80-m and 100-m isobaths, separating shelf from
Falkland Current water. Stratification is induced in spring and summer in the classical
manner, by increased insolation and decreased wind stress together with some advective
transport into the region (Piccolo, 1998); only inshore, in the macrotidal regions, does
stratification not occur. On the southern part of the shelf, just north of the Falklands,
a summer thermocline at 50 m induces density stratification in a water column that is
essentially isohaline: salinity ranges across the shelf from 32‰ near the Tierra del Fuego
coast to about 34‰ at the shelf edge north of the Falklands. In the extreme south, beyond
52
S, summer stratification is rarely observed.
The presence of the subantarctic water of the Falkland Current along the shelf edge is
very significant for the distribution of productivity in this province. Shelf-edge upwelling
brings nutrient-rich water either to the surface or, alternatively, causes it to pass westward
below the thermocline so that, with the breakdown of stratification in the autumn,
nutrients the newly mixed shelf water are replenished with nutrients at deep ocean
concentration.
Response of the Pelagic Ecosystems
Though we have little direct information from regional studies, there is no reason to
believe that the description of processes associated with midlatitude continental shelves
having active tidal frontal regions (e.g., NECS and NWCS provinces) is not also be
generally relevant to here.
It is to the serial chlorophyll images that we must turn to deduce a characteristic
climatology of productivity in this province and, once again, they do not fail us. Clearly
revealed are the effects of the mixing and stratification regime described earlier and, just
as clearly, the consequences of the shelf-edge flow of subantarctic water in the Falkland
Current. In midwinter, the entire shelf region has <1 mg chl m
−3 , weakest in the south
off Tierra del Fuego, but this is, nevertheless, higher than the offshore oceanic regions;
this value begins to increase generally, and overall, as early in the sun-year as the middle
of July. However, the first signs of major blooms occur in the north, off Bahia Blanca
(38−40
S) along the shelf break in mid-September. This linear bloom rapidly extends
southward until by early October it has reached the edge of the Falkland Plateau and has
Chapter 9: The Atlantic Ocean
The Falkland Current carries subantarctic water northward along the continental slope
from the latitude of Cape Horn, first around Birdwood Bank and then the Falkland
Plateau along the 1000-m isobath. Flow continues north along the continental slope to
the confluence with the Brazil Current at about 40
S (Peterson and Whitworth, 1989).
Cold-core eddies are shed, particularly across the Falkland plateau, and the more saline
shelf water originating in the Brazil Current is thereby modified. Eddy generation is
also especially active at the confluence zone, in the general latitude of Bahia Blanca,
because here the Falkland Current makes an abrupt cyclonic loop and returns toward
the southeast, alongside the seaward flow from the Brazil Current. This confluence then
continues eastward across the ocean as the Subtropical Convergence and stands out in
satellite imagery as a region rich in eddies.
The same interaction occurs here between tidal mixing and the seasonal cycle of
mixing and stratification that has been investigated principally in the NECS province.
The neritic area is mixed year-round by tidal stress, whereas deeper water over the shelf
is mixed in winter and stratified rapidly in spring as buoyancy is accumulated within
the upper 30 m of the water column. Because the shelf has so little relief, the tidal front
separating the mixed from stratified water in summer runs parallel to the coast along the
40-m isobath and clearly separates coastal from outer shelf water. The water column is
almost everywhere mixed down to the sediments by seasonal wind stress during winter
when the warmest and least dense water occurs near the coast, and a moderately steep
shelf-break front forms between the 80-m and 100-m isobaths, separating shelf from
Falkland Current water. Stratification is induced in spring and summer in the classical
manner, by increased insolation and decreased wind stress together with some advective
transport into the region (Piccolo, 1998); only inshore, in the macrotidal regions, does
stratification not occur. On the southern part of the shelf, just north of the Falklands,
a summer thermocline at 50 m induces density stratification in a water column that is
essentially isohaline: salinity ranges across the shelf from 32‰ near the Tierra del Fuego
coast to about 34‰ at the shelf edge north of the Falklands. In the extreme south, beyond
52
S, summer stratification is rarely observed.
The presence of the subantarctic water of the Falkland Current along the shelf edge is
very significant for the distribution of productivity in this province. Shelf-edge upwelling
brings nutrient-rich water either to the surface or, alternatively, causes it to pass westward
below the thermocline so that, with the breakdown of stratification in the autumn,
nutrients the newly mixed shelf water are replenished with nutrients at deep ocean
concentration.
Response of the Pelagic Ecosystems
Though we have little direct information from regional studies, there is no reason to
believe that the description of processes associated with midlatitude continental shelves
having active tidal frontal regions (e.g., NECS and NWCS provinces) is not also be
generally relevant to here.
It is to the serial chlorophyll images that we must turn to deduce a characteristic
climatology of productivity in this province and, once again, they do not fail us. Clearly
revealed are the effects of the mixing and stratification regime described earlier and, just
as clearly, the consequences of the shelf-edge flow of subantarctic water in the Falkland
Current. In midwinter, the entire shelf region has <1 mg chl m
−3 , weakest in the south
off Tierra del Fuego, but this is, nevertheless, higher than the offshore oceanic regions;
this value begins to increase generally, and overall, as early in the sun-year as the middle
of July. However, the first signs of major blooms occur in the north, off Bahia Blanca
(38−40
S) along the shelf break in mid-September. This linear bloom rapidly extends
southward until by early October it has reached the edge of the Falkland Plateau and has