Atlantic Trade Wind Biome
209
Synopsis
Case 5—Large-amplitude response to monsoon-like reversal of trade winds—integrated over
the entire province, Z m shows very weak distantly forced seasonality from 17 m in March–
May to 25 m in September–November, while Z eu is consistently deeper (40–50 m), so
that the thermocline is illuminated in all months. Seasonal changes in P rate are complex
but generally are inverse to changes in Z m , being highest from boreal summer to early
winter. Chlorophyll biomass closely tracks P rate change (Fig. 9.18). Herbivore biomass
is known to respond in a predictable manner to chlorophyll accumulation, confirming
the assumption that close coupling occurs between production and consumption.
South Atlantic Gyral Province (SATL)
Extent of the Province
The SATL comprises the anticyclonic circulation of the South Atlantic, excluding the
coastal boundary currents that are treated as separate provinces (see BRAZ, BENG). The
east and west boundaries of the province lie at the edge of the eddy fields associated with
these two currents: to the north, along the southern boundaries of ETRA and WTRA and
to the south, along the limit of the biological enhancement of the meandering Subtropical
Convergence Front that defines the SSTC province.
Defining Characteristics of Regional Oceanography
As will become clear, this is one of the least well-researched regions, far from the
major oceanographic research institutes and the troubles of the world. To the longhaul work of the South American institutes have been added in recent decades some
international initiatives, such as the CONFLUENCE program of Argentina, France, and
the United States of 1988–1990 (Anonymous, 1990), although these have mostly looked
at the margins of this province. We may also expect some assistance from the Atlantic
Meridional Section (AMT) project, worked by Antarctic supply vessels sailing between the
UK and Antarctica (e.g., Aiken and Bale, 2000) that pass down the western part of SATL.
WOCE, of course, contributed significantly to our general knowledge of the physics of
the entire South Atlantic, and these data are now readily available to all through Java
OceanAtlas. Integrated accounts of the circulation within this ocean basin go back to
the 1960s, although I have leaned heavily on the very accessible review of upper-level
circulation processes of Petersen and Stramma (1991). Of course, the characteristics of
the region that will be of interest to ecologists are dominated by the fact this is a major
anticyclonic subtropical gyre; consequently, the bowl-shaped isopleths for nutrients and
of other properties of interest slope upward toward the margins of the ocean.
The South Atlantic gyre is not entirely the homologue of that in the North Atlantic,
because of the geographical differences in the shapes of the two ocean basins. The
consequences of this are not trivial: the heat equator, and hence the ITCZ between the
northern and southern trade-wind systems, lies north of the true equator. Nor is the South
Atlantic fully enclosed to the east since Africa extends south only to 35
S. Perhaps most
importantly, because the Andes extend further both poleward and equatorward than do
the western mountains of North America, they form a more complete barrier to the flow
of the planetary westerlies: this extends almost 20
latitude further poleward than does
the tip of Africa.
For these reasons, the confluence between the westerlies and the trade winds across
the South Atlantic is not simple; the westerlies sweep up into the South Atlantic around
Cape Horn, to pass to the east across the ocean in the “roaring forties.” Thus, at all
seasons, westerlies dominate across the southern margin of the ocean from just north of
209
Synopsis
Case 5—Large-amplitude response to monsoon-like reversal of trade winds—integrated over
the entire province, Z m shows very weak distantly forced seasonality from 17 m in March–
May to 25 m in September–November, while Z eu is consistently deeper (40–50 m), so
that the thermocline is illuminated in all months. Seasonal changes in P rate are complex
but generally are inverse to changes in Z m , being highest from boreal summer to early
winter. Chlorophyll biomass closely tracks P rate change (Fig. 9.18). Herbivore biomass
is known to respond in a predictable manner to chlorophyll accumulation, confirming
the assumption that close coupling occurs between production and consumption.
South Atlantic Gyral Province (SATL)
Extent of the Province
The SATL comprises the anticyclonic circulation of the South Atlantic, excluding the
coastal boundary currents that are treated as separate provinces (see BRAZ, BENG). The
east and west boundaries of the province lie at the edge of the eddy fields associated with
these two currents: to the north, along the southern boundaries of ETRA and WTRA and
to the south, along the limit of the biological enhancement of the meandering Subtropical
Convergence Front that defines the SSTC province.
Defining Characteristics of Regional Oceanography
As will become clear, this is one of the least well-researched regions, far from the
major oceanographic research institutes and the troubles of the world. To the longhaul work of the South American institutes have been added in recent decades some
international initiatives, such as the CONFLUENCE program of Argentina, France, and
the United States of 1988–1990 (Anonymous, 1990), although these have mostly looked
at the margins of this province. We may also expect some assistance from the Atlantic
Meridional Section (AMT) project, worked by Antarctic supply vessels sailing between the
UK and Antarctica (e.g., Aiken and Bale, 2000) that pass down the western part of SATL.
WOCE, of course, contributed significantly to our general knowledge of the physics of
the entire South Atlantic, and these data are now readily available to all through Java
OceanAtlas. Integrated accounts of the circulation within this ocean basin go back to
the 1960s, although I have leaned heavily on the very accessible review of upper-level
circulation processes of Petersen and Stramma (1991). Of course, the characteristics of
the region that will be of interest to ecologists are dominated by the fact this is a major
anticyclonic subtropical gyre; consequently, the bowl-shaped isopleths for nutrients and
of other properties of interest slope upward toward the margins of the ocean.
The South Atlantic gyre is not entirely the homologue of that in the North Atlantic,
because of the geographical differences in the shapes of the two ocean basins. The
consequences of this are not trivial: the heat equator, and hence the ITCZ between the
northern and southern trade-wind systems, lies north of the true equator. Nor is the South
Atlantic fully enclosed to the east since Africa extends south only to 35
S. Perhaps most
importantly, because the Andes extend further both poleward and equatorward than do
the western mountains of North America, they form a more complete barrier to the flow
of the planetary westerlies: this extends almost 20
latitude further poleward than does
the tip of Africa.
For these reasons, the confluence between the westerlies and the trade winds across
the South Atlantic is not simple; the westerlies sweep up into the South Atlantic around
Cape Horn, to pass to the east across the ocean in the “roaring forties.” Thus, at all
seasons, westerlies dominate across the southern margin of the ocean from just north of