448
Chapter 12: The Southern Ocean
This characterization of regional production processes represents the situation along
170
W, a meridian that was selected specifically because here the individual fronts tend
to lie zonally and to be well separated meridionally. As far as can be managed, it
therefore represents an ideal situation from which we may reasonably extrapolate to other
regions. The complex evolution of differential nutrient relationships during the summer
growth season suggested by these JGOFS observations provides a more satisfactory basis
for further discussion in the sections that follow than was provided by the earlier,
simpler model of de Baar et al. that was conceived from a region having greater local
characterization.
A little to the east, down 150
W, several CPR tows have recently been reported
that demonstrate the partition of zooplankton species distributions in relation to the
zonal frontal structure of the Southern Ocean (Hunt and Hosie, 2005); the northern
Subantarctic Front was the strongest biogeographic boundary at the southern limits
of many subtropical and temperate zone species. Five zonally arranged species clusters
were identified by Bray-Curtis techniques, their distributions clearly determined by the
occurrence along the transect of the frontal zones discussed earlier.
Antarctic Westerly Winds Biome
South Subtropical Convergence Province (SSTC)
Extent of the Province
This province covers the entire Subtropical Convergence Zone (STCZ) that is defined
operationally by two fronts, or sharp gradients, in surface water properties that lie across
the southern side of the southern subtropical gyres of each ocean basin. The location of
these fronts is predictable in the general sense, but flow along them is meandering, so that
their instantaneous location can be observed usefully only in satellite imagery. Although
there is no coastline in this province, the STCZ does in some places pass close along the
edge of shallow shelf topography, as around the Falkland and Campbell Plateaus.
Defining Characteristics of Regional Oceanography
While heading south, George Deacon observed a steep temperature gradient at 40
S
“accompanied by a sharp decrease in salinity [that] provides clear evidence
of a sharp, though moveable and sometimes fragmented boundary between water of
subtropical origin to the north and subantarctic to the south.” I shall try to emphasize
throughout this chapter that although it is convenient (and sometimes useful) to portray
the annular fronts of the Southern Ocean as simple linear features, in reality they are, as
I have already noted, meandering and fragmented.
As Deacon remarked, the STCZ divides the anticyclonic circulations of the southern
Atlantic, Indian, and Pacific Oceans from the cyclonic circulation of the ACC and can
be traced almost continuously around the globe at about 35–45
S. It turns equatorward
when it encounters shallower water and poleward over deep water.
The STCZ occurs below a sharp decrease in the westerly winds of the Southern
Ocean and a permanent line of zero wind-stress curl, and is therefore the equatorward
limit of northward Ekman drift of the subantarctic water and of southward drift in
the subtropical gyre: strong convergence and downwelling consequently occurs here.
Sharp surface thermal gradients (14–18
C summer, 11–15
C winter) separate warm, salty
subtropical water from the colder, fresher water to the south. These are the NSTF and
SSTF of Belkin and Gordon (1996), usually separated by 4–5
in each ocean basin;
the NSTF is a shallow (200–300 m) front at the southern boundary of the warm, salty
Chapter 12: The Southern Ocean
This characterization of regional production processes represents the situation along
170
W, a meridian that was selected specifically because here the individual fronts tend
to lie zonally and to be well separated meridionally. As far as can be managed, it
therefore represents an ideal situation from which we may reasonably extrapolate to other
regions. The complex evolution of differential nutrient relationships during the summer
growth season suggested by these JGOFS observations provides a more satisfactory basis
for further discussion in the sections that follow than was provided by the earlier,
simpler model of de Baar et al. that was conceived from a region having greater local
characterization.
A little to the east, down 150
W, several CPR tows have recently been reported
that demonstrate the partition of zooplankton species distributions in relation to the
zonal frontal structure of the Southern Ocean (Hunt and Hosie, 2005); the northern
Subantarctic Front was the strongest biogeographic boundary at the southern limits
of many subtropical and temperate zone species. Five zonally arranged species clusters
were identified by Bray-Curtis techniques, their distributions clearly determined by the
occurrence along the transect of the frontal zones discussed earlier.
Antarctic Westerly Winds Biome
South Subtropical Convergence Province (SSTC)
Extent of the Province
This province covers the entire Subtropical Convergence Zone (STCZ) that is defined
operationally by two fronts, or sharp gradients, in surface water properties that lie across
the southern side of the southern subtropical gyres of each ocean basin. The location of
these fronts is predictable in the general sense, but flow along them is meandering, so that
their instantaneous location can be observed usefully only in satellite imagery. Although
there is no coastline in this province, the STCZ does in some places pass close along the
edge of shallow shelf topography, as around the Falkland and Campbell Plateaus.
Defining Characteristics of Regional Oceanography
While heading south, George Deacon observed a steep temperature gradient at 40
S
“accompanied by a sharp decrease in salinity [that] provides clear evidence
of a sharp, though moveable and sometimes fragmented boundary between water of
subtropical origin to the north and subantarctic to the south.” I shall try to emphasize
throughout this chapter that although it is convenient (and sometimes useful) to portray
the annular fronts of the Southern Ocean as simple linear features, in reality they are, as
I have already noted, meandering and fragmented.
As Deacon remarked, the STCZ divides the anticyclonic circulations of the southern
Atlantic, Indian, and Pacific Oceans from the cyclonic circulation of the ACC and can
be traced almost continuously around the globe at about 35–45
S. It turns equatorward
when it encounters shallower water and poleward over deep water.
The STCZ occurs below a sharp decrease in the westerly winds of the Southern
Ocean and a permanent line of zero wind-stress curl, and is therefore the equatorward
limit of northward Ekman drift of the subantarctic water and of southward drift in
the subtropical gyre: strong convergence and downwelling consequently occurs here.
Sharp surface thermal gradients (14–18
C summer, 11–15
C winter) separate warm, salty
subtropical water from the colder, fresher water to the south. These are the NSTF and
SSTF of Belkin and Gordon (1996), usually separated by 4–5
in each ocean basin;
the NSTF is a shallow (200–300 m) front at the southern boundary of the warm, salty