446
Chapter 12: The Southern Ocean
with the limit of winter ice cover. The STF and the PF are used here as the limits of a
Subantarctic Water Ring Province (SANT), integrating two entities: a Subantarctic Zone
(SAZ) and a Polar Frontal Zone (PFZ).
Because both the PF and, to a lesser extent, the SAF are associated with jetlike eastward
flow, each typically exists as a multiple front with active meandering and eddy shedding
(Read et al., 1995; Veth et al., 1997). In fact, one merit of the collective term Polar Frontal
Zone (PFZ) is that it recognizes the fact that—in the real ocean—PF and SAF often
merge. The steering of the ACC by deep topography in several regions induces the SAF
and PF to approach very close to each other and frequently to form a single dynamic
feature: this occurs most commonly north of the Falkland and Kerguelen Plateaus, near
32
E, and also in the vicinity of the mid-Pacific Rise (Orsi et al., 1995).
An additional deep front, the Southern ACC Front, occurs to the south of the PF
(Orsi et al., 1995). This does not separate distinct surface water masses, for it is expressed
only in the deeper density structure and will not much concern us here. Nevertheless,
you will frequently see references to the fact that the flow of the ACC is subdivided by
three, rather than the habitual two, circumpolar fronts.
Antarctic Region (ANTA and APLR). From the PF southward to the continent, there
is only a single superficial water mass, Antarctic Surface Water, and this is widely subject
to winter ice and the effects of melting within the marginal ice zone. Orsi et al. (1995)
suggest that this region is unitary, even though fronts do occur within it; nevertheless,
as was done in the first edition of this work, I continue to believe it useful to partition
the region into two provinces, of which one represents the Coastal biome. The principal
discontinuity in the Antarctic region is the Antarctic divergence (AD) that lies below the
change in direction of mean wind stress and marks the poleward limit of flow of ACC
water to the east and the limit of the Coastal biome. Less useful is the boundary of diluted
continental shelf water.
At Drake Passage, between Cape Horn and the South Shetland Islands, the West Wind
Drift is restricted to a narrow passage of only about 300 km compared with the 1200 km
between South Africa and the Antarctic continent. This requires that the annular zones
should squeeze together as they pass through the straits, although there is some loss
from the northern ACC into the southern part of the Humboldt Current. Nevertheless,
the zonal bands retain their relative positions, and only the STF itself is not identifiable
here. East of the Drake Passage, flow conforms to the continental margin and so sweeps
to the north along the edge of the Falkland Plateau before again passing eastward on
meeting the Brazil Current. Consequently, in the Atlantic sector of the Southern Ocean
the zonal boundaries take their most northerly position. Further yet to the east, the
relatively narrow passage south of Africa again somewhat constrains the northern parts
of the banded flow of the ACC.
From analytical accounts of the physical forcing of the banded flow of the ACC, it
is rather easy to obtain a false understanding of how this flow is actually manifest at
the surface of the ocean. The best way of visualizing this is to consult a collection of
satellite images of chlorophyll, which forms an excellent tracer for differential horizontal
motion of the near-surface water mass (see Color plate 20). Such images demonstrate that
textbook interpretations of the mean banded flow of the ACC are singularly misleading.
Although the frontal regions discussed earlier are prominent features in all images, they
exhibit far from an “ideal” pattern; linear but highly meandering and fragmented features
dominate the flow field and it is usually not immediately obvious whether a frontal jet
observed in this way represents the PF or the SAF, or both mingled—or even neither.
The certainty with which each front may be identified from an observing ship, properly
equipped, may suggest wrongly that these are simple linear features. It is also perhaps
significant that so much of the work at sea that has required frontal identification has
Chapter 12: The Southern Ocean
with the limit of winter ice cover. The STF and the PF are used here as the limits of a
Subantarctic Water Ring Province (SANT), integrating two entities: a Subantarctic Zone
(SAZ) and a Polar Frontal Zone (PFZ).
Because both the PF and, to a lesser extent, the SAF are associated with jetlike eastward
flow, each typically exists as a multiple front with active meandering and eddy shedding
(Read et al., 1995; Veth et al., 1997). In fact, one merit of the collective term Polar Frontal
Zone (PFZ) is that it recognizes the fact that—in the real ocean—PF and SAF often
merge. The steering of the ACC by deep topography in several regions induces the SAF
and PF to approach very close to each other and frequently to form a single dynamic
feature: this occurs most commonly north of the Falkland and Kerguelen Plateaus, near
32
E, and also in the vicinity of the mid-Pacific Rise (Orsi et al., 1995).
An additional deep front, the Southern ACC Front, occurs to the south of the PF
(Orsi et al., 1995). This does not separate distinct surface water masses, for it is expressed
only in the deeper density structure and will not much concern us here. Nevertheless,
you will frequently see references to the fact that the flow of the ACC is subdivided by
three, rather than the habitual two, circumpolar fronts.
Antarctic Region (ANTA and APLR). From the PF southward to the continent, there
is only a single superficial water mass, Antarctic Surface Water, and this is widely subject
to winter ice and the effects of melting within the marginal ice zone. Orsi et al. (1995)
suggest that this region is unitary, even though fronts do occur within it; nevertheless,
as was done in the first edition of this work, I continue to believe it useful to partition
the region into two provinces, of which one represents the Coastal biome. The principal
discontinuity in the Antarctic region is the Antarctic divergence (AD) that lies below the
change in direction of mean wind stress and marks the poleward limit of flow of ACC
water to the east and the limit of the Coastal biome. Less useful is the boundary of diluted
continental shelf water.
At Drake Passage, between Cape Horn and the South Shetland Islands, the West Wind
Drift is restricted to a narrow passage of only about 300 km compared with the 1200 km
between South Africa and the Antarctic continent. This requires that the annular zones
should squeeze together as they pass through the straits, although there is some loss
from the northern ACC into the southern part of the Humboldt Current. Nevertheless,
the zonal bands retain their relative positions, and only the STF itself is not identifiable
here. East of the Drake Passage, flow conforms to the continental margin and so sweeps
to the north along the edge of the Falkland Plateau before again passing eastward on
meeting the Brazil Current. Consequently, in the Atlantic sector of the Southern Ocean
the zonal boundaries take their most northerly position. Further yet to the east, the
relatively narrow passage south of Africa again somewhat constrains the northern parts
of the banded flow of the ACC.
From analytical accounts of the physical forcing of the banded flow of the ACC, it
is rather easy to obtain a false understanding of how this flow is actually manifest at
the surface of the ocean. The best way of visualizing this is to consult a collection of
satellite images of chlorophyll, which forms an excellent tracer for differential horizontal
motion of the near-surface water mass (see Color plate 20). Such images demonstrate that
textbook interpretations of the mean banded flow of the ACC are singularly misleading.
Although the frontal regions discussed earlier are prominent features in all images, they
exhibit far from an “ideal” pattern; linear but highly meandering and fragmented features
dominate the flow field and it is usually not immediately obvious whether a frontal jet
observed in this way represents the PF or the SAF, or both mingled—or even neither.
The certainty with which each front may be identified from an observing ship, properly
equipped, may suggest wrongly that these are simple linear features. It is also perhaps
significant that so much of the work at sea that has required frontal identification has