the zonal wind stress is balanced by bottom form
stress means that the Sverdrup balance is upset by
interactions with topography: meridional flows
driven by the wind stress curl are returned in flows
balanced by bottom pressure torques rather than
in viscous boundary layers. In addition, for a significant part of the northward Ekman flux to
return in density layers that do not intersect topography at some latitude (as models and observations indicate), a strong, deep-reaching ACC is
needed to maintain the interfacial form stress
divergence required for dynamical balance of the
return flow. Without strong top-to-bottom flow,
the ACC could not achieve a dynamical state that
is almost non-viscous, supercritical with respect
to Rossby wave propagation and meridionally
constrained in narrow fronts.
It has long been recognized that eddy fluxes
play an important part in the dynamics and thermodynamics of the Southern Ocean. In the last
decade the central role of eddies has become even
more clear. Eddies carry heat poleward and
momentum downward across density surfaces; the
momentum transfer helps establish the correlation
between pressure and topography that provides
the bottom form stress to balance the wind; and a
substantial fraction of the meridional overturning
circulation is dynamically balanced by the divergence of interfacial form stress. In this sense, the
dynamical analogy between the ACC system and
the mid-latitude troposphere is even more complete than previously appreciated.
The high-quality WOCE hydrographic and
tracer sections have provided new insights into the
formation and circulation of Southern Ocean
water masses. Buoyancy exchange with the atmosphere drives substantial water mass transformations, converting both light water to dense water
(e.g. over the extensions of the subtropical western
boundary currents, and near the Antarctic margin)
and dense to light (over much of the Southern
Ocean, where northward Ekman transport combines with heat and freshwater input to convert
deep water to intermediate water). Tracers have
been used to refine estimates of the rate of Antarctic
Bottom Water formation, to identify the main circulation pathways, and to quantify the ‘ventilation
age’ of Southern Ocean water masses.
Many of these ideas have come together to provide a new appreciation of the significance of the
Southern Ocean in the global climate system. The
ACC provides the interbasin connection required
for a global thermohaline circulation to exist. Perhaps a more important link between the Southern
Ocean and the global overturning circulation is the
transformation of water masses driven by air–sea
forcing and diapycnal mixing. Various lines of evidence suggest that diapycnal mixing rates in the
main thermocline are too slow to support the traditional view that sinking of deep water is balanced by upwelling uniformly distributed over the
ocean. Much of the conversion of cold to warm
water that is required to close the NADW overturning circulation appears to take place in the
Southern Ocean, where deep water outcrops and is
exposed to air–sea buoyancy forcing. The NADW
cell, however, is overwhelmed in the zonal integral by an even stronger deep overturning involving conversion of upper deep water to denser
lower deep and bottom water in the Southern
Ocean, and conversion of lower to upper deep
water in the deep basins of the Indian and Pacific
Oceans.
Analyis of WOCE data from the Southern
Ocean is at an early stage. We anticipate further
progress will be made as the full suite of observations collected during WOCE (e.g. hydrography
and tracers, floats, altimetry, moorings) are synthesized. Advances in theory and modelling of the
ACC also continue at a rapid rate. Among the
important open questions to be addressed by these
analyses and future observational programmes are:
What is the absolute transport of the ACC, and
how and why does it vary in time? How sensitive
are the water mass conversions taking place in the
Southern Ocean, and the overturning circulations
of which they are part, to changes in atmospheric
forcing? What are the relative contributions of
‘deep’ and ‘bottom’ waters produced along the
Antarctic margin to the ventilation of the deep
sea? How representative are the WOCE-era measurements? Can we detect and interpret changes
between measurements made during WOCE and
historical or future observations?
Many, but not all, of these questions will be
answered as analysis of the WOCE data set continues. However, while a major step forward, the
WOCE observations are still sparse in space and
time. In a region as remote as the Southern Ocean,
there will always be a strong reliance on remote
sensing and autonomous instruments. For example,
the development of profiling floats now provides
4.6 The Antarctic Circumpolar Current System
301
Rintoul, Hughes and Olbers
stress means that the Sverdrup balance is upset by
interactions with topography: meridional flows
driven by the wind stress curl are returned in flows
balanced by bottom pressure torques rather than
in viscous boundary layers. In addition, for a significant part of the northward Ekman flux to
return in density layers that do not intersect topography at some latitude (as models and observations indicate), a strong, deep-reaching ACC is
needed to maintain the interfacial form stress
divergence required for dynamical balance of the
return flow. Without strong top-to-bottom flow,
the ACC could not achieve a dynamical state that
is almost non-viscous, supercritical with respect
to Rossby wave propagation and meridionally
constrained in narrow fronts.
It has long been recognized that eddy fluxes
play an important part in the dynamics and thermodynamics of the Southern Ocean. In the last
decade the central role of eddies has become even
more clear. Eddies carry heat poleward and
momentum downward across density surfaces; the
momentum transfer helps establish the correlation
between pressure and topography that provides
the bottom form stress to balance the wind; and a
substantial fraction of the meridional overturning
circulation is dynamically balanced by the divergence of interfacial form stress. In this sense, the
dynamical analogy between the ACC system and
the mid-latitude troposphere is even more complete than previously appreciated.
The high-quality WOCE hydrographic and
tracer sections have provided new insights into the
formation and circulation of Southern Ocean
water masses. Buoyancy exchange with the atmosphere drives substantial water mass transformations, converting both light water to dense water
(e.g. over the extensions of the subtropical western
boundary currents, and near the Antarctic margin)
and dense to light (over much of the Southern
Ocean, where northward Ekman transport combines with heat and freshwater input to convert
deep water to intermediate water). Tracers have
been used to refine estimates of the rate of Antarctic
Bottom Water formation, to identify the main circulation pathways, and to quantify the ‘ventilation
age’ of Southern Ocean water masses.
Many of these ideas have come together to provide a new appreciation of the significance of the
Southern Ocean in the global climate system. The
ACC provides the interbasin connection required
for a global thermohaline circulation to exist. Perhaps a more important link between the Southern
Ocean and the global overturning circulation is the
transformation of water masses driven by air–sea
forcing and diapycnal mixing. Various lines of evidence suggest that diapycnal mixing rates in the
main thermocline are too slow to support the traditional view that sinking of deep water is balanced by upwelling uniformly distributed over the
ocean. Much of the conversion of cold to warm
water that is required to close the NADW overturning circulation appears to take place in the
Southern Ocean, where deep water outcrops and is
exposed to air–sea buoyancy forcing. The NADW
cell, however, is overwhelmed in the zonal integral by an even stronger deep overturning involving conversion of upper deep water to denser
lower deep and bottom water in the Southern
Ocean, and conversion of lower to upper deep
water in the deep basins of the Indian and Pacific
Oceans.
Analyis of WOCE data from the Southern
Ocean is at an early stage. We anticipate further
progress will be made as the full suite of observations collected during WOCE (e.g. hydrography
and tracers, floats, altimetry, moorings) are synthesized. Advances in theory and modelling of the
ACC also continue at a rapid rate. Among the
important open questions to be addressed by these
analyses and future observational programmes are:
What is the absolute transport of the ACC, and
how and why does it vary in time? How sensitive
are the water mass conversions taking place in the
Southern Ocean, and the overturning circulations
of which they are part, to changes in atmospheric
forcing? What are the relative contributions of
‘deep’ and ‘bottom’ waters produced along the
Antarctic margin to the ventilation of the deep
sea? How representative are the WOCE-era measurements? Can we detect and interpret changes
between measurements made during WOCE and
historical or future observations?
Many, but not all, of these questions will be
answered as analysis of the WOCE data set continues. However, while a major step forward, the
WOCE observations are still sparse in space and
time. In a region as remote as the Southern Ocean,
there will always be a strong reliance on remote
sensing and autonomous instruments. For example,
the development of profiling floats now provides
4.6 The Antarctic Circumpolar Current System
301
Rintoul, Hughes and Olbers