of New Zealand (Bryden and Heath, 1985) are of
the right sign and sufficient magnitude to close the
heat budget if extrapolated around the circumpolar belt. But the reliability of such an extrapolation
is obviously open to question, given the length and
heterogeneity of the ACC. With regard to eddy
momentum fluxes, both the Drake Passage and
New Zealand measurements suggest the momentum flux carried by the lateral Reynolds stresses
is small relative to the wind stress. The primary
significance of eddies in the momentum budget of
the Southern Ocean lies in their ability to transfer
momentum downward across density surfaces,
rather than horizontally, as described below.
Over the last decade, our understanding of
the eddy field and its influence on the ACC
has improved as a result of several advances: satellite altimeter observations of the ACC as a whole,
a limited number of additional current meter
measurements, and numerical models capable of
resolving (or at least ‘permitting’) eddies.
Measurements of sea-surface height variability
from satellite altimeters has permitted the eddy
energy distribution around the entire ACC to be
mapped for the first time (Wunsch and Stammer,
1995). High eddy energy is found where the ACC
interacts with topography or with poleward extensions of the subtropical western boundary currents
(e.g. the Malvinas–Brazil Current Confluence).
Morrow et al. (1994) showed that the lateral
Reynolds stresses were generally small, but on
average tended to transfer momentum into the jets
of the ACC, accelerating the mean flow, although
more recent results suggest that the eddies act to
decelerate some of the strongest jets (Hughes and
Ash, 2001).
There have been only a few in-situ measurements of eddy fluxes in the ACC during WOCE.
South of Australia, an array of four tall current
meter moorings was maintained for 2 years (Phillips
and Rintoul, 2000). The array was deployed at
the Subantarctic Front along the WOCE SR3 line
(centred on Ϸ 50.7° S in Fig. 4.6.3), in a region that
altimetry suggests is one of moderate eddy activity,
with the eddy energy increasing rapidly downstream. Although the eddy heat flux varies across
the array, the mean values show poleward eddy
heat fluxes at all depths between 300 and 2500 m
that are significant at the 95% level. The eddy
heat fluxes are larger in magnitude than the two
previous such measurements, in Drake Passage
and southeast of New Zealand. If extrapolated to
the circumpolar belt, the eddy heat flux south of
Australia would carry 0.9 PW of heat poleward
(40-h to 90-day band-passed data, ‘poleward’
defined as normal to the direction of daily shear),
more than sufficient to balance the heat loss to the
atmosphere and the export of heat in the Ekman
layer. (Note that this estimate of the eddy heat
flux contains both the divergent, dynamically
active part of the eddy heat flux and the nondivergent part, see Marshall and Shutts (1981).)
The eddy heat flux scaled by the mean vertical
temperature gradient gives the vertical momentum
flux (e.g. Johnson and Bryden, 1989). South of
Australia, fluctuations in the ‘eddy band’ (40-h to
90-day periods) carry momentum downward at
a rate of about 0.2 N m
92 (2 dyne cm
92 ) at all
depths (i.e. at about the same rate as momentum is
supplied by the wind stress).
4.6.3 Dynamics of the ACC
The absence of continental barriers in the latitude
band of Drake Passage makes the dynamics of the
ACC distinctly different in character from those of
currents at other latitudes. At levels where no
topography exists to support zonal pressure gradients, there can be no mean meridional geostrophic
flow. The vertically integrated vorticity balance in
the Sverdrup approximation, which at least qualitatively succeeds in describing the wind-driven circulation in the interior of closed basins, cannot be
used to infer zonal flows in the zonally unbounded
Southern Ocean. Even the concept of a wind-driven
circulation in the Southern Ocean is inappropriate,
as the wind- and buoyancy-forced circulations are
inextricably linked. The unique dynamics of the
zonal and meridional circulation of the Southern
Ocean have attracted the attention of theoreticians
for many years. Recent work has led to substantial
progress in understanding the heat, momentum
and vorticity budgets of the ACC, although some
questions remain a source of controversy.
4.6.3.1 Sverdrup balance arguments applied
to the ACC
Sverdrup balance holds in the interior of the subtropical gyres because the wind-driven circulation
does not penetrate deep enough to interact with
bottom topography. In spin-up calculations, the
deep circulation is effectively cut off (in the absence
SECTION 4 THE GLOBAL FLOW FIELD
280
the right sign and sufficient magnitude to close the
heat budget if extrapolated around the circumpolar belt. But the reliability of such an extrapolation
is obviously open to question, given the length and
heterogeneity of the ACC. With regard to eddy
momentum fluxes, both the Drake Passage and
New Zealand measurements suggest the momentum flux carried by the lateral Reynolds stresses
is small relative to the wind stress. The primary
significance of eddies in the momentum budget of
the Southern Ocean lies in their ability to transfer
momentum downward across density surfaces,
rather than horizontally, as described below.
Over the last decade, our understanding of
the eddy field and its influence on the ACC
has improved as a result of several advances: satellite altimeter observations of the ACC as a whole,
a limited number of additional current meter
measurements, and numerical models capable of
resolving (or at least ‘permitting’) eddies.
Measurements of sea-surface height variability
from satellite altimeters has permitted the eddy
energy distribution around the entire ACC to be
mapped for the first time (Wunsch and Stammer,
1995). High eddy energy is found where the ACC
interacts with topography or with poleward extensions of the subtropical western boundary currents
(e.g. the Malvinas–Brazil Current Confluence).
Morrow et al. (1994) showed that the lateral
Reynolds stresses were generally small, but on
average tended to transfer momentum into the jets
of the ACC, accelerating the mean flow, although
more recent results suggest that the eddies act to
decelerate some of the strongest jets (Hughes and
Ash, 2001).
There have been only a few in-situ measurements of eddy fluxes in the ACC during WOCE.
South of Australia, an array of four tall current
meter moorings was maintained for 2 years (Phillips
and Rintoul, 2000). The array was deployed at
the Subantarctic Front along the WOCE SR3 line
(centred on Ϸ 50.7° S in Fig. 4.6.3), in a region that
altimetry suggests is one of moderate eddy activity,
with the eddy energy increasing rapidly downstream. Although the eddy heat flux varies across
the array, the mean values show poleward eddy
heat fluxes at all depths between 300 and 2500 m
that are significant at the 95% level. The eddy
heat fluxes are larger in magnitude than the two
previous such measurements, in Drake Passage
and southeast of New Zealand. If extrapolated to
the circumpolar belt, the eddy heat flux south of
Australia would carry 0.9 PW of heat poleward
(40-h to 90-day band-passed data, ‘poleward’
defined as normal to the direction of daily shear),
more than sufficient to balance the heat loss to the
atmosphere and the export of heat in the Ekman
layer. (Note that this estimate of the eddy heat
flux contains both the divergent, dynamically
active part of the eddy heat flux and the nondivergent part, see Marshall and Shutts (1981).)
The eddy heat flux scaled by the mean vertical
temperature gradient gives the vertical momentum
flux (e.g. Johnson and Bryden, 1989). South of
Australia, fluctuations in the ‘eddy band’ (40-h to
90-day periods) carry momentum downward at
a rate of about 0.2 N m
92 (2 dyne cm
92 ) at all
depths (i.e. at about the same rate as momentum is
supplied by the wind stress).
4.6.3 Dynamics of the ACC
The absence of continental barriers in the latitude
band of Drake Passage makes the dynamics of the
ACC distinctly different in character from those of
currents at other latitudes. At levels where no
topography exists to support zonal pressure gradients, there can be no mean meridional geostrophic
flow. The vertically integrated vorticity balance in
the Sverdrup approximation, which at least qualitatively succeeds in describing the wind-driven circulation in the interior of closed basins, cannot be
used to infer zonal flows in the zonally unbounded
Southern Ocean. Even the concept of a wind-driven
circulation in the Southern Ocean is inappropriate,
as the wind- and buoyancy-forced circulations are
inextricably linked. The unique dynamics of the
zonal and meridional circulation of the Southern
Ocean have attracted the attention of theoreticians
for many years. Recent work has led to substantial
progress in understanding the heat, momentum
and vorticity budgets of the ACC, although some
questions remain a source of controversy.
4.6.3.1 Sverdrup balance arguments applied
to the ACC
Sverdrup balance holds in the interior of the subtropical gyres because the wind-driven circulation
does not penetrate deep enough to interact with
bottom topography. In spin-up calculations, the
deep circulation is effectively cut off (in the absence
SECTION 4 THE GLOBAL FLOW FIELD
280