that, instead, revealed the mesoscale eddy phenomenon (Crease, 1962; Swallow, 1971).
Advances in technology, principally the RAFOS
float (reverse of SOFAR; see Davis and Zenk,
Chapter 3.2), suggested to a number of investigators during the formulation of WOCE plans that it
was time to revisit the question of the deep interior
circulation. Certainly, given the numbers needed
to average out the eddy noise in the presence of
the expectedly weak mean flow, it was not feasible
to attack the global deep circulation problem.
Instead, it was decided to concentrate on a subbasin of the South Atlantic – the Brazil Basin –
where sufficient background information was
available to suggest that the ‘mean flow’ could be
measured. Prior knowledge included the Hogg
et al. (1982) computation of elevated mixing rates
(see Section 4.5.2.4), which demanded significant
upwelling and supported the Stommel notion that
meridional flows should be present. Based on computations of eddy kinetic energy from satellite
measurements of sea surface elevation, this basin
was expected to be relatively ‘quiet’ such that the
signal-to-noise ratio would be enhanced. The South
Atlantic is also a crossroads of water masses.
Below the thermocline there are three major ones:
the AAIW and AABW with southern sources are
split in the vertical by NADW from the north. A
final attribute of the Brazil Basin is that its geometry is relatively simple (Fig. 4.5.6). It is bounded
on all four sides by the coast or ridges through
which there are only a small number of passages
(four) that permit flow of the bottom water to, or
from, neighbouring basins.
The bulk heat and mass balances give basinaveraged estimates for diffusivity and fluxes across
constant property surfaces. As was pointed out by
Munk (1966), the mixing can be accomplished in a
variety of ways: intense mixing along boundaries
has become a popular alternative to Stommel’s
uniformly distributed hypothesis (e.g. Armi, 1978).
Hence, the hydrographic programme for the DBE
was designed to allow calculation of budgets
on scales small enough that one might be able
to discriminate between boundary and interior locations. In addition, a deliberate tracer release was
performed along with associated microstructure
measurements in order to observe directly the vertical mixing process at a particular location over
the rough topography of the Mid-Atlantic Ridge.
4.5.3.1 Bounding arrays and budgets and
mixing
Moored arrays were placed along the southern
boundary of the Brazil Basin, including the Vema
and Hunter Channels, and in the two equatorial
passages. As was described in Section 4.5.2, the
net inflow of AABW (potential temperatures less
than ϳ2°C) is 6.9<1.2 Sv while that flowing out
at the equator is 1.22<0.04 Sv to the east through
the Romanche-Chain complex and 2.0<0.2 Sv to
the west into the western North Atlantic. The net
inflow into the Basin is, therefore, 3.7<1.2 Sv,
which must upwell if a steady state is to be maintained. This is little different from the estimates
made by Hogg et al. (1982): Morris et al. (1997)
have used the DBE data set to redo the flux balance calculation: they found that the diffusivity is
approximately 3<0.8 cm
2 s
91 and has no significant variation with property type used for the
bounding surface.
An inert chemical, sulphur hexafluoride (SF 6 ),
was released on the western flank of the MidAtlantic Ridge near 20°S in an attempt to observe
directly and quantify the stirring of the tracer
across density surfaces (Ledwell et al., 2000, see
also Toole and McDougall, Chapter 5.2). Implied
diffusion rates vary from order 2–4 cm
2 s
91 over
the lower segments of the Mid-Atlantic Ridge
to order 10 cm
2 s
91 over the rougher segments.
St Laurent (1999) has developed a dynamically
consistent interpretation of the hydrographic,
microstructure and tracer data. Bottom-enhanced
mixing driven by the barotropic tide interacting
with the rough topography of the Mid-Atlantic
Ridge forces a secondary circulation over the various fracture zones. With large extrapolation it is
possible to account for most of the mixing and
upwelling that is implied by the net inflow to the
basin to be occurring over the rough topography
of the ridge flanks.
4.5.3.2 Lagrangian measurements
The upper water mass, AAIW, is discussed elsewhere (Davis and Zenk, Chapter 3.2; Hanawa and
Talley, Chapter 5.4), so here we will concentrate
on the two deepest water masses. With Stommel’s
scheme in mind, as well as that of Reid (1989), it
was expected that the signal (the mean flow)
would be less than 1 cm s
91 while the rms noise
from the mesoscale eddy field would be of the
4.5 Quantification of the Deep Circulation
267
Hogg
Advances in technology, principally the RAFOS
float (reverse of SOFAR; see Davis and Zenk,
Chapter 3.2), suggested to a number of investigators during the formulation of WOCE plans that it
was time to revisit the question of the deep interior
circulation. Certainly, given the numbers needed
to average out the eddy noise in the presence of
the expectedly weak mean flow, it was not feasible
to attack the global deep circulation problem.
Instead, it was decided to concentrate on a subbasin of the South Atlantic – the Brazil Basin –
where sufficient background information was
available to suggest that the ‘mean flow’ could be
measured. Prior knowledge included the Hogg
et al. (1982) computation of elevated mixing rates
(see Section 4.5.2.4), which demanded significant
upwelling and supported the Stommel notion that
meridional flows should be present. Based on computations of eddy kinetic energy from satellite
measurements of sea surface elevation, this basin
was expected to be relatively ‘quiet’ such that the
signal-to-noise ratio would be enhanced. The South
Atlantic is also a crossroads of water masses.
Below the thermocline there are three major ones:
the AAIW and AABW with southern sources are
split in the vertical by NADW from the north. A
final attribute of the Brazil Basin is that its geometry is relatively simple (Fig. 4.5.6). It is bounded
on all four sides by the coast or ridges through
which there are only a small number of passages
(four) that permit flow of the bottom water to, or
from, neighbouring basins.
The bulk heat and mass balances give basinaveraged estimates for diffusivity and fluxes across
constant property surfaces. As was pointed out by
Munk (1966), the mixing can be accomplished in a
variety of ways: intense mixing along boundaries
has become a popular alternative to Stommel’s
uniformly distributed hypothesis (e.g. Armi, 1978).
Hence, the hydrographic programme for the DBE
was designed to allow calculation of budgets
on scales small enough that one might be able
to discriminate between boundary and interior locations. In addition, a deliberate tracer release was
performed along with associated microstructure
measurements in order to observe directly the vertical mixing process at a particular location over
the rough topography of the Mid-Atlantic Ridge.
4.5.3.1 Bounding arrays and budgets and
mixing
Moored arrays were placed along the southern
boundary of the Brazil Basin, including the Vema
and Hunter Channels, and in the two equatorial
passages. As was described in Section 4.5.2, the
net inflow of AABW (potential temperatures less
than ϳ2°C) is 6.9<1.2 Sv while that flowing out
at the equator is 1.22<0.04 Sv to the east through
the Romanche-Chain complex and 2.0<0.2 Sv to
the west into the western North Atlantic. The net
inflow into the Basin is, therefore, 3.7<1.2 Sv,
which must upwell if a steady state is to be maintained. This is little different from the estimates
made by Hogg et al. (1982): Morris et al. (1997)
have used the DBE data set to redo the flux balance calculation: they found that the diffusivity is
approximately 3<0.8 cm
2 s
91 and has no significant variation with property type used for the
bounding surface.
An inert chemical, sulphur hexafluoride (SF 6 ),
was released on the western flank of the MidAtlantic Ridge near 20°S in an attempt to observe
directly and quantify the stirring of the tracer
across density surfaces (Ledwell et al., 2000, see
also Toole and McDougall, Chapter 5.2). Implied
diffusion rates vary from order 2–4 cm
2 s
91 over
the lower segments of the Mid-Atlantic Ridge
to order 10 cm
2 s
91 over the rougher segments.
St Laurent (1999) has developed a dynamically
consistent interpretation of the hydrographic,
microstructure and tracer data. Bottom-enhanced
mixing driven by the barotropic tide interacting
with the rough topography of the Mid-Atlantic
Ridge forces a secondary circulation over the various fracture zones. With large extrapolation it is
possible to account for most of the mixing and
upwelling that is implied by the net inflow to the
basin to be occurring over the rough topography
of the ridge flanks.
4.5.3.2 Lagrangian measurements
The upper water mass, AAIW, is discussed elsewhere (Davis and Zenk, Chapter 3.2; Hanawa and
Talley, Chapter 5.4), so here we will concentrate
on the two deepest water masses. With Stommel’s
scheme in mind, as well as that of Reid (1989), it
was expected that the signal (the mean flow)
would be less than 1 cm s
91 while the rms noise
from the mesoscale eddy field would be of the
4.5 Quantification of the Deep Circulation
267
Hogg