Brazil Basin Tracer Release Experiment
Brazil Basin Tracer Release Experiment - Overview
The
purpose of the Brazil Basin Tracer Release Experiment is to measure
diapycnal (across isopycnal) mixing and epipycnal (along-isopycnal)
mixing and stirring in the deep ocean. The experiment is funded by the
U.S. National Science Foundation, Division of Ocean Sciences. In early
1996, 110 kg of sulfur hexafluoride (SF6) were released on an isopycnal
surface near 4000 meters depth in the eastern part of the basin on the
flanks of the Mid-Atlantic Ridge (MAR). The location of the release was
near 21.7 S, 18.4 W. The release site was over a zonal valley that
leads to the MAR and is about 5000 m deep. The isopycnal surface of the
release was defined as the surface on which the potential density
anomaly, referenced to 4000 dbar pressure, was 45.9408 kg/m3. The
release streaks and results of initial sampling in 1996 are described
in Polzin et al. [1997].
Surveys of the tracer patch were made in 1997 and 1998, with the results
summarized in Ledwell et al. [2000]. The tracer was found to mix with
a diapycnal diffusivity of 3 cm2/s near the target density surface,
and with an epipycnal diffusivity of 100 m2/s. The diapycnal diffusivity
appears to increase toward the bottom. There also appears to be a
flow of about 1 cm/s toward the east in the region beneath the target
isopycnal surface. This deep eastward flow had carried about 30% of
the tracer east of the release site as of 1998, while the rest of
the tracer was in a patch that had drifted west and a little south
at a mean speed of about 0.3 cm/s. Tracer concentrations in the east
tend to increase with depth until a thick layer of uniform tracer
and weak stratification is found in the deep water. This layer can
be hundreds of meters thick in the valleys and deep depressions. The
concentration profiles in the west tend to be more Gaussian in shape,
and centered near the target density surface.
Click here to enlarge
1997 Section from Tracer Gulch
Measurements were made of the dissipation rate of turbulent kinetic
energy and temperature variance with the High Resolution Profiler
in 1996 and 1997. Results from this instrument are reported in Polzin
et al. [1997] and in Ledwell et al. [2000]. They indicate mixing which
is enhanced at all depths in the eastern part of the basin where the
bathymetry is rough and which increases strongly with depth. The diffusivities,
adjusted for certain sampling biases, agreed with the tracer result
at the target density surface, and increase to around 10 cm2/s near
the tops of the bathymetric ridges. Over smooth bathymetry that characterizes
the western half of the basin the diffusivity inferred from the dissipation
measurements is very low, on the order of 0.1 cm2/s. The hypothesis
for this pattern of mixing is that the tides running over the rough
bathymetry generate internal waves which propagate upward into the
water column where the associated shear intensifies, inducing turbulent
mixing events.
Overall, the mixing in the eastern part of the basin appears to be
strong enough to provide the heat flux necessary to balance the heat
budget for the abyssal water of the Brazil Basin. Hogg et al. [1982]
and Morris et al. [1997] infer from the net rate of Antarctic Bottom
Water entering the basin in various temperature and density classes
that the diapycnal diffusivity averaged over the whole basin is 2
to 4 cm2/s. The tracer release experiment has found values this large
at the target density surface in the eastern part of the basin. The
dissipation data suggest that the lower diffusivity in the western
part may well be compensated by higher values of diffusivity and of
heat flux where the isopycnal surfaces come near the bottom in the
east.
© Woods Hole Oceanographic Institution
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