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Transcript
Vertical mixing in the deep ocean also controls as-
the length scales constraining 50% to 70% of the
pects of the horizontal circulation. Both wind-driv-
tidally driven mixing. Some features that gener-
en and tidal currents generate internal waves when
ate internal waves are too small to be visible from
they flow over abyssal topography. These waves
space, but they may be predicted statistically, pro-
subsequently mix the ocean vertically through
vided that bathymetry is resolved down to lengths
wave breaking or other mechanisms. Measurements
where the statistics of roughness may be extrapo-
of vertical mixing in the Brazil Basin indicate that
lated from fractal models (8 km).
mixing rates vary with geographic location and
depth, and the energetics of dissipation depend
There are still unknown circulation pathways in the
on fine-scale topography in the deep ocean (Figure
ocean, and the best bathymetry cannot yet deter-
6). Models that predict future climate will require
mine where critical mixing is happening. A factor
accurate bathymetry in order to predict spatially
of two increase in resolution and a factor of five
varying mixing rates.
increase in the signal-to-noise ratio, possible with
the proposed satellite mission, will permit many of
Unfortunately, the best global bathymetry that is
these currently unknown pathways to be mapped,
currently available does not resolve seafloor to-
and areas with topographically enhanced mixing to
pography at all length scales. Theoretical studies
be found.
suggest that bathymetric features as small as 1 km
may influence mixing. A new mission could map
Figure 5. Mid-ocean ridges constrain flow and
mixing in a way similar to the blocking and steering of winds by mountain-pass topography. The
Mid-Atlantic Ridge generally prevents exchange
between the basins on its east and west flanks.
In the South Atlantic, the deep water is higher in
oxygen on the western side, allowing oceanographers to observe that some flow does cross the
ridge where it is cross-cut by deep troughs associated with fracture zones. Rapid and climatically
important mixing takes place in such passages,
and so mapping their locations is an important
step in realistic climate modeling.
9