Some Aspects of Turbulence Role in Oceanic Currents - Selected Topics of Computational and Experimental Fluid Mechanics - page 179

temperature, conductivity, accelerometers and pressure. The probes on the instru-
ments need to be in contact with unperturbed water, trying to be the least intrusive to
the shear layers in the water column. To achieve these conditions the instruments are
deployed and left in free-fall until they reach a terminal velocity. As they approach
the pre-programed depth, they release the weights that make them sink and the in-
strument comes back to the surface. To analyze the data it is required to make several
corrections to compensate the tilting and the rotation during the descent. From the
shear measurements it is possible to estimate dissipation from the total shear. The
buoyancy frequency is estimated from the measurements of temperature, conductiv-
ity (salinity) and pressure using the equation of state of seawater. This data sets are
direct measurements, for shear, the dissipation and diffusivity variables are obtained
from typical relations using the typical mixing efficiency values.

2.3 Tracer-Release Experiment

This experiment consists of releasing a tracer patch at a given depth. The tracers need
to be inexistent in the environment they will be released. Thus, they tend to be gases
that will dissolve in water at those high pressures. After the tracer injection, it spreads
vertically and horizontally, depending on the spatial scales of the experiment, and
the tracer is sampled over time scales of months or years. For example, the LADDER
experiment had a relatively small spatial scale and the tracer was sampled for months
after the injection. On the contrary, the DIMES experiment is quite large and it has
taken over 5years to sample the tracer. Together with sampling the tracer there have
been a lot of other instruments and research teams, making of this experiment one of
the largest in the world. For a detailed description on the estimation of the turbulent
diffusivity from the tracer injection, we refer to the appendix in Watson et al. ( 2013 ).

3 Observations and Results

The oceanic expeditions presented in Fig. 2 highlighted the importance of rough
topography as “hotspots” for high levels of mixing. This high levels are crucial to
mix the deep waters. For the EPR case, the location of the sea mounts and the ridge
for the EPR determine the spatial patterns of the turbulence (Fig. 3 ), which are of
the order of

s 2 in between the seamounts and below the crests of the
ridge. That value is consistent with the tracer release experiment carried out a year
before the fine-structure/microstructure cruise (Jackson et al. 2010 ). However, it is
important to notice that K

10 − 4 m 2 ∼ /

N 2

, therefore the smaller is the stratification,
N 2 , the larger are the values of K . However, if the stratification is high (high strain
values) the onset of instability, based on Richardson number and leading to turbulent
mixing, requires higher shear levels.

∼ O ( 1 / )