Some Aspects of Turbulence Role in Oceanic Currents - Selected Topics of Computational and Experimental Fluid Mechanics - page 177

ˁ 0 ∂ˁ

g

N 2

=−

z ,

(1)

∂

ˁ 0 is a reference density and ∂ ∂ z is the
vertical density gradient. In order to break a stable stratification or mix water masses,
it is required to input some energy into the system. In the absence of statically unstable
patches in thewater column, the energy comes from the vertical shear of the horizontal
velocity components as a shear instability mechanism as a source of turbulence. The
total vertical shear is defined as:

where g is the acceleration due to gravity,

∂

2 1 / 2

2 ∂

U
∂

V
∂

S = + . (2)

z z

N 2
S 2 , which is the
ratio of the oscillation characteristic time of a water parcel and the vertical shear
characteristic time. These two terms are constantly competing in the interior of the
ocean; Ri defines the onset of instability, which locally leads tomixing. Theoretically,
it could be shown that Ri

The ratio of them is known as Richardson number, Ri =

25 is the necessary condition for the vertical shear
characteristic time to overcome the Brunt-Väisälä oscillation characteristic time so
that turbulent mixing is expected to occur. This onset could be reached by different
ways. One of the mechanisms that has been proposed in later years is produced
by the interaction of Internal Gravity Waves, which are ubiquitous in the oceans
interior. The interaction between hydrodynamic processes (barotropic tides) and the
ocean floor topographic features such as sea mounts, ridges, submarine canyons,
etc., enhance turbulent mixing (Toole et al. 1997 ; Kunze et al. 2006 ). Internal wave
driven by mixing is the basis of the parametrizations that will be discussed in this
paper. In particular, “fine-structure parametrization” method is based on wave-wave
interactions.

< 0 .

2 In-Situ Measurements

The diapycnal mixing estimates are obtained based on several measurements. This
manuscript briefly explains “fine-structure”, “microstructure” and tracer release ex-
periments. All of them have shown that there is evidence of high mixing rates over
rough topography (Ledwell et al. 2000 ; Kunze et al. 2006 ). The two regions presented
in this manuscript are unique in the sense that they have experienced almost simul-
taneously all three methods. Figure 2 shows the regions where the measurements
were taken.
The temporal and spatial scales for both projects are quite different. However,
they were some of the first experiments used to compare and validate the existing
techniques to estimate diapycnal mixing rates. The next section will discuss the
different methods and the results of them will be addressed in the discussion.