Calculating Thorpe Scales and Vertical
Mixing From CTD Data, With Application to Juan de Fuca Strait.
Kate Stansfield, Chris Garrett and Richard Dewey
Vertical mixing in the ocean can sometimes be quantified by
measurements of the Thorpe overturning scale, LT. In regions
of weak mixing and weak density gradients such measurements may
be limited by slow sensor response times (or sampling rates)
and/or by lack of resolution and noise in the density
measurements. On the other hand, the expected Thorpe scale can be
written as LT = (Int0inftyL2 P1(L)dL)1/2,
where P1(L) is the probability of the Thorpe displacement,
L. Data from Juan de Fuca Strait, British Columbia show that
even though the probability of a small Thorpe displacement is
much greater than that of a large Thorpe displacement, it is the
large and more easily resolved values of L that dominate the
Thorpe scale. We find that it is possible to determine LT
down to a scale of 0.4 m with a conventional
Conductivity-Temperature-Depth instrument. This corresponds to
values of Kv ~ 10-4 m2 s-1 in
summertime if LT ~ (epsilon/N3)1/2, as we
confirm using velocity and temperature microstructure data.
P1(L) is a convolution of the probability distribution of
overturn height, P2(H), with the probability distribution of
the fractional displacement within each overturn, P3(L/H).
Our data show that P2(H) is dominated by small overturns,
which is consistent with previous work on the thickness of
turbulence patches. Finally, the distribution of P3(L/H) is
examined and compared with the prediction of a very simple
kinematic model. The data show a pattern similar to that
predicted by the model, though with more small L/H, and fewer
medium to large L/H than in the model.
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