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Ocean mixing has historically been estimated using Osborn's model by measuring the rate of dissipation of turbulent kinetic energy ∈ and the background density stratification N while assuming a value of the flux Richardson number Rif. A constant Rif 50:17 is typically assumed, despite mounting field, laboratory, and modeling evidence that Rif varies. This challenge can be overcome by estimating the turbulent diffusivity of heat KT using the Osborn-Cox model. This model, however, requires measuring the rate of dissipation of thermal variance χ, which has historically been challenging, particularly in energetic flows because the high wavenumbers of the temperature gradient spectra are unresolved with current technology. To overcome this difficulty, amethod is described that determines χ by spectral fitting to the inertial-convective (IC) subrange of the temperature gradient spectra. While this concept has been exploited for moored time series, particularly near the bottom boundary, it has yet to be adapted to vertical microstructure profilers such as gliders, and autonomous and shipbased vertical profilers from which there are the most measurements. By using the IC subrange, χ, and hence KT, can be estimated even in very energetic events-precisely the conditions requiring more field observations.During less energetic periods, the temperature gradient spectra can also be integrated to obtain χ. By combining these two techniques,microstrucure profiles at a field site known for its very energetic internal waves are analyzed. This study demonstrates that the spectral fitting approach resolves intensemixing events with KT≳10-2 m2 s-1. By equating the Osborn and Osborn-Cox models, indirect estimates for Rif can also be obtained.
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- 2 Finished
1/01/14 → 30/06/17