Microstructure signature of equilibrium double-diffusive convection
Caplan, Shelley D.
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Salt fingering is an oceanographic small-scale phenomenon that occurs in statically stable regions where relatively warm salty water lies above cold, fresh water. Currently, analyses and interpretation of most microstructure measurements and salt fingers in particular, are based upon the fundamental assumption that the microstructure is isotropic. While is it generally accepted that the isotropic assumption may lead to underestimates of the dissipation rates of thermal and velocity fluctuations by factors of two to four, no significant attempt has been made to take the anisotropy of microstructure into account yet. Thus, the anisotropy remains one of the key elements of uncertainty in the microstructure analysis and a major obstacle in quantifying the strength of vertical mixing in the ocean. This thesis represents an attempt to use direct numerical simulation in two and three-dimensions, in order to examine the validity of the isotropic assumption for the fully developed double-diffusive convection over a range of density ratios. Calculations are performed and the aspect ratio of the salt fingers is shown to be considerably different from unity. The anisotropy is particularly evident at higher density ratios. Based on the performed simulations, we formulate a simple method to take the anisotropy into account. The proposed technique is readily applicable to the oceanic data from free-falling and towed profilers. We expect that the reanalysis of microstructure with high density ratios would yield considerably different estimates of the diapycnal diffusivities and fluxes of heat and salt. With regard to Navy interests, it should be noted that double diffusion can exert a substantial influence on the propagation and dispersion of the acoustic signature particularly in the regions of the pronounced thermocline staircases. Whether a sound source is transmitted from inside or outside a thermohaline staircase, the acoustic propagation is affected, especially at high frequencies.
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