Instability and Turbulence in a Sheared, Diffusively Unstable Fluid

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0453140Mixing, transport and sequestration of heat in the Earth's oceans contributes a major element of uncertainty to our present understanding of the climate system. Double diffusive instability refers to a set of mixing processes that have no analogue in the atmosphere and whose existence was unsuspected before 1960. Driven by the vast difference in the molecular diffusivities of heat and salt in seawater, these processes contribute significantly to ocean mixing. In the simplest model, double diffusion takes one of two forms: salt fingering and diffusive layering, both of which form easily recognizable thermohaline staircase structures in many regions of the world ocean. Salt fingering is common in mid-latitude waters, while diffusive layering is more prominent near the poles. This theoretical study of shear / double diffusion interactions will illuminate mixing mechanisms active in the large regions of the ocean interior that are subject to both shear and double diffusive instability. The research will employ a combination of primary and secondary stability analyses and direct numerical simulations. These will focus on inflectional shear flows (with and without inertial rotation) and fingering-favorable stratification. Broader Impacts: Besides furnishing new physical insights into this oceanographically important regime of stratified turbulence, the work will result in improved parameterizations of diapycnal fluxes and anisotropy statistics for use both in the analysis of observational data and in large-scale modeling. The proposed work will benefit society by improving the representation of small-scale fluxes in large-scale ocean and climate models, thereby addressing one of the main obstacles to reliable climate forecasting. The project will provide partial support for a scientific programmer and full support for a graduate student, who will learn the latest techniques for the theoretical study of ocean mixing. The research will be done in collaboration with Professor Eric. Kunze of the University of Victoria, Canada.