Parameterizations of Salt Fingers in the Ocean

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The ultimate goal of this research is to advance our understanding of double diffusion from salt fingers in the ocean so that field observations can be properly interpreted and new representations of this phenomenon can be developed for inclusion in large-scale ocean circulation models. Salt fingering is an ocean mixing process that occurs in tropical and sub-tropical regions where warm and salty water overlies colder and fresher water. It is driven by the fact that, in water, heat diffuses faster than salt. A small parcel of warm, salty water sinking downwards into a colder, fresher region will lose its heat before losing its salt, making the parcel of water increasingly denser than the water around it and sinking further. Likewise, a small parcel of colder, fresher water will be displaced upwards and gain heat by diffusion from surrounding water, which will then make it lighter than the surrounding waters, and cause it to rise further. Locations where salt fingers are present may increase the ability of the oceans to absorb and transport heat, freshwater and nutrients than turbulent mixing in the abyss. Thus, it is very important to develop the tools necessary to quantify salt finger fluxes. The evidence that salt finger intensity is a strong function of density ratio and can feedback on the ocean's stratification is now quite abundant. This project aims to build on new theoretical results and high quality data sets to better quantify the mixing due to salt fingers in the ocean and hasten the progress toward a plausible representation in numerical models of the ocean circulation, which have a demonstrated sensitivity to double diffusion. This project will also contribute to the general knowledge of double-diffusive systems which are now recognized to be important in fields as diverse as astrophysics and magmas. The investigator plans to participate in a proposed multi-disciplinary workshop on "The Mathematics of Layers and Interfaces" in 2015. The project will also contribute to the educational development of undergraduate Summer Student Fellows at the Woods Hole Oceanographic Institution who will participate in the project as part of their summer research projects. Many of the excellent students in that program go on to become very successful graduate students.Analysis of finestructure, microstructure and tracer data will conducted along with theoretical analysis and numerical simulations in order to better understand the role of salt fingers in oceanic mixing. Evidence for the importance of salt fingers is now clear; it is the dominant thermocline mixing process in some regions and a significant contributor in many others. The fact that heat and salt are transported at different rates has a variety of consequences for thermohaline structure and ocean dynamics. There are four main elements to this project. First, a parameterization of salt fingers using microstructure data from the Salt Finger Tracer Release Experiment (SFTRE) as well as other cruises will be developed to test consistency with the new salt finger equilibration model. Second, the properties of the SFTRE thermohaline staircase will be analyzed using the Moored Profiler time series data. The analysis of layer properties in Theta-S space will be used to evaluate the viability of an existing flux divergence model. In addition, this analysis will help test the layer formation and thickness models which rely on a decreasing finger flux ratio with increasing density ratio. Third, a study of shear dispersion and lateral mixing in the staircase will be conducted using the tracer data in conjunction with the remarkably consistent Theta-S properties of the layers. This will locate any one layer observation within the overall meridional gradients of the region and permit the investigator to establish the lateral dispersion of the deliberate tracer. This analysis will allow the assessment of the displacement of layers relative to one another, and improve estimates of both vertical and lateral diffusivities. Fourth, an analysis of fingers in unstable stratifications will be done. Theory and numerical modeling will be used to help understand how a new parameter regime based on recent experimental results has the potential to affect "mixed" layer stratifications in the ocean. Overall, this project will contribute to improvement of the general circulation models used for climate prediction and ecosystem dynamics, both tasks of great importance to society.