Routes to vertical mixing in the Equatorial Under Current: quantification through high-resolution numerical simulations

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Intellectual Merit: Understanding and quantifying the vertical fluxes of heat and momentum in the tropical Pacific equatorial undercurrent (EUC) system is one of the central problems linking the small-scale physics of flow instabilities and turbulent mixing to larger scale climatic variability such as ENSO. Significant progress has been made in the last two decades, spurred primarily by simultaneous observations of mean and turbulent vertical profiles combined with classical linear stability analyses. Nevertheless, significant uncertainty remains and, in particular, the mechanisms by which a unique mix of high-frequency internal waves and coherent turbulence patches is initiated and sustained at depths where the background shear is linearly stable are only poorly understood. In this project, a high-resolution numerical investigation will be conducted to quantify the pathways through which energy sources - surface wind, buoyancy flux and mean kinetic energy of the jet - feed into turbulent dissipation. The improved modeling capability in this project, based primarily on high-resolution LES will complement on-going observational programs. These three-dimensional, non-hydrostatic simulations performed on large-scale NSF computational facilities will directly resolve the flow instabilities leading to turbulence and vertical fluxes. The analysis of the space-time dataset will focus on elucidating the finite amplitude, nonlinear mechanisms responsible for sustaining turbulence in regions where background conditions are linearly stable. The modeling will incorporate the new observations in the form of background profiles of shear and stratification with corresponding wind stress and heat flux. The principal objective of the effort will be to understand the physical mechanisms of turbulence generation and transport operative in the EUC under a variety of conditions and to quantify the associated energy transport and vertical fluxes.Broader Impacts: Scientifically, the proposed effort will contribute to the development of physically based parameterizations and predictive capabilities of broad societal interest. The project will directly support the training of one post-doctoral scholar of Vietnamese origin, an under-represented community in science. It will contribute to the development of computational and information technology infrastructure through collaboration with an existing NSF funded project on database design for computational fluid dynamics simulations. The project will also directly support graduate education at UCSD by incorporating analytical and numerical techniques and results into existing courses in environmental and computational fluid dynamics cross-listed between Mechanical and Aerospace Engineering and the Scripps Institution of Oceanography at UCSD. The cross-disciplinary nature of the PI team will ensure broad dissemination of the results among the geosciences and engineering communities.