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.
Identifying the routes to vertical exchange of heat across the surface mixed layer in the Pacific equatorial ocean is crucial to the understanding and prediction of events such as El Nino and La Nina that greatly affect the Americas. Field observations at the site in the past three decades indicate that turbulence enhances mixing of heat and momentum in the upper 100 m of the ocean especially during night time with the turbulence intensity several orders of magnitude larger than the average. The turbulence events cool the surface mixed layer by entraining the cold water from the ocean interior into the surface layer. Although many hypotheses have been suggested, a definitive understanding of the mechanisms leading to the mixing is lacking. Consequently, accurate turbulence parameterizations necessary for quantitatively accurate predictions of El Nino and La Nina have been elusive. We have performed numerical simulations with unprecedented resolution that enable us to track the evolution of turbulence events, link them to meteorological forcing and background ocean conditions, and thus provide a detailed picture of the life-cycle of turbulent mixing. The flow is typically in a marginal state of instability determined by an interplay of large-scale currents and density stratification. We find that a combination of wind forcing and the diurnal cycle of heat flux results in shear instabilities that drives the flow away from marginal instability towards turbulence. During day time, when solar heat flux warms the surface mixed layer, the near-surface stratification increases and the wind momentum is trapped in the surface layer increasing the near-surface shear. In the late afternoon as the heat flux is reduced, the near-surface stratification is reduced nudging the state towards instability and shear instabilities develop into turbulence. The turbulence further generates a downward momentum flux which is manifested as a downward propagating burst of shear that accelerates the current below the surface mixed layer. The acceleration tips the state of instability in this region, generates shear instabilities and turbulence well below the mixed layer. The process repeats itself over night time until the upper 100 m of the equatorial ocean becomes turbulent and mixed with cold fluid from the ocean interior. The project fostered discussions and collaborations among researchers coming from different backgrounds and institutions. Specifically, specialists in fluid mechanics from the Department of Mechanical and Aerospace Engineering interacted with oceanographers from Scripps Institution of Oceanography and Oregon State University to advance the research. A postdoctoral researcher from engineering was also trained in ocean science over the tenure of the project thus adding to the expertise in computational ocean sciences.
