This study is motivated on the observation that ocean mixed layer fronts are common features throughout the world ocean and are currently ignored in boundary layer representations. Circulations associated with frontal instabilities are capable of transporting material from the surface into the ocean interior and are likely a key element in the ocean general circulation. This research will yield quantitative estimates of these transport processes that can be used to improve parameterizations for ocean models used in climate studies. Participation by a graduate student for the duration of the project is planned and results will be published in recognized peer-reviewed journals. Elements of this investigation will be incorporated into Oregon State University graduate curriculum as well as in public learning efforts through a summer internship program involving high school students.
This research focuses on the properties and dynamics of coherent structures in the turbulent ocean boundary layer and on the conditions that support their development. This work will extend and complement previous studies that have examined or invoked Langmuir or symmetric instability processes to explain ocean boundary layer observations. It is hypothesized that the coherent, large-eddy structures observed in preliminary large-eddy simulation numerical experiments may be best understood as non-symmetric linear instabilities of a basic state that incorporates surface stress and buoyancy flux modification of a geostrophically balanced, horizontal density gradient. The first goal of this work will be specifically to address this hypothesis. The second, broader goal is, with the results of the first part as context, to use the large-eddy simulation to explore and characterize the dependence of ocean boundary layer turbulent processes, including especially the vertical fluxes and mixing associated with coherent structures, on the strength and character of mean horizontal gradients. A range of large-eddy simulations will be conducted to assess the relative influence of frontal gradients and surface forcing on coherent structure development and characteristics. Specific forcing parameters examined will include surface stress and buoyancy (heat) flux, surface waves (Langmuir circulation), and frontal (horizontal density or temperature gradient) strength, including where appropriate the derived Ekman buoyancy flux as a scaling parameter. The numerical simulations will be complemented by theoretical studies in the form of linear stability analysis for a generalized basic state that will provide a conceptual and quantitative framework for characterization of instabilities in the ocean boundary layer. Specific objectives include: 1) Development of a generalized linear instability theory for comparison with large-eddy simulations and testing of the initial hypothesis; 2) Characterization of the dependence of turbulent structures and flux scaling regimes on the imposed surface fluxes and mean horizontal gradient, using large-eddy simulations; 3) Physical interpretation and rationalization of the large-eddy simulation regimes in terms of the results of the generalized linear instability theory.
