Topographic Dynamics of the Gulf Stream

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This is a theoretical and numerical study of the subsurface Gulf Stream from the Charleston Bump to the Cape Hatteras region. The Gulf Stream is a central element of the North Atlantic circulation that has proven subtle to model in ocean general circulation settings. This work will solidify current understanding of Gulf Stream dynamics, particularly with regards to the robust separation from the coast at Cape Hatteras which remains a stubborn problem for numerical models. In addition, this study will isolate and define the locations in the vicinity of Cape Hatteras where the Gulf Stream comes out of geostrophic balance between the pressure gradients and forces due to the rotation of the Earth. The loss of energy from balanced flows to smaller scales, dissipation, and mixing, will also be quantified in these areas, an important step to closing the oceanic energy cycle. The identification of these locations can inform site selection for future observational studies of processes involved in loss of balance and non-hydrostatic phenomena. In addition to the usual methods for disseminating results by means of journal publications and presentations at national and international meetings, a graduate student will be trained in multi-scale computational oceanography and large and mesoscale ocean dynamics. Their Ph.D. research will be drawn from one of the topics outlined in this proposal. Also, a connection with the FSU Women in Mathematics, Science and Engineering (WIMSE) program will be engaged to recruit an undergraduate to work on the development of topographic datasets, and the development of analysis software. This will result in publications joint with the undergraduate in major oceanographic journals. A website will be created to host and maintain the modeling results and software, thereby making them available to the broader community for their use. The goal is to have all the numerical data available for download within six months of completing the experiments.The motivations for the study come from recent results emphasizing the dynamical importance of this region to the North Atlantic general circulation and open questions about Gulf Stream energetics, local topographic controls and mixing. The presented hypotheses are: (1) Gulf Stream -topography interaction promotes subsurface ''loss of balance'' and subsequently the generation of submesoscale phenomena, (2) the isobath convergence occurring at Cape Hatteras is a major control on Gulf Stream separation from the coast and (3) non-hydrostatic phenomena, including density inversions, are routinely generated in this area. This project will examine these hypotheses within the confines of a numerical and theoretical study of the Gulf Stream. The primary tools to be employed are regional embedded and downscaled numerical models, combined with analytical and process-oriented analyses. The goals of the project are to locate and describe the phenomena associated with regional loss of balance and submesoscale generation and provide quantitative estimates of the regional energy drain from the Gulf Stream to unbalanced, submesoscale motions.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.