Graduate Thesis Or Dissertation
 

Linear theory of roll instabilities in the ocean surface layer

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/vh53x110x

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  • This thesis describes the results of a linear stability analysis conducted on a simulated wind-forced ocean front. The ocean surface boundary layer regulates exchanges between the ocean and atmosphere, and the presence of turbulence in the surface layer can greatly impact climate and ocean life. Past research has shown that the combination of horizontal density gradients and wind stress can trigger turbulent roll instabilities in the boundary layer, but the underlying instability mechanisms are not well understood. In this thesis I assess the dominant instability mechanisms of roll instabilities from several Large Eddy Simulation (LES) numerical model solutions. I relate the numerical solutions to a simplified analytical model to draw conclusions about the underlying instability mechanisms. The results show that roll instabilities in the LES solutions convert energy from both ageostrophic and geostrophic shear of the mean flow. Both symmetric instability and Ekman inflection point instability contribute to roll growth, and in some cases, the combination of symmetric instability and Ekman instability form a mixed-mode instability with roll axis oriented obliquely to the front, consistent with the LES solutions. I conclude that wind-forced frontal zones are unstable to baroclinic, symmetric, and Ekman inflection point instabilities depending on the strength of the wind stress and the frontal temperature gradient. Future studies of wind-forced fronts should recognize the impact of Ekman inflection point instability on boundary layer energy dissipation and mixing, and emphasize its importance even in the presence of symmetrically unstable flow conditions.
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