Unstable jet flow along zonal ridge topography Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/02870z08x

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  • Motivated by observations of locally enhanced sea level variability near steep topographic slopes in regions of strong time-mean flows, effects of zonal ridge topography on zonal oceanic jet flow are investigated. Unstable wave properties and the dynamics of eddy-mean flow interaction are evaluated as a function of topographic geometry using a 2-layer, quasigeostrophic, nonlinear, finite-difference, numerical model and two-dimensional linear stability analysis. During model spinup, the timing of the transition from nearly laminar to more turbulent flow and properties of developing linearly unstable waves vary with the height and width of the zonal ridge. Mean shear at this transition, and growth rate, phase speed, and wavelength of the most unstable mode depend primarily on topographic slope. The structure of the most unstable mode differs, however, for short and narrow ridges, as compared with tall, wide ridges. This shift affects properties and dynamics of the most unstable mode during the linear phase of wave growth. An evaluation of energy and momentum budgets during the finite-amplitude stage of wave development indicates that effects of topography on mean shear at the onset of turbulence and on the location of wave development control the time-evolution of eddy-mean flow interaction. Results from zonally uniform topography can be generalized to cases with along-stream variations in ridge geometry. The structure of the time-mean flow, distribution of time-dependent eddies and contribution of time-dependent eddies to the maintenance of time-mean energy and vorticity balances are considered as functions of along-stream variations in ridge height and width during the fully turbulent, statistically steady phase of numerical simulations. Along-stream variations in topography steer the time-mean jet, inducing along-stream modulations in ambient potential vorticity gradient at the location of the jet. As a result, the stability of the jet, the distribution of time-dependent eddies, and locations of significant eddy feedbacks on the time-mean flow vary along-stream. Topographic steering may therefore play a major role in the geographical distribution of sea level variability observed by altimetry.
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