Horizontal dispersion of near-inertial oscillations in a turbulent mesoscale eddy field
We study the dispersion of wind-induced near-inertial oscillations (NIOs) in a fully turbulent baroclinic mesoscale eddy field characterized by a continuous wavenumber spectrum. The influence of the eddy field on the horizontal dispersion of the different NIO modes is analyzed using a vertical normal mode expansion. Previous studies have identified two dispersion regimes: trapping and strong dispersion. We examine the modes in physical and spectral space to assess which regime prevails. Numerical and analytical results show the prevalence of a trapping regime. For each NIO mode, there exists a critical horizontal wavenumber, kc, that separates large-scale NIO structures, where trapping dominates, from the much less energetic small-scale NIO structures, where strong dispersion dominates. The maximum efficiency of dispersion for scales close to kc concentrates NIO kinetic energy at these scales. The wavenumber kc results from a balance between refraction and dispersion. This balance first occurs at the highest wavenumber. Thereafter, kc, which has dimensional expression k2c = /(ftR2m), decreases with time at a rate inversely proportional to the radius of deformation, Rm, of the baroclinic NIO mode considered. As a consequence, at any given time, higher NIO baroclinic mode energy can mostly be found in small-scale negative vorticity structures, such as filaments near sharp vorticity fronts, whereas lower NIO mode energy is concentrated within the core of mesoscale anticyclonic vortices. For large times, a saturation mechanism stops the time-evolution of kc at a value close to the peak of the kinetic energy spectrum of the QG flow field.
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Document Type: Research Article
Publication date: September 1, 2001
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