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Instabilities of an Eastern Boundary Current with and without Large-scale Flow Influence

Jinbo Wang, Ph.D., 2011
Paola Rizzoli & Michael Spall, Advisors

This thesis studies the stability of an eastern boundary current with and without the large-scale flow influence in an idealized framework represented by  barotropic quasi-geostrophic dynamics.

The linear stability analysis of a meridional current shows that meridional eastern and western boundary currents support a limited number of radiating modes with long meridional and zonal wavelengths and small growth rates. However, the linearly stable, long radiating modes of an eastern boundary current can become nonlinearly unstable by resonating with short trapped unstable modes, showing zonal jet structures in the interior.

The large-scale flow modifies the structure of the boundary current to stabilize or destabilize the unstable modes, leading to a meridionally localized maximum in the perturbation kinetic energy field. The shortwave mode is accelerated or decelerated by the meridional velocity adjustment of the large-scale flow to have an elongated or a squeezed meridional structure. The squeezed or elongated unstable mode detunes the nonlinear resonance with the longwave modes, which then become less energetic. These two modes show different meridional structures in kinetic energy field because of the different mechanisms.

These mechanisms are rather generic, and expected to remain relevant even in physically more complex systems.