Mean flow stability in a model of the eastern North Pacific Ocean

Abstract

Mean flow and stratification data obtained from an eight-layer quasigeostrophic model of the California Current System are used to feed a linearized model, from which a stability problem is solved. Frequencies, phase speeds, group velocities, growth rates and vertical structures are obtained after solving a complex eigenvalue problem. Seven runs are implemented to study the role played by different vertical boundary conditions (flat and sloping bottom), mean flow direction and amplitude, latitude and frictional mechanisms on the Rossby wave characteristics. For values used of buoyancy frequency as found in the Northeastern Pacific, the vertical resolution is crucial in determining the effects of bottom topography and bottom friction on stability of the basic flow. The vertical structure of the mean flow has an important effect on the determination of the e-folding-time. The e-folding times of different areas in the California Current System region range from 144 to 374 days for the most unstable waves, as found by other authors using different data and models. The vertical structure of our wave solutions (amplitudes and phases) are noticeably affected by dissipation. The first baroclinic mode stable waves show a good qualitative and quantitative agreement with those obtained from hydrographic data for the California Current by Kang et al. (1982). The inclusion of bottom topography leads to a moderate redistribution of frequencies in the wavenumber space and to higher group velocities.