Nonlinear Resonance and Instability of Planetary Waves and Low-Frequency Variability in the Atmosphere.

Details Nonlinear Resonance and Instability of Planetary Waves and Low-Frequency Variability in the Atmosphere. Nonlinear Resonance and Instability of Planetary Waves and Low-Frequency Variability in the Atmosphere.

Nov 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993jats...50.3590w&link_type=abstract

Journal of Atmospheric Sciences, vol. 50, Issue 21, pp.3590-3607

Physics

6

Scientific paper

It is demonstrated in this work that linearly unstable planetary waves can be resonantly excited to finite amplitude in a nonlinear barotropic atmosphere with vorticity forcing and dissipation. In a weakly forced/dissipated atmosphere, it is shown that barotropic instability may provide a dynamic mechanism of transition between different `flow regimes.' It is suggested that nonlinear resonance and instability of planetary waves, which seen to be common with finite amplitude steady free solutions, may play a role in producing low- frequency variability observed in the real atmosphere.A type of nonlinear free solution with realistically sheared zonal flow is described in spherical geometry, and normal-mode instability analysis is performed to examine its stability. A time-dependent spectral model is employed to demonstrate the resonant excitation and breakdown of unstable planetary waves that results in low-frequency variability. The steady free solution is barotropically unstable to large-scale, low-frequency perturbations. Even though unstable, it can be resonantly excited from many initial conditions by a stationary vorticity forcing that projects onto it. The leading low-frequency oscillation has a period of about 20 days and the model flow hemispherically alternates between an almost zonal flow and a blocking regime, reminiscent of the multiple equilibria found by Charney and DeVore.

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