Intraseasonal Oscillations in the Extratropics: Hopf Bifurcation and Topographic Instabilities

A potential vorticity model in a β-channel is used to analyze the resonant response of equivalent-barotropic flow to topography in the presence of a forced zonal jet with arbitrary meridional structure. The nonlinear dynamics near different resonances is studied considering both wave-wave and wave-zonal flow interactions. It is shown that Hopf bifurcations from stationary to periodic flows are possible due to the nonlinear instability of nonzonal, topographically forced flow. Low-frequency, finite-amplitude oscillations arise due to a combination of two factors: (i) nonlinear wave-wave interactions, which tend to reduce the Rossby wave frequency; and (ii) wave-zonal flow interactions, which reflect the importance of wave momentum transport in shifting the westerly jet and of the topographic form drag. The physical mechanism of atmospherically realistic Hopf bifurcations depends crucially on the meridional profile of the mean zonal flow giving rise to a dipole-shaped resonance. The bifurcation phenomena studied here might give some insight into the inherent dynamics of intraseasonal oscillations in the Northern Hemisphere extratropics.