Low-frequency finite-amplitude oscillations in a near resonant topographically forced barotropic flow

Abstract

Abstract In this paper, an inviscid, barotropic, nondivergent potential vorticity equation with topography in a β-plane channel is used to investigate nonlinear interaction between a travelling Rossby wave in a uniform basic flow and a single-wave topography with the same zonal wavenumber. It is found that for the CDV-type monopole near resonance, when the uniform zonal basic westerly wind is near the resonant basic westerly wind, 30–60 day low-frequency oscillations (LFO) can arise from the wave-topography interaction. It has both travelling and standing components. In mid-latitudes (30°–45°N), westward-travelling 30–60 day LFO are dominated by Zonal Wavenumbers 1–3, while eastward-travelling 30–60 day LFO are dominated by Zonal Wavenumbers 1 and 3. In high latitudes (near 60°N), 30–60 day LEO are observed to travel westward, and are dominated by Zonal Wavenumber 1. However, for the LG-type (Legras and Ghil, 1985) dipole near resonance it requires a weaker, more realistic basic flow. When the uniform zonal basic westerly wind is near the resonant basic westerly wind, 30–60 day LFO can be excited by the wave-topography interaction. In the mid-bigh latitudes(30°–60°N), 30–60 day LFO are observed to slowly travel either eastward or westward, depending on the intensity of the near resonant westerly wind, and are dominated by Zonal Wavenumbers 1–3. This demonstrates that nonlinear wave-topography interaction appears to be able to induce low-frequency finite-amplitude oscillations with periods of 30–60 days in the Northern Hemisphere (NH) extratropics.