Abstract: The recent work of Cai and Huang¹ presents an entirely new view on the physical origin of Rossby waves: the presence of the latitudinal variation of the Coriolis parameter and/or a sloped bottom terrain serves as a mechanical barrier that causes a mass convergence (divergence) when a geostrophic fl ow crosses it uphill (downhill). Part of the mass convergence (divergence) causes pressure rising (falling) along the uphill (downhill) pathway while the remaining part is detoured as an unbalanced fl ow that crosses isobars. This mechanically excited unbalanced fl ow is subject to a “half-cycle” Coriolis force that only turns it to the direction parallel to isobars without continuing to turn it further back to its opposite direction because of the balance nature of the fl ow parallel to isobars. Such oscillation, involving a barrier-induced mass convergence, a mechanical deflection, and a halfcycle Coriolis deflection, is referred to as a mechanical-Coriolis oscillation with a “barrier-induced half-cycle Coriolis force” as its restoring force. Through a complete cycle of the mechanical-Coriolis oscillation, a new pattern of the geostrophic fl ow, which is in balance with a new pressure pattern, emerges to the left of the existing fl ow when facing the uphill (downhill) direction of the barrier in the Northern (Southern) Hemisphere. Due to the latitudinal variation of the Coriolis parameter, the β-barrier is always sloped towards the pole in both hemispheres, responsible for the westward propagation of Rossby waves.