Abstract: The present work using a rotating annulus with a thermally driven baroclinic fluid made a modeling investigation on the mechanism of low frequency oscillation in the index cycle of atmospheric circulation. By comparison of the experiments with and without topography, the role of topography in formation of the flow regimes is discussed. The topography used in the experiments consists of two ridges and two valleys around the annulus. Two sets of experiments have been done. One is the experiment with topography, and the other is the control experiment with no topography. The experiments show that the traveling waves strongly vacillate with time due to the topographic effects. Otherwise, in the experiments under the same imposed external conditions but with no topography, the baroclinic waves with wavenumber 6 would travel regularly with almost no vacillation. It was found that a prominent feature of experiments with topography is large amplitude vacillation of the waves with a period of approximately 127 annulus rotations, which is equivalent to approximatelly 26 days low frequency oscillation period in the Earth atmosphere. The role of topography in the experiment is to modulate the unstable baroclinic waves both in space and time. The reason that the vacillation exists in the experimental fluid is due to the forcing of the two sinusoidal wave-shape topography to the slowly moving wave flow.