Abstract: A filtered model for the tropical atmosphere is investigated. Starting from the equations describing the atmosphere motion in the tropics and using the semi-geostrophic and quasi-geostrophic models high frequency waves are filtered leaving low frequency Rossby waves. It is shown that the semi-geostrophic approximation can filter the high frequency waves to have the Rossby waves with k-0. The quasi-geostrophic approximation can not only filter the high frequency waves but also obtain undistorted Rossby waves.On the basis of filtering, the low frequency oscillation caused by the Rossby waves is analyzed.