For the stratified fluids, based on the quasi-geostrophic potential vorticity equation, an inhomogeneous modified Korteweg-de Vried (mKdV) equation including topographic forcing is derived by employing the perturbation method and stretching transforms of time and space. With inspection of the evolution of the amplitude of Rossby waves, it is found that Coridis effect, topography effect and Vaisala-Brunt frequency are the important factors, that induce the solitary Rossby wave, and it is induced even though the basic stream function has not a shear. Assuming that there is a balance between nonlinear and topographic effects, an inhomogeneous mKdV equation is derived, the results show that the topography and Rossby waves interact in the stratified flows. The inhomogeneous mKdV equation describing the evolution of the amplitude of solitary Rossby waves as a function of the change of Rossby parameter β(y) with latitude y, topographic forcing and the Vaisala-Brunt frequency is obtained.