In this paper, starting from the primitive equations, the authors derive the K-dV equation which describes the solitary Rossby waves in the tropical atmosphere, and indicate that since these waves are ageostrophic, they differ from the quasi-geostrophic solitary Rossby waves studied by Redekopp etc. Owing to the non-linear action,these waves are also different from the linear waves of the tropical atmosphere. The authors believe that the persistency of some tropical atmospheric waves reflects the characteristic of the solitary waves that energy does not disperse.