Abstract: Three numerical experiments have been made by using a spectral barotropic primitive equation model. It is found that the results obtained are quite similar to those with the barotropic filtered model. The main results read as follows.1. In the case of flat surface or symmetric orography, if the motion is symmetric (with respect to the equator; the same is true hereinafter) at the initial instant, then it would be symmetric forever.2. Antisymmetric orography distribution could generate antisymm motion. So the initial symmetric motion might become asymmetric afterwards owing to it.In order to explain the above results, it has been proved that they are the-oretical valid. And it is found that if the motion is antisymmetric initially,then it would become asymmetric. Therefore, no pure antisymmetric motion could be maintained.