Abstract: The thermal forcings of annual and interannual periodic variations are introduced into the barotropic vorticity equation, by using low order spectral model of the equation,more than 40 numerical experiments whose integration time is larger than 100 model years are performed in order to investigate vanations of arge-scale flow patterns arised from both external interannual thermal forcing and internal dynamical processes. In certain parametric region, when the frequency of the forcing term with interannual period equals to the frequency which is created by the internal dynamical processes alone, the amplitude of interannual variations of flow patterns increases obviously, and the period becomes double. In other parametric region, amplitude jump of interannual variations of flow patterns and other nonlinear behavious can exhibit, along with gradual changes of interannual forcing parameters.