Resumen
A simple low-order nonlinear model with forcing and dissipation is used to demonstrate that inter-monthly oscillations may be caused by the nonlinear interaction. The length of the period depends on the intensity of the forcing. The model used for the integrations has three components. Another low-order model based on the barotropic vorticity equation with Newtonian forcing is used to show that similar oscillations appear also in this model. It is proposed that the models may describe the basic mechanism for the observed atmospheric intermonthly variations although the models are too simple to account for details in the spatial distributions of the variations.