In this study, we examine the nonlinear effect in the evolution of the Rossby wave propagating in the jet stream using a nonlinear barotropic model. We focus primarily on the formation of isolated eddy from the Rossby wave. The Rossby wave is induced by localized vorticity forcing in a zonally-varying basic flow. For weak, steady forcing we obtain the Rossby wavetrain trapped in the jet stream, as in the linear model. For strong, steady forcing it is occasionally seen that an isolated eddy with

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... negative vorticity is formed from some domain in the Rossby wavetrain, when the zonal asymmetry of the basic flow is large. For strong forcing of a finite period, a strong isolated eddy with negative vorticity can be formed from some domain when the zonal asymmetry of basic flow is large. This eddy breaks down to finally emanate Rossby wave downward.