The impact of nonlinear stability and instability on the validity of tangent linear model (TLM) is investigated by comparing its results with those produced by the nonlinear model (NLM) with the identical initialperturbations. The evolutions of different initial perturbations superposed on the nonlinearly stable andunstable basic flows are examined using the two-dimensional quasi-geostrophic models of doubleperiodic-boundary condition and rigid boundary condition. The results indicate that the valid time p...