| Understanding the dynamics of low-frequency variability in the atmosphere is crucial for the improvement of long-range climate prediction. In this study we investigate the internal dynamics of large-scale, extratropical, low-frequency variability using a global barotropic model with a zonally asymmetric basic state. A particular focus is on the simulation and analysis of the traveling low-frequency patterns in the Northern Hemisphere higher latitudes discovered by Branstator and Kushnir. The Branstator-Kushnir type retrograde waves appear in the leading normal mode of the barotropic system. The retrograde waves survive nonlinear interaction in the supercritical nonlinear equilibrium states for a wide range of periodic and chaotic solutions. The frequencies of the retrograde waves in the nonlinear solutions are close to the normal mode frequency. A normal mode instability-nonlinear equilibration scenario is suggested for the generation of the retrograde disturbances under supercritical conditions.; Unforced, subcritical nonlinear simulations fail to produce meaningful variability, indicating that forcing is necessary in maintaining the low-frequency variability in the barotropic system under subcritical condition. Stochastically forced experiments are conducted using spatially homogeneous forcing. In the weakly subcritical cases, the model produces spatially and temporally coherent variability, with the first two EOFs of the perturbation field resembling the phase quadrature of the leading normal mode. In this case, the normal mode and retrograde wave structures emerge without normal mode instability. In the strongly subcritical cases, the stochastically forced solutions generally exhibit poor coherence. A stochastically-driven scenario works only in the weakly subcritical regime. The characteristics of the optimal perturbations in the barotropic system are examined. A more general type of spectrally-constrained optimal perturbation is discussed. |
