Low-Frequency Variations and Optimal Excitation in a Simple Barotropic Model with Zonal Asymmetry

[摘要] Fundamental dynamics of extra-tropical low-frequency variations were investigated by numerical experiments based on a simple spherical barotropic model with forcing, dissipation and topography. Nonperiodic solutions that show low-frequency variations were obtained for realistic parameter ranges. The time average of the solutions has a zonally varying jet structure under the influence of the topography, as the climatological mean state has in the upper troposphere. Some disturbances, which are defined as a deviation from the time average, grew rapidly in a favorable area of upstream and equatorward of the jet maximum. Linear characteristics of the time-averaged basic states, such as eigenmodes, optimal modes, and linear responses of isolated initial perturbations, were examined in detail and were compared with the rapidly growing disturbances in the nonlinear nonperiodic solutions; the rapid growth is explained well by the linear evolution of optimal modes rather than that of unstable eigenmode. The wave activity diagnostics reveal the importance of the propagation of Rossby waves in the zonally varying basic state to the growth of disturbances around the jet exit region.

[学科分类] 大气科学