A study of low-frequency unstable planetary waves in realistic zonal and zonally varying basic states*

Abstract

This study investigates low-frequency, unstable planetary waves in realistic basis states of January 1979, using three-dimensional spectral primitive equations derived by orthonormal vertical structure functions and Hough harmonics. Eigenfrequencies, modal structures, and energetics are contrasted for the zonal basic state and the zonally varying basic state. Three selected unstable modes are extensively examined. One is the Green mode in the planetary waves, and the other two are deep Charney modes having different meridional structures. We find that the Green mode of wavenumber 1 in the zonally varying basic state shows notable transient growth in the first internal vertical component during its life-cycle. One of the deep Charney modes becomes stationary at a preferred geographical location with nearly barotropic structure. The other deep Charney mode, which shows a monopole structure in the zonal basic state, becomes a dipole structure in the zonally varying basic state. The northern part of the dipole structure shows westward propagation with transient growth in amplitude at a preferred location. The results suggest that the large-scale Pacific blockings during the winter are, at least in part, related to the low-frequency Charney mode having a dipole structure. A possible connection between the amplification of planetary waves and the Green mode in zonally varying basic states is also discussed. DOI: 10.1111/j.1600-0870.1989.tb00375.x