Spectral Evolution of a Blocking Episode and Comparison with Wave Interaction Theory

Abstract

The structure and evolution of planetary-scale waves in the 500 mb geopotential height field over the Northern Hemisphere during December 1976 and January 1977 are studied with the techniques of one- and two-dimensional Fourier analysis and spherical harmonic analysis. The incipient stages of ?split-flow? blocking episodes in this period are associated with an increase in amplitude in two-dimensional wave components having zonal wavenumbers 1 and 2 and meridional wavenumber 1. At the time of observed split-flow blocking, the wave component with zonal wavenumber 3 and meridional wavenumber 2 achieves peak amplitude, while the amplitudes of the other wave components decline. In addition, the height field reconstructed from the sum of these three components is found to simulate well the observed 500 mb height pattern during this episode. It is suggested that these observations may be explained by nonlinear resonant wave interaction theory. A streamline configuration resembling the observed split-flow blocking pattern is found to be generated in the two-layer quasi-geostrophic channel model through the mechanism of resonant triad interactions. The favorable comparison between observations and theory in this particular case study suggests that resonant interactions among planetary-scale waves may be an important physical mechanism at work during the evolution of atmospheric blocking.