Ultra-Long Waves and Two-Dimensional Rossby Waves

Abstract

The characteristics of planetary wave dispersion in the wintertime troposphere are investigated on the basis of 5 day mean 500 mb height data for 30 winters, making use of simple analysis techniques involving lag-correlation maps for individual gridpoints and for Fourier coefficients of zonal wavenumbers 1 and 2 on 50°N. It is shown that the time evolution of the planetary-waves is dominated by energy dispersion through longitudinally localized wavetrains with “great circle route” orientations, revealed most clearly by the lag-correlation maps for individual gridpoints. When the polarity of these localized patterns is such that large anomalies of like (opposing) sign appear in the Atlantic and Pacific sectors near 50°N, a strong zonal wavenumber 2 (1) pattern results. These wavenumber 1 and 2 patterns do not retain their identity from one 5 day period to the next as distinctly as the localized wavetrains do. The conceptual model of Rossby-wave propagation along latitude circles still appears to be valid for the wintertime stratosphere, where the waves have the same two-dimensional scale as the polar vortex itself, and for external Rossby-modes such as those described by Madden (1978). It may also be valid at times in the Southern Hemisphere troposphere.