A Numerical Study of the Interaction between Stationary Rossby Waves and Eastward-Traveling Waves in the Southern Hemisphere Stratosphere

Abstract

Quasi-stationary Rossby waves with a wavenumber 1 component (wave 1) and eastward-traveling waves with a wavenumber 2 component (wave 2), whose period lies in 10?20 days, have been frequently observed during the late winter and early spring in the stratosphere of the Southern Hemisphere. Temporal variations of these amplitudes are remarkably enhanced during this period.The interaction among wave 1, wave 2, and the zonal mean flow was simulated using a low-order truncated semispectral model. It was assumed that both the stationary wave 1 and eastward-traveling wave 2 are forced at the tropopause. When the wavenumbers from 1 to 3 or 4 were included in the model with use of a reasonable forcing amplitude and the observed monthly mean zonal wind, the following results were obtained. 1) The amplitude of wave 1 reaches a maximum value when the ridge of wave 2 overlaps that of wave 1. 2) The amplitude of wave 2 is negatively correlated with wave 1. 3) In many cases the large amplification of wave 1 occurs only once for every two passages of wave 2 over wave 1.From the analysis of energetics, it was found that the amplification of wave 1 arises from the abrupt decrease of kinetic energy conversion to the zonal mean flow. The analysis of potential enstrophy conversion revealed more clearly that the amplification of wave 1 is caused by the wave-mean flow interaction. Eliassen-Palm (E?P) flux of wave 1 focuses to the high-latitude stratosphere during the amplification stage. The focusing is caused by appearance of the negative refractive index for wave 1 in the midlatitudes, that is, formation of waveguide in the high latitudes.The interaction between wave 1 and wave 2 causes the periodic variation of wave amplitudes through their energy/enstrophy exchange and controls degree of the interaction with zonal mean flow through the horizontal phase movement. In the amplification stage, the wave-wave interaction emphasizes the focus of E?P flux.