Zonal flow generation by internal gravity waves in the atmosphere

A novel mechanism for the generation of low-frequency large-scale zonal flows by higher-frequency, small-scale, finite-amplitude internal gravity (IG) waves is analyzed in the atmosphere from the troposphere to the ionosphere E layer. The nonlinear generation mechanism is based on the parametric excitation of convective cells by finite-amplitude IG waves. A set of coupled equations describing the nonlinear interaction of IG waves and zonal flows is derived. The generation of zonal flows is due to the Reynolds stress and mean stratification forces produced by finite-amplitude IG waves. The onset mechanism for the instability is governed by a generalized Lighthill instability criterion. Explicit expressions for the maximum growth rate as well as for the optimal spatial dimensions of the zonal flows are derived. The growth rates of zonal flow instabilities and the conditions for driving them are determined. A comparison with existing results is carried out. The present theory can be used for the interpretation of IG wave observations in the Earth's atmosphere and laboratory experiments. Some earthquake-related phenomena are briefly discussed.