Nonlinear Propagation of Zonal Winds in an Atmosphere with Newtonian Cooling and Equatorial Wavedriving

Abstract

Advection of angular momentum by the mean meridional circulation is important in the quasi-biennial and semiannual oscillations of the tropical middle atmosphere. The advection is nonlinear, implying a finite horizontal or vertical displacement of angular momentum surfaces. Horizontal advection contributes to the easterly phase of the semiannual oscillation, and is sensitive to extratropical body forces. The mean meridional circulation may be thought of as a hybrid Hadley/body-force circulation driven by radiative heating and Eliassen-Palm flux convergence. Realistic steady states are obtained when a mesospheric friction layer, representing gravity wave drag, is included in the problem. This device resolves an ambiguity in the inviscid theory of the middle atmosphere Hadley circulation. Nonlinear advection is also important in the quasi-biennial oscillation; it is responsible, in part, for the strong asymmetry between east and west phases. Diabatic advection of westerly shear displaces angular momentum surfaces downward at the equator in agreement with observations. From this initial condition, it is shown that a self-propagating westerly jet is excited that differs substantially from the linear-diffusive propagation discussed by Dickinson. These results are derived from high-resolution, two-dimensional models of the atmosphere. Realistic simulations of the quasi-biennial and stratopause semiannual oscillations are obtained without ad hoc forcing of semiannual easterlies. It is argued, however, that a spectrum of Kelvin or gravity waves may be necessary for the westerly acceleration phase. A novel result is that the period of the quasi-biennial oscillation is increased by extratropical body forces, due to the time mean Brewer-Dobson circulation.