Radiative-Photochemical Damping of Equatorial Waves in the Middle Atmosphere

Convection in the tropical equatorial troposphere spawns a broad spectrum of waves that propagate vertically into the middle atmosphere. The damping of these waves results in a body force that drives the zonal-mean circulation, including the two most striking features of the equatorial middle atmosphere: the quasi-biennial oscillation (QBO) and the semi-annual oscillation (SAO). To faithfully represent these wave-driven circulation features requires a precise description of the damping processes operating on the waves. With this in mind, we examine the relative importance of Newtonian cooling (NC) and ozone heating (OH) on the damping of Kelvin, Rossby-gravity, inertia-gravity and equatorial Rossby waves using an equatorial beta-plane model of the middle atmosphere. The OH is comprised of contributions from ozone photochemistry and ozone advection (vertical and meridional). The model waves are forced from the lower boundary and propagate vertically on a steady, zonal-mean basic state. An analytical analysis yields explicit expressions for the (local) spatial propagation and spatial damping rate for each wave type. Guided by observed wave spectra, spatial damping rates are calculated for zonal-mean basic states consistent with the easterly and westerly phases of the QBO and SAO.

Irrespective of wave type, the OH due to vertical ozone advection dominates over meridional ozone advection. The vertical ozone advection may augment or oppose the damping due to NC, depending on altitude and meridional wave structure. Damping effects due to ozone photochemistry increase with height and always augment NC. In the vicinity of a critical layer, ozone photochemistry always dominates over advection. The effect of OH on the spatial damping rates is typically maximized in the mid to upper stratosphere. For Kelvin, Rossby-gravity, and equatorial Rossby waves corresponding with observations, the OH may contribute as much as 45% to the spatial damping rate. Because inertia-gravity waves span a wide range of zonal-wave scales and meridional structures, and may be of high or low frequency, the effects of OH on their spatial damping rate are more complicated than for the other wave types. For inertia-gravity waves propagating with the mean current, OH may contribute as much as 35% to the spatial damping rate. Counter-propagating waves have damping rates that are typically about ~10% less than for waves that propagate with the current.

Because the effects of OH on the spatial damping rates depend on wave type, zonal scale, meridional structure, and propagation, devising a simple parameterization that accounts for the radiative-photochemical damping of equatorial waves is problematic. Thus chemistry climate models that seek to accurately represent equatorial wave damping will not only have to resolve the broad spectrum of waves that drive the circulation, they will need to account for the interaction between the waves and the zonally asymmetric ozone field.