Observed temperature two-day wave and its relatives near the stratopause

The two-day wave is observed in the Upper Atmosphere Research Satellite Mic rowave Limb Sounder temperature data around 40-58 km. Between December 1991 and September 1994, the two-day wave temperature signature is most signifi cant after each solstice when the derived easterly winds near the stratopau se extend across the equator to at least 20 degrees latitude in the winter hemisphere, and the zonal mean winds near the equator are inertially unstab le with observed inertial instability disturbances. The observed two-day wa ve consists of a 2.0-day period zonal wavenumber-3 and a 1.8-day period zon al wavenumber-4 component, named (3, 2.0) and (4, 1.8), respectively. The ( 3, 2.0) component is dominant during two of the three available austral sum mers, but its amplitude is much weaker than the (4, 1.8) component during t he two observed boreal summers. During the austral summers, correspondence between amplification of the two -day wave temperature signatures, regions of reversed potential vorticity g radient due to meridional curvature of the zonal mean flow, and the critica l lines for the (3, 2.0) and (4, 1.8) modes suggest barotropic instability as a source of both wave components. Momentum redistribution by observed in ertial instability appears to barotropically destabilize the equatorward fl ank of the easterly jet where the wave components subsequently grow. During the boreal summers, the (4, 1.8) component appears to be excited by instab ility that is associated with vertical shear and curvature of the flow seat ed above the observational domain. The boreal (3, 2.0) mode appears unrelat ed to the zonal flow instability within the observational domain and may re flect a normal-mode-like response during these periods.