Observed Temperature Two-Day Wave and Its Relatives near the Stratopause

The two-day wave is observed in the Upper Atmosphere Research Satellite Microwave Limb Sounder tem-perature data around 40–58 km. Between December 1991 and September 1994, the two-day wave temperature signature is most significant after each solstice when the derived easterly winds near the stratopause extend across the equator to at least 208 latitude in the winter hemisphere, and the zonal mean winds near the equator are inertially unstable with observed inertial instability disturbances. The observed two-day wave consists of a 2.0-day period zonal wavenumber-3 and a 1.8-day period zonal 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 summers, but its amplitude is much weaker than the (4, 1.8) component during the two observed boreal summers. During the austral summers, correspondence between amplification of the two-day wave temperature signatures, regions of reversed potential vorticity gradient due to meridional curvature of the zonal mean flow, and the critical 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 inertial instability appears to barotropically destabilize the equatorward flank of the easterly jet where the wave components subsequently grow. During the boreal summers, the (4, 1.8) component appears to be excited by instability that is associated with vertical shear and curvature of the flow seated above the observational domain. The boreal (3, 2.0) mode appears unrelated to the zonal flow instability within the observational domain and may reflect a normal-mode-like response during these periods. 1