Abstract:
The two-day wave is observed in the Upper Atmosphere Research
Satellite Microwave Limb Sounder temperature 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 20o 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 wave
number 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
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