Abstract:

    The two-day wave is observed in the Upper Atmosphere Research Satellite (UARS) Microwave Limb Sounder (MLS) temperature and water vapor data. During a 3-year period (Dec.1991-Sep.1994), the wave signature is prominent semiannually after each solstice and is comprised of a zonal wavenumber 3 component with ~2.0-day period and a wavenumber 4 component with ~1.8-day period. Intra-annually, the wavenumber 3 amplitude during the austral summer is nearly twice as strong as during the boreal summer. The wavenumber 4 component however can be equally strong in both summers. The wavenumber 3 signal tends to be dominant during the austral summer while the wave 4 component is dominant during the boreal summer. The derived zonal wind structure suggests that the Charney-Stern inflection instability mechanism is responsible for generating the two-day wave whose amplitude resides mainly on the equatorward flank of the summer easterly jet.
    In some seasons, momentum redistribution by inertial instability appears to destabilize the easterly jet from which the barotropically unstable two-day wave grows. In these instances, an independent study using the UARS Cryogenic Limb Array Etalon Spectrometer (CLAES) temperature (Hayashi et al., 1998) coincidently identifies "pancake" structures associated with inertial instability. A possible connection between inertial instability and the two-day wave has been discussed by Hitchman (1985) and Orsolini et al. (1997).
    The National Center for Atmospheric Research (NCAR) Community Climate Model version 2 (CCM2) mechanistic model is used to simulate this connection. Model experiments show that, for a prescribed initial wind condition with strong horizontal wind shear in the tropics, inertial instability can trigger the two-day wave. An increase in easterly wind curvature fostered by inertial instability circulation destabilizes the jet in low summer latitudes and allows first the growth of the wavenumber 4 then the wavenumber 3 component of the two-day wave. Near the stratopause, the two-day wave energy propagation is directed equatorward away from the wave's critical line source and westerly momentum is fluxed into the easterly jet core by the wave. While much of the wavenumber 4 activity is confined near the stratopause, the wavenumber 3 energy can propagate upward well into the mesosphere where strong Rayleigh damping is imposed. The simulated waves' spatial and spectral characteristics are fairly realistic.

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