Zonal-Mean Quasi Bi-Monthly Oscillations (BMO) in the Mesosphere
Abstract
In the zonal mean (m = 0), oscillations are observed with the UARS spacecraft that have periods of months. Based on measurements from the High Resolution Doppler Imager (HRDI) instrument, Quasi Bi-Monthly Oscillations (BMO) appear in the meridional winds at equatorial latitudes and at 95 km altitude, as reported by Huang and Reber (JGR, 2003). Such oscillations are also observed in temperatures at altitudes of about 55 km, which are measured with the Microwave Limb Sounder (MLS) and the Cryogenic Limb Array Etalon Spectrometer (CLAES) on UARS. Considering that it takes 36 days to cover all solar local times, a two-dimensional Fourier least squares analysis is carried out to delineate in the temperature variations (a) the tides (diurnal and semidiurnal) and (b) the zonal-mean component. The inferred zonal-mean temperature variations have periods between 2 and 3 months, and they are larger away from the equator. The amplitudes of this BMO are about 4 K, and the oscillations are in opposite phase in the two hemispheres. We shall investigate the BMO with the temperature and wind measurements from the TIMED spacecraft, which are available at common altitudes throughout the mesosphere and thus can provide a more definitive picture of this phenomenon. Oscillations of the kind discussed have been identified in a modeling study (Mayr et al., JATP, 2003). It is shown there that the meridional momentum source from parameterized small-scale gravity waves generates variations in the meridional winds with periods between one and three months that have the character of a non-linear auto-oscillator -- the apparent counterpart to the Quasi-Biennial Oscillation (QBO) in the zonal circulation. The meridional wind oscillations are largely confined to equatorial latitudes, and they generate through dynamical heating and cooling temperature oscillations in the two hemispheres on either side of the equator.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSMSA13A..04H
- Keywords:
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- 3332 Mesospheric dynamics;
- 3334 Middle atmosphere dynamics (0341;
- 0342);
- 3337 Numerical modeling and data assimilation;
- 3367 Theoretical modeling