Long-term oscillations of the wind field in the tropical mesosphere and lower thermosphere from Hawaii MF radar measurements

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Abstract

[1] Wind measurements in the mesosphere and lower thermosphere obtained with a medium frequency (MF) radar in Hawaii (22°N, 160°W) spanning ∼16 years are employed to examine the intra-annual and interannual variability of the mean and tidal motions at altitudes between 84 and 94 km. Intra-annual periodicities range from ∼3 to 12 months, with significant coherence in altitude and between the zonal and meridional components of each motion field. Interannual variations confirm the dominant periodicities identified previously at this site and elsewhere, in particular, the significant diurnal, and less significant semidiurnal, tidal responses at periods of ∼28 and ∼48 months. Amplitudes of these long-period oscillations of the diurnal tide increase with altitude below 92 km and are larger than the amplitudes of the 12 month oscillations above ∼90 km. Phases of the ∼28 and ∼48 month oscillations show a downward progression with a slightly larger altitude variation in the meridional diurnal tide for the ∼28 month oscillation and a significantly larger altitude variation in the zonal diurnal tide for the ∼48 month oscillation. The long and nearly continuous Hawaii data set also enables characterization of the responses of the wind fields to the 11 year solar cycle. Both wind and tidal fields exhibit this periodicity, but these responses display interesting and different relations to the phase of the solar cycle. The 11 year oscillation of the meridional wind is nearly in phase with the solar cycle, while the 11 year oscillation of the zonal wind is in approximate quadrature with the solar cycle. The 11 year oscillations of the semidiurnal tidal amplitudes and the meridional diurnal amplitude are all in approximate quadrature with the solar cycle (with the tidal amplitudes leading by ∼29 to 37 months), while the 11 year oscillation of the zonal diurnal amplitude is somewhat nearer an antiphase than a quadrature relation to the solar cycle (leading by ∼54 months).

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