Intraseasonal Wind Variability in the Equatorial Mesosphere and Lower Thermosphere: Long-Term Observations From the Central Pacific

摘要

Analysis of intraseasonal (10-100 days) oscillations in the equatorial mesosphere and lower thermosphere (MLT) is presented, based on over five years of velocity data acquired by a radar system at Christmas Island (2 deg N, 157 deg W), in the central Pacific. Strong peaks in the zonal winds are found at periods of approx. 60 days, approx. 35-40 days, and approx. 22-25 days. These peaks, as well as the mean annual variations of the activity within the various period ranges, are similar to 30-60 day and 20-25 day oscillations that occur in the equatorial troposphere. Weaker (but nonetheless clear) periodicities are also found in the meridional winds at approx. 60 days and approx. 35 days. A strong quasi-60-day variation is detected in gravity-wave variances, with much weaker signals at approx. 40 days and approx. 25 days. Strong variations in diurnal tidal amplitudes are observed with periods of approx. 60 days, approx. 40 days, and approx. 25 days. These observations lead us to propose the following explanation for the observed intraseasonal variability of the equatorial MLT region. Intraseasonal cycles in tropical tropospheric convection produce intraseasonal variations in the intensity of gravity waves and nonmigrating diurnal tides impinging upon the mesosphere. This accounts for the intraseasonal peaks we observe in gravity-wave and tidal activity. This intraseaonally modulated wave activity induces similar periodicities in the wave-induced driving of the zonal MLT flow, which in turn forces the observed intraseasonal peaks in the zonal MLT winds. If this explanation is valid, these observations provide an unusually clear example of the driving of MLT flow patterns by waves emanating from tropospheric systems, and highlight the importance of convectively generated waves in understanding the dynamics of the equatorial middle atmosphere.