Intraseasonal wind variability in the equatorial mesosphere and lower thermosphere: Long-term observations from the central Pacific

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 degrees N, 157 degrees W), in the central Pacific. Strong peaks in the zonal winds are found at periods of similar to 60 days, similar to 35-40 days, and similar to 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 similar to 60 days and similar to 35 days. A strong quasi-60-day variation is detected in gravity-wave variances, with much weaker signals at similar to 40 days and similar to 25 days. Strong variations in diurnal tidal amplitudes are observed with periods of similar to 60 days, similar to 40 days, and similar to 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 intraseasonally 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. (C) 1997 Elsevier Science Ltd.