INTRASEASONAL WIND VARIABILITY IN THE EQUATORIAL MESOSPHERE AND LOWERTHERMOSPHERE - 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 f ive years of velocity data acquired by a radar system at Christmas Isl and (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, simila r to 35-40 days, and similar to 22-25 days. These peaks, as well as th e mean annual variations of the activity within the various period ran ges, are similar to 30-60 day and 20-25 day oscillations that occur in the equatorial troposphere. Weaker (but nonetheless clear) periodicit ies are also found in the meridional winds at similar to 60 days and s imilar to 35 days. A strong quasi-60-day variation is detected in grav ity-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 followin g explanation for the observed intraseasonal variability of the equato rial MLT region. Intraseasonal cycles in tropical tropospheric convect ion produce intraseasonal variations in the intensity of gravity waves and nonmigrating diurnal tides impinging upon the mesosphere. This ac counts for the intraseasonal peaks we observe in gravity-wave and tida l activity. This intraseasonally modulated wave activity induces simil ar periodicities in the wave-induced driving of the zonal MLT flow, wh ich in turn forces the observed intraseasonal peaks in the zonal MLT w inds. If this explanation is valid, these observations provide an unus ually clear example of the driving of MLT flow patterns by waves emana ting from tropospheric systems, and highlight the importance of convec tively generated waves in understanding the dynamics of the equatorial middle atmosphere. (C) 1997 Elsevier Science Ltd.