| Dynamic processes in the mesosphere and lower thermosphere(MLT)region provide a significant key to understanding coupling processes between the lower atmosphere and the thermosphere/ionosphere system.These tides are predominantly generated by water vapor absorption in the troposphere and ozone absorption in the stratosphere and then propagate vertically to the MLT,where they reach large amplitudes,forcing dynamic processes in the MLT region.Meteor radar can provide single-point high temporal resolution and continuous MLT horizontal wind measurements at a single point.In recent decades,a large number of studies have been carried out on tides through various observation methods and numerical simulations.Although some studies based on meteor radar observations have studied the atmospheric tidal variability in the MLT region,the short-term changes in migrating and nonmigrating tides still have shortcomings,especially at low latitudes or equatial regions.Based on this problem,we decomposed the migrating and nonmigrating tidal winds observed by meteor radars and studied the modulation of migrating tides by some short-time scale events in the lower atmosphere,such as sudden stratospheric warming(SSW)and the Madden-Julian oscillation(MJO),as well as the nonlinear interaction between planetary waves and tides.1.The radars located in Cariri,Brazil(7.4° S,36.5° W),Kototabang,Indonesia(0.2° S,100.3° E),Ascension Island,United Kingdom(7.9° S,14.4° W),and Darwin,Australia(12.3° S,130.8° E),are used jointly to decompose the migrating and nonmigrating tidal winds in the equatorial MLT region.Harmonic analysis was used to obtain amplitudes and phases for diurnal and semidiurnal solar migrating tides between 82 and 98 km altitude during the period 2005 to 2008.To verify the reliability of the tidal components calculated by the four meteor radar wind measurements,we also present a similar analysis for the Whole Atmosphere Community Climate Model(WACCM)winds,which suggests that the migrating and nonmigrating tides are well observed by the four different radars.The tides include the important tidal components of diurnal westward-propagating zonal wavenumber 1(DW1),semidiurnal westwardpropagating zonal wavenumber 2(SW2),diurnal eastward-propagating zonal wavenumber 3(DE3)and diurnal eastward-propagating zonal wavenumber 2(DE2).In addition,the results based on observations were compared with the Climatological Tidal Model of the Thermosphere(CTMT)and a High-Altitude version of the Navy Global Environmental Model(NAVGEM-HA).In general,in terms of climatic features,our results for the major components of migrating tides are qualitatively consistent with the CTMT models derived from satellite data,and the nonmigrating tides,including DE3 and DE2,are consistent with the NAVGEM-HA outputs.2.In addition,the migrating tidal amplitudes observed show significant short-time scale variability.All migrating tidal amplitudes are unusually stronger in JanuaryFebruary 2006.According to the results of Monte Carlo tests,these unusual enhancements are probably because tides were enhanced by the 2006 northern hemisphere stratospheric sudden warming(NH-SSW)event.Moreover,highly correlated intraseasonal variations with periods from 30 to 80 days are recognized in the DW1 wind amplitudes and zonal wind in the MLT region,meaning that the DW1 tides modulate the zonal mean wind.Using the Real Multivariate MJO(RMM)index and outgoing longwave radiation(OLR)anomalies to explore MJO impacts on the DW1 tides in the MLT region.The analysis suggests that the diurnal migrating tides respond strongly to the MJO during both boreal winter(～20-25%)and summer(-25%).The response of DW1 tides in both wind components strongly depends on the MJO phase,possibly due to the variability in convective forcing and GW forcing.3.By using the MLT horizanal wind data observed from meteror radars located at Mohe,China(53.5° N,122.3° E),Beijing,China(40.3° N,116.2° E),Wuhan,China(30.5° N,1 14.2° E)and Sanya,China(18.4° N,109.6° E),the features of the seasonal variations in the quasi-6-day wave(6DW)at different latitudes are analyzed,and the modulation of the 6DW by the diurnal tides is discussed.The data used in the analysis are based on a wind dataset from a meteor radar chain from December 2008 to November 201 7.Interestingly,the annual oscillations of 6DW in the mid-low latitudes are modulated by the quasibiennial oscillation in the diurnal tide,resulting in seasonal features that are different from other latitudes.To verify the nonlinear interactions between the 6DW and DW1 tides,the 6DW with westward-popagating zonal wavenumber 1 and 2(Q6W1 and Q6W2)and the secondary planetary waves are decomposed jointly from multiple meteor radars in four longitudinal sectors situated in the equatorial MLT region.Considering that the nonmigrating diurnal tides are strong in the equatorial region,the nonlinear interactions between 6DW and nonmigrating diurnal tides can also contribute to the formation of these secondary waves. |
