Mesosphere and Lower Thermosphere Zonal wind Variations over Tropics: Relation to local stratospheric zonal winds and Global Circulation anomalies

Abstract:

Long-term (2001-20012) meteor radars observations, spatially well separated in tropics, are used to study mesosphere and lower thermosphere (MLT) zonal wind variations in relation to the stratospheric winds. The tropical MLT zonal winds show clear seasonal variations with eastward flow during the solstices and westward flow during the equinoxes. In addition to the seasonal variation, MLT zonal winds show clear interannual variation mainly during boreal spring equinox. Mesospheric spring equinox enhancements (MSEE) in the zonal winds appear with a periodicity of 2–3 years, suggesting a modulation by the quasi-biennial oscillation. These enhancements are identified during the weak stratospheric westward winds. The underlying mechanism is a local forcing of the MLT region by gravity waves (GWs) due to less filtered westward waves enforcing large westward winds. Although the stratospheric westward winds are weak during 2004 and 2006, the spring equinox westward winds at MLT show small amplitudes. These unexpected small winds occur in these years are due to early spring transition, which is connected to the global circulation anomalies associated with strong sudden stratospheric warmings. Spatial distribution of meteor radar observations enables us to study the latitudinal variation of the mesospheric semiannual oscillation (MSAO) and the QBO modulation on MLT winds. Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA) runs also support the latitudinal behavior of MSAO and QBO modulation on MLT winds.