Mesosphere and lower thermosphere zonal wind variations over low latitudes: Relation to local stratospheric zonal winds and global circulation anomalies

摘要

Long-term observations from medium-frequency and meteor radars (1993–2012) and rocket\udsoundings (1979–1990 and 2002–2007) are used to study mesosphere and lower thermosphere (MLT)\udzonal wind variations in relation to the stratospheric winds over northern low latitudes. The combined data\udset provides a complete height profile of amplitude of semiannual oscillation (SAO) up to 100 km, with an\udexception around 75–80 km. The SAO signal has maxima around 50 km and 82 km and a minimum around\ud65 km. The MLT zonal winds show remarkable interannual variability during northern hemispheric spring\udequinox and much less during fall equinox. Zonal wind mesospheric spring equinox enhancements (MSEE)\udappear with a periodicity of 2–3 years, suggesting a modulation by the quasi-biennial oscillation, which we\udidentified with the strength of stratospheric westward winds. Out of 20 years of observations, the stratospheric\udwestward winds are strong during 11 years (non-MSEE) and weak during 9 years. Six of these 9 years show large\udMLT winds (MSEE), and 3 years (1999, 2004, and 2006) show small MLT winds (missing MSEE). These unexpected\udsmall winds occur in years with global circulation anomalies associated with strong sudden stratospheric\udwarmings and an early spring transition of zonal winds. With the proposed three MSEE classes, we take into\udaccount local and global forcing factors.