Longitudinal variations in mesospheric winds: Evidence for gravity wave filtering by planetary waves

Mesospheric horizontal wind data from the High Resolution Doppler Imager (HRDI) satellite instrument are used to investigate the longitudinal variation of nontidal winds in the upper mesosphere and their relationship with winds in the stratosphere. The period covered by the study is late winter (February-mid-March) for 1992-1994. The results indicate that there is a negative correlation between longitudinal variations of the zonal winds in the stratosphere and those in the mesosphere, and between longitudinal variations of the meridional winds in the stratosphere and mesosphere. The data are examined to determine which of several possible causes is responsible. The possibilities are 1) that planetary-scale disturbances are produced in situ by gravity wave drag in the mesosphere that is not zonally symmetric because of filtering by the planetary waves in the stratosphere, 2) that asymmetries are caused by longitudinal variations in gravity wave drag due to azonal source distributions that are coincidentally tied to the standing wave patterns in stratospheric winds; or 3) that a planetary Rossby wave propagates from the stratosphere to the mesosphere and undergoes a phase shift. The observations are overall most consistent with the first hypothesis, that is, that filtering of gravity waves by zonal asymmetries in the stratospheric winds leads to in situ generation of planetary-scale structures in the winter upper mesosphere.