  We present direct observations of zonal wave numbers 1 and 2 planetary wave
activity in the mesopause region derived from a longitudinal chain of
high-latitude Northern Hemisphere (51-66$^{\circ}$N) Super Dual Auroral Radar
Network radars. Over a 9 year period (2000-2008), the planetary wave activity
observed shows a consistent increase around the fall equinox. This is shown to
be coincident with a minimum in the magnitude of the stratospheric winds and
consequently a minimum in the stratospheric gravity wave filtering and the
subsequent momentum deposition in the mesopause region. Despite this, the
observed meridional winds are shown to be perturbed poleward and mesopause
temperatures rise temporarily, suggesting that westward momentum deposition
from planetary waves temporarily becomes the dominant forcing on the mesopause
region each fall equinox.
