Two high-precision CEDAR instruments, an OH Mesospheric Temperature Mapper (MTM) and a Na Temperature Lidar, have been used to investigate seasonal variability in the mid-latitude temperature at ~87 km altitude over the western USA. Here we report the observation of a large perturbation in mesospheric temperature that occurs shortly after the autumnal equinox in close association with the penetration of planetary-wave energy from the troposphere into the mesosphere. This perturbation has been observed on three occasions and exhibits a departure of up to ~25--30 K from the nominal seasonal trend during a disturbed period of ~2 weeks. Such behavior represents a dramatic transient departure from the seasonal trend expected on the basis of current empirical models. These novel results coupled with a recent TIME-GCM modeling study [Liu et al., 2000> provide important insight into the role of planetary waves in mesospheric variability during the equinox periods. ¿ 2001 American Geophysical Union