Longitudinal variability of mesospheric temperatures during equinox at middle and high latitudes

Airglow emission and temperature observations by the wind imaging interferometer (WINDII) on the upper atmosphere research satellite (UARS) and ground-based stations revealed a rapid 2-day rise in the nighttime emission rate in springtime followed by a subsequent decrease in the magnitude, which was termed 'springtime transition'. A rapid temperature enhancement was also revealed in the average annual temperature at 87 kin height, employing observations from 1991 to 1997 and more recent temperature data from 1998 to 1999. Large amplitude perturbations in mesospheric Na-lidar and OH rotational temperatures at 87 km around the autumnal equinox at mid-latitudes were also reported. Recently it has been suggested that the observed fall perturbation is a signature of mesospheric planetary waves of zonal wavenumber 1 (hereafter a wave 1) in the Northern hemisphere. The current study extends earlier work on mesospheric temperatures from the WINDII/UARS experiment and complementary ground-based observations to examine global spatial and temporal characteristics of the equinox transition at 87 km height. A 7-year climatology at middle and high latitudes, 40-60degreesN is presented revealing a V-shaped signature of temperature decrease with amplitude of similar to25 K below the annual mean and a duration of 3-4 weeks depending on the year, following fall equinox (Day 257-284). Satellite and ground-based temperature observations were successfully combined to infer tidal information, a key to the interpretation of the results obtained. After accounting for the diurnal tidal perturbation a distinct planetary wave structure was revealed at latitudes from 45degreesN to 65degreesN. Longitudinal temperature variations show a drop at 140-180degreesE on Day 264 at 50degreesN which extends to at least 60degreesN accompanied by wave signatures. Spectral analysis indicated planetary waves with waves 1 and 2 planetary perturbations maximizing on Day 263-265 (September 20-22). Various waveforms were also identified (waves 3-6) from which a waveform with a wavenumber 4 appeared to be the third most persistent wave signature during the period with a period of about 4 days. The spectral analysis also revealed a depressed planetary wave activity following fall equinox. It is concluded that the observed temperature perturbation is the likely result of in situ generated gravity waves at 87 km superposed on the planetary wave 1 propagating upwards from below enhanced by the reversal of zonal mean wind flow at equinox.

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