A comparative study of winds and tidal variability in the mesosphere/lower-thermosphere region over Bulgaria and the UK

Meteor radars located in Bulgaria and the UK have been used to simultaneous ly measure winds in the mesosphere/lower-thermosphere region near 42.5 degr eesN, 26.6 degreesE and 54.5 degreesN, 3.9 degreesW, respectively, over the period January 1991 to June 1992. The data have been used to investigate p lanetary waves and diurnal and semidiurnal tidal variability over the two s ites. The tidal amplitudes at each site exhibit fluctuations as large as 30 0% on time scales from a few days to the intra-seasonal, with most of the v ariability being at intra-seasonal scales. Spectral and cross-wavelet analy sis reveals closely related tidal variability over the two sites, indicatin g that the variability occurs on spatial scales large compared to the spaci ng between the two radars. In some, but not all, cases, periodic variabilit y of tidal amplitudes is associated with simultaneously present planetary w aves of similar period, suggesting the variability is a consequence of non- linear interaction. Calculation of the zonal wave number of a number of lar ge amplitude planetary waves suggests that during summer 1991 the 2-day wav e had a zonal wave number of 3, but that during January-February 1991 it ha d a zonal wave number of 4.