Evidence for nonlinear coupling of planetary waves and tides in the lower thermosphere over Bulgaria

Analyses of hourly values of zonal and meridional wind near 95 km observed by meteor radar at Yambol (42.5 degrees N, 26.6 degrees E) during January 1 991-June 1992 indicate the presence of planetary waves with prevailing peri ods of 1.5-2.5, 4-6, 9-10 and 16-18 days. About 20% of the whole power of a tmospheric motions is connected with these waves, so they play an important role in the dynamics of the mesosphere-lower thermosphere (MLT) region. By dynamic spectral analysis applied to the hourly neutral wind and to the ca lculated hourly values of tidal amplitudes it has been demonstrated that th ere is considerable modulation of tidal amplitudes by planetary waves in th e neutral wind, as this process is better expressed in the semidiurnal tide s. The nonlinear interaction between tides and planetary waves is studied b y bispectral analysis. The results of these analyses indicate again that th e nonlinear interactions between semidiurnal tides and planetary waves with periods 2-20 days are stronger than those of the diurnal tides and planeta ry waves. A peculiar feature of dynamics in the MLT region above Bulgaria i s the presence of strong oscillations with periods of 20 and 30 h, which in dicate significant nonlinear coupling between them. (C) 2000 Elsevier Scien ce Ltd. All rights reserved.