Wind measurements made using ground-based mesosphere and lower thermosphere radar systems at Sheffield, UK (2°W, 53°N) and Saskatoon, Canada (107°W, 52°N) show an increase in long-period planetary wave (PW) activity at the end of August until the middle of September 1993. Some of these waves can be identified with the well-known normal atmospheric modes; however, the westward propagating wave with zonal wave number one (m = 1) and a period of about 6.5 days observed during this time interval cannot be explained using the classical theory of free atmospheric oscillations. Analysis of the UKMO assimilated fields shows that in the stratosphere the 7-day wave with m = 2 increases in amplitude during the time interval considered, and there exists a strong stationary planetary wave (SPW) with m = 1. A steady state two-dimensional numerical model of planetary waves has been used to simulate the free atmospheric oscillations and secondary planetary wave, which arises from a nonlinear interaction between the second asymmetric normal mode with m = 2 (the 7-day wave) and SPW with m = 1. The results of simulation show that an increase in long-period PW activity during August/September can be explained by seasonal changes of the zonal mean flow, which influence the wave propagation conditions, and that one of the possible sources for the 6.5-day wave is a nonlinear interaction between the 7-day wave and SPW with m = 1. It is suggested that the 6.5-day wave with m = 1 be referred to as a “secondary normal mode,” which has been generated by random meteorological motions in the lower atmosphere and involving interaction with a SPW of wave number 1.