Correlations of gravity waves and tides in the mesosphere over Saskatoon

Measurements in the Mesosphere and Lower Thermosphere (MLT), from the Saskatoon MF (medium frequency) radar (52 degrees N, 107 degrees W), are used for the years 1992 and 1993 to study gravity waves (GW) and their variability. GW-hand time series (10-100 min; 2-6 h), their spectra and their 12, 24, 48 h oscillations are used. Fifty days of data from three seasons are selected, which are themselves marked by strong differences in middle atmospheric winds and solar tides. Encouragingly consistent modulations of the GW variances at 12, 24, 48 h periods are noted during the summer months; for both short events (a few days) and for the entire 50 days (for the 2-6 h). The inferred GW propagation directions (toward the NE) are internally consistent with calculations of GW propagation-directions using a new correlation method and:consistent with independent measurements of gravity waves discussed in earlier articles from Saskatoon. In this season, all three oscillations are-comparable and moderately large in the hourly-mean winds. During the winter, when the 12 h tide is dominant, there is apparent modulation of the GW variances for short events only, despite the size of the tidal wind oscillations (larger than summer). The inferred GW propagation directions are closest to eastward, although the inference is not strong, due to the weakness and variability-of the modulations. Surprisingly, in the autumn months, when the 12 h tide is at its annual maximum, the modulation is very weak. However, an event of a few days was identified when some consistency was identified. Spectra from a Lomb-Scargle spectral analysis of the variances for nine years are also used to provide a climatology. There is dominance of peaks near 24, 12 and even 48 h during summer months with a much weaker tendency for peaks to occur in winter and autumn. The existence of peaks at 6 h, although possible when the propagation directions of the GW fluxes are isotropic, is actually a rare event. This also confirms earlier:results from Saskatoon, that indicate anisotropy of the GW directions is usually quite strong. The intermittent nature of the GW modulation at tidal periods suggests strongly that the wave sources are intermittent in strength and direction and that the background wind at lower-heights also contributes variability. (C) 1998 Elsevier Science Ltd. All rights reserved.