Long-term trends and decadal variability of upper mesosphere/lower thermosphere gravity waves at midlatitudes

Abstract

Mesosphere/lower thermosphere (MLT) winds over Germany as measured with a low-frequency spaced receiver system at Collm 1984–2007 have been analysed with respect to variations at the time scales of gravity waves. Background winds are also registered to analyse possible gravity wave-mean flow interactions at decadal and interdecadal time scales. In both winter and summer an increasing mesospheric zonal wind jet with time is registered, which is accompanied with increasing gravity wave variances. At greater altitudes in summer, the mean wind jet trend reverses, and negative trends of gravity wave variances are found. This connection between gravity waves and mean wind is also observed on a quasi-decadal scale: during solar maximum stronger mesospheric zonal wind jets as well as larger gravity wave amplitudes are observed. This results in a solar cycle modulation of gravity waves with larger amplitudes during solar maximum. The observed positive correlation between gravity wave amplitudes and the mean zonal wind may follow the theory of saturated waves in the atmosphere, such that stronger mesospheric zonal winds are connected with larger gravity wave amplitudes.