Behaviour of monthly tides from meteor radar winds at 22.7°S during declining phases of 23 and 24 solar cycles

Abstract

Atmospheric tides dominate the mesosphere and lower thermosphere (MLT) dynamics and show seasonal, interannual as well as long term variability. In the present study, MLT wind data obtained between 1999 and 2019 from meteor radar observations at Cachoeira Paulista (22.7∘S, 45.0∘W), Brazil, have been used to investigate similarities and differences in the diurnal, semidiurnal and terdiurnal tide amplitudes during the declining phases of solar cycles 23 and 24. The equinoctial peaks of the terdiurnal and semidiurnal tide amplitudes decay from solar maximum to solar minimum, while diurnal tide amplitudes present high variability, but no obvious long-term trend or decadal variability. The deseasonalized MLT terdiurnal and semidiurnal tidal amplitudes show higher positive correlation with solar activity during solar cycle 24 (ρ=0.97 and 0.90) than during solar cycle 23 (ρ=0.85 and 0.67) and an insignificant correlation with the quasi-biennial oscillation (QBO), during both solar cycles, in disagreement to some results reported in the literature. The deseasonalized MLT diurnal tidal amplitudes show QBO signals whose amplitudes decrease with solar activity during the declining phase of solar cycle 23, suggesting a modulation of the QBO amplitude of the MLT diurnal tide by solar activity. However, in contrast to solar cycle 23, quasi-biennial amplitudes of the diurnal tide increase during the declining phase of the very weak solar cycle 24.