Interannual Variability of the 12-hr Tide in the Mesosphere and Lower Thermosphere in 15 Years of Meteor-Radar Observations Over Rothera (68°S, 68°W)

The solar tides of the mesosphere and lower thermosphere (MLT) show great variability on time scales of days to years, with significant variability at interannual time scales. However, the nature and causes of this variability remain poorly understood. Here, we present measurements made over the interval 2005–2020 of the interannual variability of the 12-hr tide as measured at heights of 80–100 km by a meteor radar over Rothera (68°S, 68°W). We use a linear regression analysis to investigate correlations between the 12-hr tidal amplitudes and several climate indices, specifically the solar cycle (as measured by F10.7 solar flux), El Niño Southern Oscillation (ENSO), the Quasi-Biennial Oscillation (QBO) at 10 and 30 hPa and the Southern Annular Mode (SAM). Our observations reveal that the 12-hr tide has a large amplitude and a clearly defined seasonal cycle with monthly mean values as large as 35 m s⁻¹. We observe substantial interannual variability, with monthly mean 12-hr tidal amplitudes at 95 km exhibiting a two standard-deviation range (2σ) in spring of 13.4 m s⁻¹, 11.2 m s⁻¹ in summer, 18.6 m s⁻¹ in autumn, and 7.0 m s⁻¹ in winter. We find that F10.7, QBO10, QBO30, and SAM all have significant correlations to the 12-hr tidal amplitudes at the 95% level, with a linear trend also present. Whereas we detect very minimal correlation with ENSO. These results suggest that variations in F10.7, the QBO and SAM may contribute significantly to the interannual variability of 12-hr tidal amplitudes in the Antarctic MLT.