Interannual variability of the 12-hour tide in the mesosphere and lower
thermosphere in 15 years of meteor-radar observations over Rothera (68S,
68W)
Abstract
The solar tides of the mesosphere and lower thermosphere (MLT) show
great variability on timescales of days to years, with significant
variability at interannual timescales. However, the nature and causes of
this variability remain poorly understood. Here, we present measurements
made over the interval 2005 to 2020 of the interannual variability of
the 12-hour tide as measured at heights of 80 to 100 km by a meteor
radar over Rothera (68S, 68W). We use a linear regression analysis to
investigate correlations between the 12-hour 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 hPa and 30 hPa and the Southern
Annular Mode (SAM). Our observations reveal that the 12-hour tide has a
large amplitude and a clearly defined seasonal cycle with monthly mean
values as large as 35 ms-1. We observe substantial interannual
variability, with monthly mean 12-hour tidal amplitudes at 95 km
exhibiting a two standard-deviation range (2σ) in spring of 13.4 ms-1,
11.2 ms-1in summer, 18.6 ms-1 in autumn and 7.0 ms-1 in winter. We find
that F10.7, QBO10, QBO30 and SAM all have significant correlations to
the 12-hour 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-hour tidal amplitudes
in the Antarctic MLT.