Interannual variability of the 12-hour tide in the mesosphere and lower thermosphere in 15 years of meteor-radar observations above Rothera (68°S, 68°W)

The wind field in the mesosphere and lower thermosphere (MLT), at heights between 80 and 100 km, is dominated by the global scale oscillations of atmospheric tides. The 12-hour tide is crucial to the middle and upper atmosphere dynamics and hence vital to understanding the coupling between the lower atmosphere and space. The 12-hour tide shows considerable variability on timescales of days to years, with significant variability at interannual timescales. However, the nature and causes of this interannual 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 – 100 km by a meteor radar over the British Antarctic Survey base at Rothera (68°S, 68°W). We use 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, the Southern Annular Mode (SAM) and also investigate any linear trends.

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 tidal amplitudes at 95 km exhibiting a difference between the 10th and 90th percentiles in spring of 17.2 ms-1, 12.6 ms-1 in summer, 23.6 ms-1 in autumn and 9.0 ms-1 in winter. We find that F10.7, QBO10, QBO30 and SAM have significant correlations at the 95% level, with a linear trend also apparent at the 95% significance level. In contrast, we detect very minimal correlation with ENSO. There is a significant negative correlation between F10.7 solar flux and tidal amplitudes in summer, implying that an increase in solar flux is related to a decrease in monthly mean 12-hour tidal amplitudes in the MLT. These results suggest that the solar cycle, QBO and SAM can modulate the amplitude of the polar 12-hour tide.