​​Long-term Variability and Tendencies of the Semidiurnal Tide in Mesosphere and Lower Thermosphere from Meteor Radar Observations Over Esrange (67.9°N, 21.1°E)

Long-term variability and tendencies in monthly mean semidiurnal tide (12-hr) in zonal (U12) and meridional (V12) winds are investigated in northern polar mesosphere and lower thermosphere (MLT; ∼80–100 km) using meteor radar observations during 1999–2022 over Esrange (67.9°N, 21.1°E). The climatological mean of U12 and V12 amplitudes peak (up to ∼35 m/s) in winter (December–February) above ∼90–95 km with secondary maxima in late summer/early autumn (August–September), however the amplitude of V12 is larger than U12. The U12 and V12 exhibit strong interannual variability that varies with altitude and month/season. The responses of U12 and V12 anomalies (from 1999–2003) to solar cycle (SC), Quasi Biennial Oscillation (QBO) at 10 hPa and 30 hPa, El Niño-Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), ozone (O3) and carbon dioxide (CO2) are analyzed using multiple linear regression. From the analysis, significant correlations are found between monthly tidal amplitudes and the above potential drivers, and the correlations vary with altitude and month. The U12 and V12 responses to O3 are positive and significantly large (∼60–80 m/s/ppmv) below ∼85–90 km in February–March and above ∼95 km in January-March. The tidal response to ENSO is significantly negative during August-October (above ∼90 km) and positive in November-December (above ∼85 km) in both components. The cumulative trend in U12 is positive below ∼93 km and negative above this height peaking at ∼97 km. A positive trend in V12 increases above ∼93 km and maximize at ∼98 km. The significant monthly trends vary with altitude in both tidal components.