Influence of diurnal tide on the low-latitude UMLT mean zonal wind: Evidence from momentum flux estimation using ICON-MIGHTI winds

The influence of dissipating solar diurnal tides in driving the mean zonal wind in the upper mesosphere and lower thermosphere (UMLT) is investigated using the zonal and meridional winds observed by the Michelson Interferometer for Global High-Resolution Thermospheric Imaging (MIGHTI) instrument onboard the Ionospheric Connection Explorer (ICON) satellite over the region of interest having a latitudinal and longitudinal extent of 5° N - 15°N and 67.5°E - 90°E, respectively, for the years 2020, 2021 and 2022. The mean zonal wind exhibits consistent seasonal variation with large westward winds at 91-103 km during January-March and September-December, however with varying intensity (20-40 m/s) in all the three years. The diurnal tidal amplitude in meridional wind (DTV) also displays similar seasonal variation with maximum amplitudes reaching ~80–100 m/s. The seasonal variation of westward acceleration due to diurnal tide momentum deposition is found to be maximum during January-March (18-43 m/s/day) and September-December (40-55 m/s/day) and reveals similar seasonal variation and intensity of the mean westward winds. This clearly indicates that the potential role of diurnal tide in driving the mean zonal flow.  The westward acceleration induced by the vertical gradient of meridional flux of zonal momentum (F_meridional) due to diurnal tide exceeds the convergence of vertical flux of zonal momentum (F_zonal) due to diurnal tide during January-March, while the westward acceleration induced by both F_zonal and F_meridional are larger and comparable during September-December.