Influence of Diurnal Tide on the Low-Latitude UMLT Mean Zonal Wind: Evidence From Momentum Flux Estimation Using ICON/MIGHTI Winds

Abstract

The influence of migrating 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 a distinct semiannual oscillation (SAO) with large westward winds found during January–March and September–December, varying in intensity (20–40 m/s) across all three years from 91 to 103 km. The diurnal tidal amplitude in meridional wind (DTV) reported equinoctial maximum (∼80–100 m/s) and solstitial minimum (∼10–30 m/s), revealing similar SAO found in mean zonal wind. The seasonal variation of westward acceleration, induced by the vertical gradient of meridional flux of zonal momentum (F_meridional), peaks during January-March (18–43 m/s/day) and September-December (40–55 m/s/day), exhibiting an equinoctial enhancement analogous to the westward wind intensity in mesospheric SAO. This quantitatively demonstrates the significance of momentum flux deposition by diurnal tides in driving the MSAO above 91 km using ICON/MIGHTI wind observations for the first time. The magnitude of westward acceleration (m/s/day) induced by F_meridional exceeds the convergence of vertical flux of zonal momentum (F_zonal) due to diurnal tides from January to March, while the westward acceleration induced by both F_zonal and F_meridional are found to be larger and comparable during September–December.