Seasonal Variation in Gravity Wave Momentum and Heat Fluxes in the Mesopause Region Observed by Sodium Lidar

The seasonal variations in gravity wave (GW) vertical fluxes of zonal momentum and heat in the mesopause region are calculated using nearly 10 years (from 2012 to 2021) of temperature and wind observations by a narrowband sodium lidar located in Hefei, China (32 degrees N, 117 degrees E). Except in summer, the GW zonal momentum fluxes are mostly westward, with similar to 80% contributed by the short-period (10 min-1 hr) GWs, and anticorrelate with the eastward zonal wind. The mean westward zonal momentum fluxes in spring and winter are -2.55 and -1.78 m(2)/s(2), respectively, which are much stronger than -0.98 m(2)/s(2) in fall. The momentum flux in summer is primarily eastward, dominated by long-period (1-16 hr) GWs. These seasonal variations in zonal momentum fluxes are mainly due to the filtering of GWs by the mean background wind at lower altitudes. The annual mean zonal momentum flux is westward with a value of -1.08 m(2)/s(2). The calculated acceleration from flux divergence is westward below 93 km, opposite to the eastward mean zonal wind. The heat flux below 94 km is downward throughout the year, with no apparent seasonal variation. The annual mean heat flux is primarily downward, with a cooling rate of similar to-35 K/day between 87 and 95 km.