Role of the Diurnal Tide in Driving the Westward Phase of the Mesospheric Semiannual Oscillation Around March Equinox

Abstract The mesospheric semiannual oscillation (MSAO) in tropical zonal‐mean zonal winds exhibits a strong westward phase around March equinox. It has been thought that the migrating diurnal tide (DW1) plays a role in the initiation of this phase because DW1 peaks around this time and satellite observations have shown it to undergo wave breaking. The high‐resolution (HR) Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension successfully simulates the strong westward phase of the MSAO in contrast with its coarser resolution counterpart. In this work, we show that the wave breaking of DW1, between about 85 and 100 km, serves as the dominant westward acceleration mechanism. However, between about 80 and 85 km, diurnal wave broadening and DW1 nonlinearity is responsible for the westward acceleration. These results demonstrate the utility of HR whole atmosphere modeling, which explicitly resolves small‐scale gravity waves, in revealing multiscale wave dynamics.