Migrating diurnal tide anomalies during QBO disruptions in 2016 and 2020: morphology and mechanism

Abstract. The stratosphere Quasi-Biennial Oscillation (QBO) modulates the migrating diurnal tide (DW1) in the mesosphere and lower thermosphere (MLT). DW1 amplitudes are larger during QBO westerly (QBOW) than during easterly (QBOE) phases. Since QBO's discovery in 1953, two rare QBO disruption events occurred in 2016 and 2020. During these events, anomalous westerly winds propagate upward, disrupting normal downward propagation of easterly phase and producing a persistent westerly wind layer. In this study, global responses of DW1 amplitudes and phases in MLT to these QBO disruptions, as well as their underlying mechanisms are investigated, using SABER/TIMED observations, MERRA-2 reanalysis and SD-WACCM-X simulations. Similarity of the DW1 responses to these two events is that DW1 phases and wavelengths exhibit close results to QBOW, whereas amplitudes lie between QBOW and QBOE. DW1 amplitudes in equatorial MLT (maximum difference) vary by 20.5 % and −10.2 % relative to QBOE and QBOW in 2016 event, but by 6.0 % and −21.1 % in 2020 event. In 2016 event, water-vapor radiative and latent heating, jointly modulated by the event and ENSO, increase significantly relative to QBOE. Less dissipation and less tidal energy removal in stratosphere along with gravity-wave (GW) drag in mesosphere tend to enhance DW1 amplitudes. In contrast, in 2020 event, water-vapor heating exhibits a 5 % increase. The dissipation and tidal energy removal has adverse effect on DW1, while GW drag exerts a weaker influence. The enhancement of water vapor heating together with the weaker GW drag likely accounts for the weaker enhancement of DW1 during this event.