Contributions of equatorial waves and small-scale convective gravity waves to the 2019/20 quasi-biennial oscillation (QBO) disruption

In January 2020, unexpected easterly winds developed in the downward-propagating westerly quasi-biennial oscillation (QBO) phase. This event corresponds to the second QBO disruption in history, and it occurred 4 years after the first disruption of 2015/16. According to several previous studies, strong midlatitude Rossby waves propagating from the Southern Hemisphere (SH) during the SH winter likely initiated the disruption; nevertheless, the wave forcing that finally led to the disruption has not been investigated. In this study, we examine the role of equatorial waves and small-scale convective gravity waves (CGWs) in the 2019/20 QBO disruption using Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA-2) global reanalysis data. In June-September 2019, unusually strong Rossby wave forcing originating from the SH decelerated the westerly QBO at 0-5 degrees N at similar to 50 hPa. In October-November 2019, vertically (horizontally) propagating Rossby waves and mixed Rossby-gravity (MRG) waves began to increase (decrease). From December 2019, the contribution of the MRG wave forcing to the zonal wind deceleration was the largest, followed by the Rossby wave forcing originating from the Northern Hemisphere and the equatorial troposphere. In January 2020, CGWs provided 11% of the total negative wave forcing at similar to 43 hPa. Inertia-gravity (IG) waves exhibited a moderate contribution to the negative forcing throughout. Although the zonal mean precipitation was not significantly larger than the climatology, convectively coupled equatorial wave activities were increased during the 2019/20 disruption. As in the 2015/16 QBO disruption, the increased barotropic instability at the QBO edges generated more MRG waves at 70-90 hPa, and westerly anomalies in the upper troposphere allowed more westward IG waves and CGWs to propagate to the stratosphere. Combining the 2015/16 and 2019/20 disruption cases, Rossby waves and MRG waves can be considered the key factors inducing QBO disruption.

Automated summary

The unexpected easterly winds in the downward-propagating westerly quasi-biennial oscillation (QBO) phase in January 2020 marked the second QBO disruption in history, occurring 4 years after the first disruption in 2015/16. Equatorial waves and small-scale convective gravity waves were found to play key roles in the 2019/20 QBO disruption, with different wave forcings contributing at various stages. Increased convectively coupled equatorial wave activities and barotropic instability at QBO edges were identified as factors inducing QBO disruption in both 2015/16 and 2019/20 cases, with Rossby waves and mixed Rossby-gravity waves considered crucial in this process.