In January 2020, unexpected easterly winds developed in the westerly quasi-biennial oscillation (QBO) phase. This event corresponds to the second QBO disruption in history, which occurred four years after the first disruption. According to several pre...
In January 2020, unexpected easterly winds developed in the westerly quasi-biennial oscillation (QBO) phase. This event corresponds to the second QBO disruption in history, which occurred four years after the first disruption. According to several previous studies, strong midlatitude Rossby waves propagating from the Southern Hemisphere (SH) during the austral winter likely initiated the disruption. However, the wave forcing that finally led to the disruption has not been investigated. Here, we examine the role of equatorial waves and small-scale convective gravity waves (CGWs) in the 2019/20 QBO disruption using 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˚N at ~50 hPa. In October-November 2019, wave forcing by the vertically (horizontally) propagating Rossby waves and mixed Rossby-gravity (MRG) waves began to increase (decrease). From December 2019, contribution of the MRG wave forcing to the zonal wind deceleration was the largest, followed by the Rossby wave forcing propagating from the Northern Hemisphere and the equatorial troposphere. In January 2020, CGWs provided 11% of the total negative wave forcing at ~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 waves were increased during the 2019/20 disruption. As in the 2015/16 QBO disruption, increased barotropic instability at the QBO edges generated more MRG waves in the lower stratosphere. In addition, westerly anomalies in the upper troposphere allowed more westward IG waves and CGWs to propagate into the QBO jet.