Abstract
Abstract. 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∘ N at ∼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 ∼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.
Highlights
The quasi-biennial oscillation (QBO) was first recorded through radiosonde wind observations in 1953 (Ebdon, 1960; Reed et al, 1961; Naujokat, 1986)
The enhanced equatorial wave forcing contributed to the 2015/16 QBO disruption, which was first mentioned by Lin et al (2019) and analyzed in detail by Kang et al (2020; KCG20 hereafter), who investigated each type of equatorial wave and small
We provide a comprehensive overview of the 2019/20 QBO disruption by examining all the equatorial waves (Kelvin, Rossby, mixed Rossby–gravity (MRG), and IG waves) and smallscale convective gravity waves (CGWs) as in KCG20
Summary
The quasi-biennial oscillation (QBO) was first recorded through radiosonde wind observations in 1953 (Ebdon, 1960; Reed et al, 1961; Naujokat, 1986). Because the QBO phase is highly correlated with extratropical and tropospheric phenomena, the impact of the disarrangement of the westerly QBO phase by the sudden development of the easterly winds was not limited to the equatorial stratosphere (Tweedy et al, 2017). The 2015/16 QBO disruption was primarily caused by equatorially propagating Rossby wave forcing. The large magnitude of the Rossby wave flux in the Northern Hemisphere (NH) midlatitudes (Osprey et al, 2016; Coy et al, 2017; Hirota et al, 2018) and its increased amount of equatorward propagation by the strong subtropical westerlies in the lower stratosphere (Barton and McCormack, 2017) likely induced the QBO disruption. The enhanced equatorial wave forcing contributed to the 2015/16 QBO disruption, which was first mentioned by Lin et al (2019) and analyzed in detail by Kang et al (2020; KCG20 hereafter), who investigated each type of equatorial wave and small-
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