Preprints
https://doi.org/10.5194/acp-2021-85
https://doi.org/10.5194/acp-2021-85

  01 Mar 2021

01 Mar 2021

Review status: this preprint is currently under review for the journal ACP.

Contributions of equatorial planetary waves and small-scale convective gravity waves to the 2019/20 QBO disruption

Min-Jee Kang and Hye-Yeong Chun Min-Jee Kang and Hye-Yeong Chun
  • Department of Atmospheric Sciences, Yonsei University, Seoul, South Korea

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 four years after the first disruption that occurred in 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 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, 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.

Min-Jee Kang and Hye-Yeong Chun

Status: open (until 26 Apr 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-85', Anonymous Referee #1, 24 Mar 2021 reply
    • CC1: 'Reply on RC1', Min-Jee Kang, 25 Mar 2021 reply
  • RC2: 'Comment on acp-2021-85', Anonymous Referee #2, 30 Mar 2021 reply
    • CC2: 'Reply on RC2', Min-Jee Kang, 31 Mar 2021 reply

Min-Jee Kang and Hye-Yeong Chun

Min-Jee Kang and Hye-Yeong Chun

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Short summary
In winter 2019/20, the westerly quasi-biennial oscillation (QBO) phase was disrupted again by easterly winds. It is found that strong Rossby waves from the Southern Hemisphere weaken the jet core in early stages, and strong mixed Rossby–gravity waves reverse the wind in later stages. Inertia-gravity waves and small-scale convective gravity waves also provide negative forcing. These strong waves are attributed to an anomalous wind profile, barotropic instability, and slightly strong convection.
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