01 Mar 2021
01 Mar 2021
Contributions of equatorial planetary waves and small-scale convective gravity waves to the 2019/20 QBO disruption
- Department of Atmospheric Sciences, Yonsei University, Seoul, South Korea
- 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.
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Min-Jee Kang and Hye-Yeong Chun
Status: open (until 26 Apr 2021)
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RC1: 'Comment on acp-2021-85', Anonymous Referee #1, 24 Mar 2021
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The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2021-85/acp-2021-85-RC1-supplement.pdf
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CC1: 'Reply on RC1', Min-Jee Kang, 25 Mar 2021
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The authors would like to thank reviewer1 for providing valuable comments.
We will post responses to the reviewer's comments and the manuscript modified accordingly at the end of the discussion period.
Sincerely,
Min-Jee Kang
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CC1: 'Reply on RC1', Min-Jee Kang, 25 Mar 2021
reply
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RC2: 'Comment on acp-2021-85', Anonymous Referee #2, 30 Mar 2021
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The paper "Contributions of equatorial planetary waves and small-scale convective gravity waves to the 2019/20 QBO disruption" by Kang et al. investigates which waves contribute to the 2019/20 QBO disruption during the different stages of the disruption. It turns out that in the first phase Rossby waves from the Southern Hemisphere are the leading contribution, and in the later stage tropical MRG waves and Rossby waves from the Northern Hemisphere are the main contributions.
The paper is a follow-up work of a previous paper on the 2015/16 QBO disruption and structured in a similar way for better comparability.
The paper is well written and fits in the scope of ACP and is therefore recommended for for publication in ACP after minor revisions.
Minor Comments:(1) At the beginning of the introduction you should mention the papers by Ebdon (1960) and Reed et al. (1961), who independently discovered the QBO.
Ebdon, R. A.:
Notes on the wind flow at 50 mb in tropical and subtropical regions in January 1957 and in 1958,
Q. J. R. Meteorol. Soc., 86, 540-542, 1960.Reed, R. J., Campbell, W. J., Rasmussen, L. A., and Rogers, R. G.:
Evidence of a downward propagating annual wind reversal in the equatorial stratosphere,
J. Geophys. Res., 66, 813-818, 1961.
(2) Most parameters in Eq.(1) and Eq.(2) are not explained.
(3) l.95-104: please describe briefly how the k-omega spectra are calculated
(4) l.129/Fig.1b: The positive wind shear anomaly compared to the climatology at pressures 150-100hPa does no longer hold for January 2020 - in January 2020 the wind shear has a negative anomaly.
And for the other months July-December 2019 at pressures 150-100hPa the wind anomaly is easterly, not westerly, compared to the climatology!
(5) l.145/146 (Fig.2e): Not clear which Rossby wave forcing you exactly mean. There are no anomalously strong negative values of EPFD at 0-5N in magenta stippeled regions. There are several anomalies (magenta stippled regions) which, however, do not really fit to your statement:
5S/50hPa - medium strong EPFD, -2...-4 m/s/month
5N/30hPa - there is only very weak EPFD of -1...-2 m/s/month
10-20N/50-80hPa - very strong EPFD, stronger than -10 m/s/month
Do you mean strong negative, but not anomalously strong EPFD?
If yes, why would this be relevant?
Please explain in more detail.
(6) l.243 / Fig.7:
The MRG waves are deduced from antisymmetric k-omega spectra, and in Fig.7 symmetry relative to the equator would be expected. Nevertheless, the EPF and EPFD in Fig.7 are asymmetric. Where does this come from? Because zonal wind and stability are different in the different hemispheres?
(7) l.265/266: Your statement is not correct!
The barotropic term is only the second term in Eq.(3), not the first two terms.
The first term in Eq.(3) is "beta", which is always positive and acts to stabilize the zonal flow.
The second term is the barotropic term. If this term dominates and leads to negative dQ/dphi, this is an indication for barotropic instability.
(8) l.268-271 / Fig.9: You should mention that the largest difference relative to the climatology is in September 2019!
In this month at 5S-5N precipitation is much weaker (by 2...3 sigma!) than the climatology.
Do you think this strong anomaly plays a role in the QBO disruption?
(9) In Fig.10 there are many regions that are above the climatology by more than 1 sigma (blue stippled), but that are at the same time in the light blue range of the color code that is considered insignificant by the t-test.
Still, these enhancements are discussed as "widening of the spectrum" in December (l.278/279).
Can you comment on this?Technical Comments:
l.74:
The 2019/20 QBO disruption was originally in the westerly QBO phase.
->
The QBO was originally in the westerly phase when the 2019/20 QBO disruption happened.
l.76: is greater than -> is more westerly than
l.79: downward QBO phase transition -> downward QBO phase progression ??
l.92-94: in l.92 Fz consists of three terms! Please check definitions of Fz1 and Fz2! Probably one pair of parentheses is missing in l.92
Fig.4a: the dotted line is hard to distinguish from the solid line
Fig.7: for consistency, please add the multiplication factors to the respective panels in Fig.7, similar as in Fig.5d.
caption of Fig.8: barotropic instability -> meridional potential vorticity gradient
l.278:
The spectrum more than 1sigma stronger than the climatology
->
The areas of the spectrum where values are more than 1sigma stronger than the climatology
l.280:
the strong power is evident in the spectrum related
->
areas of strong power that are evident in the spectrum are related
l.314: slightly strong -> slightly stronger
l.319/320:
The maximum negative CGWD
->
At pressures above 40hPa the maximum negative CGWD
l.369:
oval structure of the zonal wind was ...
->
the oval-shaped structure of the QBO westerlies that is seen in latitude-altitude cross sections was ...-
CC2: 'Reply on RC2', Min-Jee Kang, 31 Mar 2021
reply
The authors would like to thank reviewer1 for providing valuable comments.
We will post responses to the reviewer's comments and the manuscript modified accordingly at the end of the discussion period.
Sincerely,
Min-Jee Kang
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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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