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https://doi.org/10.5194/acp-2020-791
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-791
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  17 Aug 2020

17 Aug 2020

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A revised version of this preprint is currently under review for the journal ACP.

Role of equatorial planetary and gravity waves in the 2015–16 quasi-biennial oscillation disruption

Min-Jee Kang1, Hye-Yeong Chun1, and Rolando R. Garcia2 Min-Jee Kang et al.
  • 1Departmentof Atmospheric Sciences, Yonsei University, Seoul, South Korea
  • 2National Center for AtmosphericResearch, Boulder, Colorado, USA

Abstract. In February 2016, the descent of the westerly phase of the quasi-biennial oscillation (QBO) was unprecedentedly disrupted by the development of easterly winds. Previous studies have shown that extratropical Rossby waves propagating into the deep Tropics were the major cause of the 2015–16 QBO disruption. However, a large portion of the negative momentum forcing associated with the disruption still stems from equatorial planetary and small-scale gravity waves, which calls for detailed analyses by separating each wave mode compared with climatological QBO cases. Here, the contributions of resolved equatorial planetary waves [Kelvin, Rossby, mixed-Rossby gravity (MRG), and inertia-gravity (IG) waves] and small-scale convective gravity waves (CGWs) obtained from an offline CGW parameterization to the 2015–16 QBO disruption are investigated using MERRA-2 global reanalysis data from October 2015 to February 2016. In October and November 2015, anomalously strong negative forcing by MRG and IG waves weakened the QBO jet at 0°–5° S near 40 hPa, leading to Rossby wave breaking at the QBO jet core in the southern hemisphere. From December 2015 to January 2016, exceptionally strong Rossby waves propagating horizontally (vertically) continuously decelerated the southern (northern) flank of the jet. In February 2016, when the westward CGW momentum flux at the source level was much stronger than its climatology, CGWs began to exert considerable negative forcing at 40–50 hPa near the equator, in addition to the Rossby waves. The enhancement of the negative wave forcing in the Tropics stems mostly from strong wave activity in the troposphere associated with increased convective activity and the strong westerlies (or weaker easterlies) in the troposphere, except that the MRG wave forcing is more likely associated with increased barotropic instability in the lower stratosphere.

Min-Jee Kang et al.

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Min-Jee Kang et al.

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Latest update: 21 Oct 2020
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Short summary
In 2015/2016 winter, the descent of the westerly QBO jet was unprecedentedly interrupted by the easterly winds. We find that mixed-Rossby gravity and inertia-gravity waves weaken the jet core in the early stages, and small-scale convective gravity waves, as well as horizontal and vertical components of Rossby waves, reverse the wind sign in the later stages. The strong negative wave forcing in the Tropics results from the enhanced convection, anomalous wind profile, and barotropic instability.
In 2015/2016 winter, the descent of the westerly QBO jet was unprecedentedly interrupted by the...
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