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Discussion papers
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 02 Sep 2019

Submitted as: research article | 02 Sep 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

On the forcings of the unusual QBO structure in February 2016

Haiyan Li1,2,3, Robin Pilch Kedzierski1, and Katja Matthes1,4 Haiyan Li et al.
  • 1Marine Meteorology Department, GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
  • 2School of Atmospheric Sciences, Sun Yat-Sen University, Zhuhai, China
  • 3School of Electronic Information, Wuhan University, Wuhan, China
  • 4Faculty of Mathematics and Natural Sciences, Christian-Albrechts-Universität zu Kiel, Germany

Abstract. The westerly phase of the stratospheric Quasi-Biennial Oscillation (QBO) was reversed during Northern Hemisphere winter 2015/2016 for the first time since records began in 1953. Recent studies proposed that Rossby waves propagating from the extratropics played an important role during the reversal event in 2015/2016. Building upon these studies, we separated the extratropical Rossby waves into different wavenumbers and time-scales by analyzing the combined ERA-40 and ERA-Interim reanalysis zonal wind, meridional wind, vertical velocity and potential vorticity daily mean data from 1958 to 2017. We find that both synoptic and quasi-stationary Rossby waves are dominant contributors to the reversal event in 2015/2016 in the tropical lower stratosphere. By comparing the results for 2015/2016 with two additional events (1959/1960 and 2010/2011), we find that the largest differences in Rossby wave momentum fluxes are related to synoptic-scale Rossby waves of periods from 5–20 days. We demonstrate for the first time, that these enhanced synoptic Rossby waves at 40 hPa in the tropics in February 2016 originate from the extratropics as well as from local wave generation. The strong Rossby wave activity in 2016 in the tropics happened at a time with weak westerly zonal winds. This coincidence of anomalous factors did not happen in any of the previous events.

In addition to the anomalous behavior in the tropical lower stratosphere in 2015/16, we explored the forcing of the unusually long-lasting westerly zonal wind phase in the upper stratosphere (at 20 hPa). Our results reveal that mainly enhanced Kelvin wave activity contributed to this feature. This was in close relation with the strong El Niño event in 2015/2016, which forced more Kelvin waves in the equatorial troposphere. The easterly or very weak westerly zonal winds present around 30–70 hPa allowed these Kelvin waves to propagate vertically and deposit their momentum around 20 hPa, maintaining the westerlies there.

Haiyan Li et al.
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Haiyan Li et al.
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Short summary
The QBO westerly phase was reversed by an unexpected easterly jet near 40 hPa and the westerly zonal wind lasted an unusual long time at 20 hPa during winter 2015/16. We find that quasi-stationary Rossby wave W1 and faster Rossby wave W2 propagating from the northern extratropics and a locally generated Rossby wave W3 were important contributors to the easterly jet at 40 hPa. Our results suggest that the unusual zonal wind structure at 20 hPa could be caused by enhanced Kelvin wave activity.
The QBO westerly phase was reversed by an unexpected easterly jet near 40 hPa and the westerly...