Treffer: The Effect of Barotropic Instability on Mixed Rossby‐Gravity Wave Variability During the QBO Phases.

The traditional view posits that vertically propagating mixed Rossby‐gravity (MRG) waves generated in the troposphere partly contribute to driving the Quasi‐Biennial Oscillation (QBO) in the tropical lower stratosphere. However, recent studies suggest that MRG waves may be generated locally within the QBO region, potentially via barotropic instability. This study supports this alternative mechanism by showing that the tropical zonal mean flow at 30 hPa in ERA5 reanalysis satisfies the necessary conditions for barotropic instability approximately in 60% of cases, increasing to about 80% during the westerly QBO phase − ${-}$ twice as frequent as during the easterly phase. Additionally, the MRG wave kinetic energy spectra at 30 hPa show increased energy at zonal wavenumbers k>5 $k > 5$ in the westerly QBO phase compared to the easterly phase. The linear analysis within the QBO region reveals that MRG waves contribute between 4 and 14 times more to the total energy of the unstable modes during the westerly QBO phase, for zonal wavenumbers 6≤k≤12 $6\le k\le 12$. Idealized numerical simulations of the QBO flow demonstrate that the enhanced MRG wave spectral power at synoptic and sub‐synoptic scales during the westerly QBO phase results from barotropic instability development. Together, these results suggest that the stronger MRG wave generation observed during the westerly QBO arises from both the greater frequency of unstable flow conditions and the higher MRG wave energy in the unstable modes. Plain Language Summary: The Quasi‐Biennial Oscillation (QBO) can influence tropical lower stratospheric wind and significantly impacts seasonal weather prediction. Previous studies suggest that mixed Rossby‐gravity (MRG) waves, which propagate vertically from the upper troposphere, help drive the QBO. However, modeling studies indicate that instability in the QBO‐related zonal flow can also generate local MRG waves, potentially affecting MRG wave variability in the QBO region, alongside vertical wave propagation. Since it is unclear how often the QBO‐related zonal flow becomes unstable, we conducted a 10‐year survey and found that the QBO region is unstable in about 60% of cases, with twice as many instability events during the westerly QBO phase than the easterly phase. Furthermore, our linear analysis of unstable modes reveals a greater likelihood of synoptic‐ and sub‐synoptic‐scale MRG wave growth during the westerly QBO phase. These MRG wave scales, which are sensitive to the instability of the QBO flow, are also confirmed by idealized numerical simulations. Our results may help explain differences in MRG wave energy between the QBO phases observed in reanalysis data and provide evidence for local MRG wave generation in the stratosphere. Key Points: The synoptic‐scale MRG waves at 30 hPa in ERA5 data are more energetic during the westerly than the easterly QBO phaseThe mean zonal flow at 30 hPa is frequently barotropically unstable, especially during the westerly QBO phaseBarotropic instability development greatly enhances the MRG wave growth in idealized simulations with realistic westerly background jets [ABSTRACT FROM AUTHOR]

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