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

Abstract 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 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 . 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.