Revisiting the Quasi-Biennial Oscillation

The quasi-biennial oscillation (QBO), as the dominant mode of interannual variability in the tropical stratosphere, is revisited using ERA5 reanalysis dataset. The data cover a 40-year period (1979 to 2018), including 18 QBO cycles, and has a high vertical resolution through the lower and middle stratosphere. This study aims to provide a general overview of the structure and dynamics of the QBO, and a deeper insight into the wave-mean flow interaction in the stratosphere. It is shown that the Kelvin waves amplify the QBO through their activity in the westerly shear zones by producing strong upward fluxes of westerly momentum, while the MRG waves cause the QBO to propagate downward by producing downward flux of westerly momentum. The extratropical Rossby waves erode the QBO-related zonal wind perturbations by extracting westerly momentum from the flanks of westerly regimes and transporting it poleward.