The Buffer Zone of the Quasi-Biennial Oscillation: Formation and Variability

The quasi-biennial oscillation (QBO) is a pattern of descending easterly and westerly winds in the tropical stratosphere. QBO power decreases sharply below 70 hPa, forming a buffer zone between the lower extent of the QBO and the source of the vertically-propagating waves that drive it. Previous studies have shown that in a 1D QBO model, mean upwelling in the tropical lower stratosphere could form a buffer zone. The mean-upwelling mechanism for buffer zone formation has been invoked to argue that increasing trends in mean upwelling with global warming have caused a weakening trend in the QBO power in the buffer zone. Yet, we find that QBO power has recovered significantly since low values in the 2000s, undermining the case for a secular decreasing trend. Furthermore, there is not clear covariability between upwelling and QBO power. Revisiting buffer zone theory, we find three mechanisms in the 1D QBO model that can form a buffer zone: upwelling (as previously reported), diffusivity, and damping. But, upwelling and diffusivity only form buffer zones by exchanging momentum with the zero-wind lower boundary condition of the model; if imposed away from the artificial zero-wind boundary condition, upwelling and diffusivity fail to form buffer zones. Only damping forms a buffer zone spontaneously. In light of our updated mechanistic understanding, we investigate the formation of the buffer zone in MERRA-2 and ERA-Interim, finding that damping-like momentum fluxes sharply increase below 70 hPa and are therefore implicated in the formation of the buffer zone. Horizontal eddy momentum fluxes play a particularly strong role in damping QBO angular momentum anomalies in the buffer zone. Tropospheric expansion associated with global warming is expected to lift the profile of damping-like horizontal momentum fluxes to lower pressure, decreasing the QBO power in the buffer zone. We investigate whether a signal due to tropospheric expansion has emerged above the large interannual variability of momentum fluxes and QBO power in the buffer zone.