The QBO Buffer Zone: Insights from a Hierarchy of Models

The Quasi-Biennial Oscillation (QBO) is a pattern of alternating zonal wind regimes in the tropical stratosphere which are forced by upward propagating waves. It extends from around 17km to 50km above the surface, leaving a gap, the “buffer zone”, between its lower limit and the effective source of the waves that provide the forcing (the upper troposphere). The explanation for this buffer zone has been a subject of recent research, notably by Match and Fueglistaler (2019, 2020). They use an idealised one-dimensional model of the QBO to conclude that the buffer zone is formed by mean-flow damping (likely due to horizontal momentum fluxes). We explore this mechanism in two- and three-dimensional models. By imposing mean-flow damping of various shapes and sizes on an idealised 2D (height-latitude) QBO model, we can induce formation of a buffer zone, as well as interesting behaviour not found in the 1D model. We also investigate whether the same behaviour occurs in a 3D GCM, where the horizontal momentum fluxes are due to resolved waves, rather than being imposed as an ad hoc damping. Our results thus far seem to strengthen recent theories on the buffer zone formation mechanism, and will contribute to better understanding of the dynamics at work.