Mean-flow damping forms the buffer zone of the quasi-biennial oscillation: 1D theory

The quasi-biennial oscillation (QBO) is a descending pattern of alternating easterly and westerly winds in the tropical stratosphere. Upwelling is generally understood to counteract the descent of the QBO. The upwelling hypothesis holds that where upwelling exceeds the intrinsic descent rate of the QBO, the QBO cannot descend and a buffer zone forms. Descent-rate models of the QBO, which represent a highly simplified evolution of a QBO wind contour, support the upwelling hypothesis. Here, we show that the upwelling hypothesis and descent-rate models only correctly describe buffer zone formation in the absence of wave dissipation below critical levels. When there is wave dissipation below critical levels, the 1D QBO response to upwelling can be either to 1) reform below the upwelling, 2) undergo period-lengthening collapse, or 3) expand a preexisting buffer zone. The response depends on the location of the upwelling and the lower boundary condition. Mean-flow damping always forms a buffer zone. A previous study of reanalyses showed that there is mean-flow damping in the buffer zone due to horizontal momentum flux divergence. Therefore, the 1D model implicates lateral terms in buffer zone formation that it cannot self-consistently include.