The quasi-biennial oscillation (QBO) of the zonal mean wind is the primary mode of variability of the lower tropical stratosphere. It is understood that the QBO is forced by a combination of large scale and smaller-scale waves, however, the relative contributions from different types of waves are not clear. This study examines in detail the forcing of the QBO.
A better understanding of the QBO driving is important to the understanding of the variability of the stratosphere and the troposphere. We find here that Kelvin waves provide most of the momentum needed to drive the transition from easterly to westerly winds, whereas small-scale waves, which are parameterized in a general circulation model, provide the majority of the forcing for the westward phase of the QBO.
The QBO is a primary mode of variability in the tropical lower stratosphere. It’s forcing, especially the relative contributions from different wave types, are not well understood. It was found in this study that the increase in vertical resolution alone is not enough to generate a realistic QBO. Small-scale (~ 100 km) waves are needed to provide a portion of the QBO driving. Kelvin waves are the primary driver of the eastward phase, whereas parameterized small-scale gravity waves provide almost the entire forcing for the westward phase of the QBO.