Kelvin waves and mixed Rossby--gravity waves play an important role in driving the observed quasi-biennial oscillation (QBO) in the tropical lower stratosphere. Based on observed estimates of their amplitudes, these two wave types alone are sufficient to drive a QBO in the absence of tropical upwelling. In the presence of upwelling, it is believed that small-scale gravity waves provide the additional momentum flux needed to drive the oscillation. Simulating the QBO in general circulation models (GCMs) using only resolved waves has been a notoriously difficult problem. For a variety of reasons, the wave driving associated with Kelvin and mixed Rossby--gravity-waves is generally underrepresented in current GCMs. Recently, QBOs have been generated in a number of GCMs through parameterized gravity waves in the tropics. However, this implies a more central role of small-scale gravity waves in driving the QBO than is thought to actually occur. This raises the question: What are the consequences of skewing the QBO wave driving away from the Kelvin and mixed Rossby--gravity waves to the small-scale gravity waves? One adverse effect may be a degradation of the structure of the semi-annual oscillation in the upper stratosphere and mesosphere.

In this study we use a mechanistic model to address this question by producing a QBO with two different types of wave driving. The first type, which is representative of the observed QBO, comprises realistic wave driving for the Kelvin and mixed Rossby--gravity waves as well as that from parameterized small-scale gravity waves needed to overcome tropical upwelling. The second type, which is representative of typical GCMs, comprises wave driving that is skewed toward the small-scale gravity waves. In order to gauge the sensitivity of our results to the manner in which the gravity waves are represented, two commonly used parameterizations are employed. The ultimate goal of our present line of research is to test the viability of parameterizing the underrepresented wave driving associated with Kelvin and mixed Rossby--gravity waves in GCMs and, ultimately, drive a more realistic QBO.