  The Quasi-Biennial Oscillation (QBO) is understood to result from
wave-mean-flow interactions, but the reasons for its relative stability remain
a subject of ongoing debate. In addition, consensus has yet to be reached
regarding the respective roles of different equatorial wave types in shaping
the QBO's characteristics. Here we employ Holton-Lindzen-Plumb's quasilinear
model to shed light on the robustness of periodic behavior in the presence of
multiple wave forcings. A comprehensive examination of the various dynamical
regimes in this model reveals that increased vertical wave propagation at
higher altitudes favors periodicity. In the case of single standing wave
forcing, enhanced vertical propagation is controlled by the wave attenuation
length scale. The occurrence of non-periodic states at high forcing amplitudes
is explained by the excitation of high vertical unstable modes. Increasing the
attenuation length scale prevents the emergence of such modes. When multiple
wave forcing is considered, the mean flow generated by a dominant primary wave
facilitates greater vertical propagation of a perturbation wave. Raising the
altitude where most of the wave damping occurs favors periodicity by preventing
the development of secondary jets responsible for the aperiodic behavior. This
mechanism underscores the potential role of internal gravity waves in
supporting the periodicity of a QBO primarily driven by planetary waves.
