  The quasi-biennial oscillation (QBO) of equatorial winds on Earth is the
clearest example of the spontaneous emergence of a periodic phenomenon in
geophysical fluids. In recent years, observations have revealed intriguing
disruptions of this regular behaviour, and different QBO-like regimes have been
reported in a variety of systems. Here we show that part of the variability in
mean flow reversals can be attributed to the intrinsic dynamics of wave-mean
flow interactions in stratified fluids. Using a constant-in-time monochromatic
wave forcing, bifurcation diagrams are mapped for a hierarchy of simplified
models of the QBO, ranging from a quasilinear model to fully nonlinear
simulations. The existence of new bifurcations associated with faster and
shallower flow reversals, as well as a quasiperiodic route to chaos are
reported in these models. The possibility for periodicity disruptions is
investigated by probing the resilience of regular wind reversals to external
perturbations.
