  The Quasi-Biennial Oscillation (QBO) of stratospheric winds is the most
striking example of mean-flow generation and reversal by the non-linear
interactions of internal waves. Previous studies have used an idealized
monochromatic forcing to investigate the QBO. Here we instead force a more
realistic continuous wave spectrum. Unexpectedly, spreading the wave energy
across a wide frequency range leads to more regular oscillations. We also find
that different forcing spectra can yield the same QBO. Multi-modal wave forcing
is thus essential for understanding wave/mean-flow interactions in nature.
