The quasi-biennial oscillation (QBO) is a quasi-periodic
oscillation of the equatorial zonal wind between easterlies and
westerlies in the tropical stratosphere with a mean period of 28
to 29 months. The alternating wind regimes develop at the top of
the lower stratosphere and propagate downwards at about 1 km
(0.6 mi) per month until they are dissipated at the tropical
tropopause. Downward motion of the easterlies is usually more
irregular than that of the westerlies. The amplitude of the
easterly phase is about twice as strong as that of the westerly
phase. At the top of the vertical QBO domain, easterlies
dominate, while at the bottom, westerlies are more likely to be
found. The QBO was discovered in the 1950s, but its cause
remained unclear for some time. Rawinsonde soundings showed that
its phase was not related to the annual cycle, as is the case
for all other stratospheric circulation patterns. In the 1970s
it was recognized by Richard Lindzen and James Holton that the
periodic wind reversal was driven