The quasi-biennial oscillation (QBO) of tropical stratospheric winds was
disrupted during the 2019/20 Northern Hemisphere winter. We show that
this latest disruption to the regular QBO cycling was similar in many
respects to that seen in 2016, but initiated by horizontal momentum
transport from the Southern Hemisphere. The predictable signal
associated with the QBO’s quasi-regular phase progression is lost during
disruptions and the oscillation reemerges after a few months
significantly shifted in phase from what would be expected if it had
progressed uninterrupted. We infer from an increased wave-momentum flux
into equatorial latitudes seen in climate model projections that
disruptions to the QBO are likely to become more common in future.
Consequently it is possible that in future the QBO could be a less
reliable source of predictability on lead times extending out to several
years than it currently is.