Why was the quasi-biennial oscillation disrupted in 2016 and 2020?
In plain words
High above the equator, stratospheric winds flip between east and west roughly every 28 months, a pattern so regular it was used for forecasting. This rhythm broke unexpectedly in 2016 and again in 2020, and the reason and future frequency of such breaks are unclear.
Precise statement
Explain the 2015 to 2016 and 2019 to 2020 disruptions of the stratospheric quasi-biennial oscillation (QBO) as a response to extratropical Rossby-wave momentum fluxes reaching the equator, determine the preconditioning required, and predict whether disruptions become more frequent with warming. An answer is a mechanism reproduced in models with a realistic internally generated QBO and a projected disruption frequency.
What would settle it
Ensembles of high-top models with internally generated QBOs that reproduce both disruptions from observed initial conditions and agree on projected disruption statistics.
Status in the literature
Both events are attributed to equatorward Rossby-wave momentum fluxes from the Northern and Southern winter stratospheres respectively, but future frequency remains unknown.