Revision 1

Accepted · first revision · record

Title: Why was the quasi-biennial oscillation disrupted in 2016 and 2020?
Plain: 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: 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.
Settled by: Ensembles of high-top models with internally generated QBOs that reproduce both disruptions from observed initial conditions and agree on projected disruption statistics.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: partially-resolved
Status note: Both events are attributed to equatorward Rossby-wave momentum fluxes from the Northern and Southern winter stratospheres respectively, but future frequency remains unknown.
Posed since: 2016