An unprecedented disruption of the Quasi-Biennial Oscillation (QBO) started to develop from late 2015. The early development of this event is analyzed using the space-time spectra of eddies from reanalysis data. While the extratropical waves propagating horizontally into the tropics were assumed to be the main driver for the disruption, it was not clear why these waves dissipated near the jet core instead of jet edge as linear theory predicts. This study shows that the drastic deceleration of the equatorial jet was largely brought about by a single strong wave packet, and the local winds experienced by the wave packet served as a better indicator of the wave breaking latitude than the zonal mean winds.

Surprisingly, tropical mixed Rossby gravity waves also made an appreciable contribution to the deceleration of the equatorial westerly jet by the horizontal eddy momentum fluxes, especially before January 2016. The horizontal eddy momentum fluxes associated with the tropical waves arise from the deformation of the wave structure when background westerlies increase with height. These horizontal eddy momentum anomalies from the tropical waves are commonly observed in the reanalysis data, but are typically much weaker than those in the 2015/2016 winter. The possibility exists that exceptionally strong equatorially trapped waves precondition the flow to disruption by an extratropical disturbance.