Normal Mode Perspective on the 2016 QBO Disruption: Evidence for a Basic State Regime Transition

A normal mode decomposition of the dynamical structure of the quasi-biennial oscillation (QBO) is presented in this article. It was found that the QBO is predominantly described by Rossby modes as well as westward inertio-gravity modes with a low meridional mode index. We show that the 2016 QBO disruption was accompanied by an extreme amplitude event of an asymmetric zonal Rossby mode (with meridional index 4) consisting of the largest amplitude in 40 years. This component of the zonal flow favors the convergence of eastward momentum in the equatorial region, controlling the perturbations to the QBO. The zonal winds associated with this mode are extremely intermittent with heavy tail distributions. Our statistical analysis indicates a regime transition in this zonal mode in the recent decades, favoring stronger perturbations to the QBO.