Influence of ENSO on the QBO: Results froman ensemble of idealized simulations

Abstract

The El Niño-Southern Oscillation (ENSO) changes convection in the tropics and therefore the excitation of equatorial waves. Equatorial waves propagate from the troposphere upward where they drive the quasi-biennial oscillation (QBO) in the stratosphere. In this work, we analyze the effect of ENSO on the QBO utilizing an atmospheric general circulation model in a comprehensive experimental setup. We construct two ensembles of different QBO initial conditions, with the onset of a westerly (QBOW) and easterly (QBOE) jet at 10 hPa. In the course of an 18 months simulation period, the two sets of initial conditions experience each El Niño (EL) and La Niña (LA) boundary conditions. Due to the increased tropospheric temperatures during EL conditions compared to LA conditions, the experiments show an increase in tropospheric wave activity which increases QBO forcing in the stratosphere in EL. The underlying easterly jet in QBOW is weaker during EL compared to LA, while the underlying westerly jet in QBOE is stronger during EL compared to LA. On one hand, the weaker underlying jet in QBOW and the increase in QBO forcing due to waves cause a faster downward propagation for the westerly jet in QBOW during EL. On the other hand, the stronger underlying jet in QBOE opposes the increased QBO forcing due to waves for QBOE during EL. Therefore, the downward propagation speed of the easterly jet in QBOE is similar during EL and LA conditions. Changes in stratospheric tropical upwelling associated with EL and LA do not affect QBO properties in the simulation.