How ENSO Modifies the Quasi-Biennial Oscillation in a General Circulation Model

El Nino / Southern Oscillation (ENSO) and the Quasi-biennial Oscillation (QBO) in the stratosphere are two prominent tropical phenomena on the interannual timescale. 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 a 18 months simulation period, the two sets of initial conditions experience each El Nino (EL) and La Nina (LA) SSTs as boundary conditions.

Due to the increased tropospheric temperatures during EL conditions compared to LA conditions, the simulation shows an increase in tropospheric wave activity which increases QBO forcing in the stratosphere in EL. The underlying easterly jet of QBOW is weaker during EL compared to LA, while the underlying westerly jet of 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 of the westerly jet of QBOW during EL. On the other hand, the stronger underlying jet of QBOE opposes the increased QBO forcing due to waves for QBOE during EL. Therefore the downward propagation speed of the easterly jet of 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.