The influence of the tropical troposphere on the QBO in model simulations

The equatorial Quasi-Biennial Oscillation (QBO) and the El Niño-Southern Oscillation (ENSO) are major patterns of climate variability. Mutual interactions between these two modes have been highlighted in the observational record, but this has not been much tested using climate model simulations. In observations, the amplitude is larger during cold and smaller during warm events, while the descent rate is slower during cold and faster during warm events. Here we discuss how climate models represent this relationship, in terms of the changes of the QBO properties. The results for a multi-model ensemble, with atmosphere-only and ocean-atmosphere simulations, are discussed. The phase alignment of the QBO after the 1997/98 warm ENSO, already documented in previous works, is also present in a large ensemble of uncoupled experiments. It is found that a high horizontal resolution is needed to realistically reproduce the observed modulation of the QBO descent rate under strong ENSO events. The QBO amplitude response is instead weak at any horizontal resolution. Robust changes emerging in the projections and aspects of the modelled stratosphere-troposphere coupling in the tropics are presented.