Experiment design, nudging protocol, and models participating in Phase 2 of the APARC Quasi-Biennial Oscillation initiative (QBOi)

Abstract. We describe the protocol for coordinated experiments in phase-2 of the Quasi-Biennial Oscillation initiative (QBOi) and the participating models. The experiments involve nudging tropical stratospheric zonal-mean zonal winds in the models toward observations, enabling analysis of the impact of QBO biases on simulated teleconnections. Additionally, nudging the tropical winds allows investigation of the origins of QBO biases by examining the behaviour of resolved and parameterized equatorial waves under realistic equatorial wind shear conditions. We document here the scientific rationale and design of the nudging experiments, summarize the QBOi data request, and provide an overview of participating models. An initial evaluation is given of tropical stratospheric winds simulated by the multi-model ensemble for both nudged and free-running cases, and the overall impact of nudging on climatological aspects of the atmospheric circulation is examined.