QBO Phase Synchronization in CMIP6 Historical Simulations Attributed to Ozone Forcing

Tropical stratospheric variability is investigated in ensembles of historical simulations performed by nine models for the sixth phase of the Coupled Model Intercomparison Project (CMIP6). Realizations from all the models feature a reasonable quasi‐biennial oscillation (QBO). Variability in the zonal mean zonal winds at the equator is found to be coherent among realizations when a model prescribes the CMIP6 ozone forcing. No such coherence is found when a model simulates ozone. The coherence results in an ensemble mean QBO signal with amplitude, depending on the model, 50%–80% of the mean QBO amplitude from a single realization from the same model. The ensemble mean signal is due to synchronization of QBO phases, attributed to the CMIP6 protocol including a QBO signal in the ozone forcing. Coherence in models using the CMIP6 ozone is, therefore, artificial and individual realizations from these models are not completely independent. Plain Language Summary High above the equator winds in the stratosphere at altitudes from ∼16–40 km repeatedly switch direction from eastward to westward, and back again, roughly every 14 months. This so‐called quasi‐biennial oscillation (QBO) featured in only a subset of the simulations of the historical period that provided input for the latest international assessments of climate change. Variability in the stratospheric winds at the equator in these simulations is found to be remarkably coherent across ensembles of realizations from the individual models, but only for those models that did not simulate ozone variability. Instead, these models followed a common protocol that specified prescribing an ozone distribution that included a QBO in the tropical stratosphere. For each of the models this causes the simulated QBOs in the winds to synchronize across many realizations of the historical period. Consequently variability in the equatorial stratosphere in these realizations becomes coherent. However, this coherence is not real as it is merely an artifact of the simulations following the agreed ozone protocol. Likewise, due to the design of the protocol, individual realizations from these models can not be considered as been entirely independent in the tropical stratosphere. Key Points Coupled Model Intercomparison Project phase 6 (CMIP6) historical simulations from several models feature a realistic quasi‐biennial oscillation (QBO) in the equatorial stratosphere QBO phases synchronize across ensembles of realizations from mod