The Stratosphere–troposphere Processes And their Role in Climate (SPARC) Quasi-Biennial Oscillation initiative (QBOi) aims to improve the fidelity of tropical stratospheric variability in general circulation and Earth system models by conducting coordinated numerical experiments and analysis. In the equatorial stratosphere, the QBO is the most conspicuous mode of variability. Five coordinated experiments have therefore been designed to (i) evaluate and compare the verisimilitude of modelled QBOs under present-day conditions, (ii) identify robustness (or alternatively the spread and uncertainty) in the simulated QBO response to commonly imposed changes in model climate forcings (e.g. a doubling of CO2 amounts), and (iii) examine model dependence of QBO predictability. This paper documents these experiments and the recommended output diagnostics. The rationale behind the experimental design and choice of diagnostics is presented. To facilitate scientific interpretation of the results in other planned QBOi studies, consistent descriptions of the models performing each experiment set are given, with those aspects particularly relevant for simulating the QBO tabulated for easy comparison.

The design of the experiments described here grew out of community discussions at the first QBOi workshop in March 2015 in Victoria, Canada. Funding for the workshop from the UK Natural Environment Research Council (NE/M005828/1), the World Climate Research Programme (WCRP), Stratosphere– troposphere Processes And their Role in Climate (SPARC) activity, and the Canadian Centre for Climate Modelling and Analysis is gratefully acknowledged. We further acknowledge the scientific guidance of the WCRP for helping motivate this work, coordinated under the framework of the SPARC QBO initiative (QBOi) led by James Anstey, Neal Butchart, Kevin Hamilton, and Scott Osprey. The Centre for Environmental Data Analysis (CEDA) have very kindly offered to host the QBOi data archive. Neal Butchart and Adam Scaife were supported by the Joint UK BEIS/Defra Met Office Hadley Centre Climate Programme (GA01101). Scott Osprey and Lesley Gray were supported by NERC projects NE/M005828/1 and NE/P006779/1. Shingo Watanabe and Yoshio Kawatani used the Earth simulator for QBOi simulations and were supported by the SOUSEI programme, MEXT Japan, and the Japan Science and Technology Agency (JST) as part of the Belmont Forum. Yoshio Kawatani was supported by Grant-in-Aid for Scientific Research B (26287117), joint international research (15KK0178) from the Japan Society for the Promotion of Science, and the Environment Research and Technology Development Fund (2-1503) of the Ministry of the Environment, Japan. Francois Lott and Scott Osprey were supported by the ANR/JPI-Climate/Belmont Forum project GOTHAM (ANR-15-JCLI-0004-01). Federico Serva was supported by the European Commission under grant StratoClim-603557-FP7-ENV.2013.6.1-2, with computing resources for the ECHAM5sh simulations provided by an ECMWF special project. Young-Ha Kim was supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (NRF-2015R1C1A1A02036449). Holger Pohlmann was supported by the German Federal Ministry for Education and Research (BMBF) project MiKlip (FKZ 01LP1519A) and thanks Elisa Manzini for providing additional information on the MPI model. BSC contribution is supported by the Spanish MINECO-funded DANAE project (CGL2015-68342-R) and Red Española de Supercomputación (RES project AECT-2017-3-0015).

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