QBOi – Towards Improving the Quasi-Biennial Oscillation in Global Climate Models

Few global climate models (GCMs) simulate realistic tropical stratosphere variability, most conspicuously the quasi-biennial oscillation (QBO) – believed to have the longest naturally occurring timescale within the climate system that is due to internal atmospheric processes. This tropical phenomenon is important for the redistribution of ozone and other trace gases, and for teleconnections linked with high latitude weather, such as the North Atlantic Oscillation (NAO), and the tropics, including the Madden-Julian Oscillation (MJO). Due to the QBO’s long timescale, QBO teleconnections could provide an additional source of predictability in models that can simulate them accurately.

Of the climate models participating in the Chemistry-Climate Model Validation Activity Phase 2, only two reported an internally generated QBO, seven chose nudging toward observations, while the remainder had no realistic QBO variability. In the WCRP Coupled Model Intercomparison Project – Phase 5 (CMIP5), only five models captured a QBO, while this number increased to 15 in CMIP6 (Richter et al. 2020). In a key study exploring the effects of future climate change on tropical stratosphere variability, Kawatani and Hamilton (2013) reported a weakening of the QBO amplitude into the 21st Century in CMIP5 models, a result that has been upheld in later generations of models (Richter et al. 2020, Butchart et al. 2020). In contrast, climate models do not agree on projected changes to the QBO period.

The goal of QBOi is to improve simulations of the QBO and its teleconnections in GCMs. This is achieved primarily by conducting coordinated experiments with multi-model ensembles and coordinated analyses of these experiments. QBOi aims to be a community-driven activity, soliciting the participation of modelling groups to design numerical experiments to better understand modelling uncertainties that affect simulation of the QBO (and its teleconnections). These modelling sensitivities include factors such as vertical and horizontal resolution, parameterisations of gravity wave drag and tropical convection, and ozone chemistry.

QBOi is focused on modelling studies, but benefits from other APARC activities including Gravity Waves (e.g., for constraining parameter estimates within GCMs) and the APARC Reanalysis Intercomparison Project  (A-RIP; e.g., for estimates of observed QBO wave driving from reanalyses). Output from QBOi may also benefit programmes such as the Working Group for Numerical Experimentation (evaluating process uncertainty), the Working Group on Seasonal to Interannual Prediction (identifying pathways for predictability). Other related activities within APARC include CCMI (tracer transport), SNAP (predictability), and DynVar (stratosphere-troposphere coupling).

QBO Phase 1 began with a workshop that agreed on a set of coordinated multi-model experiments and essential diagnostics (QBO Modelling and Reanalysis Workshop, 16-18 March 2015, Victoria, Canada, reported in SPARC Newsletter 45). Second and third workshops were held in 2016 (Oxford) and 2017 (Kyoto, joint with SATIO-TCS and FISAPS), as reported in SPARC newsletters no. 48 and 50. These workshops focused on analysis of the Phase-1 experiments, and an extension of these experiments to examine ENSO impacts on the QBO and its teleconnections. Publication of the core papers in a QJRMS Special Issue on QBO modelling intercomparison completed QBOi Phase 1. A brief overview of results from these studies is given in SPARC Newsletter 57.

Planning for Phase 2 of QBOi began at the 2018 SPARC General Assembly, motivated by the need to address outstanding questions identified in Phase 1. An online workshop during the COVID-19 pandemic resulted in a new set of coordinated experiments that constrain the tropical stratospheric zonal-mean zonal winds to be realistic by nudging toward reanalysis (similar to the nudging methodology used in the SNAPSI experiments, but nudging only in the tropics). The first post-pandemic QBOi workshop, joint with the Gravity Waves and CCMI activities, was held in March 2023 in Oxford, UK. This workshop focused on analysis of the Phase-2 experiments, the ENSO extensions of the Phase-1 experiments, and provided impetus for the emergence of the QUOCA project examining the QBO in ozone and its feedbacks on the dynamical QBO. The March 2023 workshop attracted considerable community interest, with approximately 65 in-person attendees. This number was surpassed by the subsequent QSQ workshop in March 2025 in Cambridge, UK, held jointly with the SNAP and QUOCA projects, which attracted approximately 100 participants, over half of which were early-career researchers (see workshop report in APARC Newsletter 65).

QUOCA Working Group

A working group has been established to study radiative and dynamical feedbacks from stratospheric ozone which impact the QBO. Please find more details here.