Constraining the circulation in general circulation models by nudging the model variables toward reanalysis provides a rather powerful tool for investigating the sensitivities of predictions and simulations to specific processes or phenomena and enables the impact of model biases to be better quantified. A shared methodology for applying nudging to stratospheric variables has been developed by the World Climate Research Programme (WCRP) Stratosphere–troposphere Process And their Role in Climate (SPARC) Quasi-Biennial Oscillation initiative (QBOi) and Stratospheric Nudging And Predictable Surface Impacts (SNAPSI) activities. This methodology involves nudging (relaxing) only the zonal mean, while atmospheric waves are allowed to evolve freely, though publications that adopt other nudging methodologies or that explore sensitivity to the nudging methodology are encouraged to submit. For SNAPSI the nudging is applied throughout the stratosphere, whereas QBOi applies nudging to a localized region of the equatorial stratosphere. The reliability and robustness of the conclusions obtained from this new approach are further enhanced by the use of coordinated multi-model experiments.

The purpose of the special issue is to provide a common location for reporting results from the SNAPSI and QBOi coordinated experiments utilizing stratospheric nudging, along with results from other related studies that nudge the atmosphere, whether nudging just the zonal mean or the full field. A number of modelling centres have now carried out the SNAPSI and QBOi experiments. The special issue would collect papers analysing these datasets that have been produced by participating modelling centres.

The SNAPSI experiments are designed to study the role of sudden stratospheric warmings (SSWs) in surface predictability; they are hindcast-type experiments, specifying 45-day runs covering three recent SSW events, with a large ensemble size of 50–100 members for each case. Analysis topics include the role of stratospheric variability in surface extremes and predictability and stratosphere–troposphere coupling mechanisms. The QBOi experiments are designed to study the impact of biases in the quasi-biennial oscillation (QBO) on QBO teleconnections and the forcing of the QBO; they are climate-type experiments, specifying 40-year runs with one to three ensemble members. Analysis topics include the QBO teleconnections with the stratospheric polar vortex and the relationship between the QBO and Madden–Julian oscillation (MJO). Analysis of both sets of coordinated experiments, SNAPSI and QBOi, is currently in progress, with papers expected to be submitted in the coming 1–2 years.