The Response of the QBO to External Forcings: Implications for Disruption Events

Abstract The response of the Quasi‐Biennial Oscillation to changing concentrations of anthropogenic greenhouse gases, aerosols, and ozone, and also to volcanic eruptions and solar variability, is explored using 65,000 years of model output contributed by four modeling centers to the Large Ensemble Single Forcing Model Intercomparison Project (LESFMIP). The large ensemble size (at least 10, and in many cases 50) allows for the isolation of weak signals that are usually hidden by internal variability. Increasing greenhouse gas concentrations lead to weakening of the Quasi‐biennial Oscillation (QBO) and an increased likelihood of a disruption event, with the effect most pronounced in the lower stratosphere. Increasing aerosols lead to a strengthening of the QBO. Explosive volcanic eruptions lead to a weakening, and for some phases also a stalling, of the QBO. Volcanic eruptions can also help trigger a QBO disruption. The ozone forcing used for LESFMIP helps synchronize the QBO phase across ensemble members, and also increases the strength of the QBO. Finally, the solar forcing has the smallest impact on the QBO of the five forcings. The large ensemble sizes also allow for exploring the QBO phases and the time of year most prone to a QBO disruption: disruptions preferentially occur around 8 months after the phase with easterlies near 50 hPa, and are most common in late boreal winter and early spring.