Modeling the MJO–QBO Relationship in a Reforecast Model

Recent observational results have demonstrated a remarkably robust connection between the stratospheric quasi-biennial oscillation and the intra-seasonal Madden-Julian oscillation. The hallmark of this relationship is a strengthening of MJO convection during the easterly phase of the QBO in boreal winter. In this study, we explore whether the European Centre for Medium Range Weather Forecasting’s state-of-the-art reforecast model is capable of capturing the MJO-QBO relationship. We first diagnose the MJO-QBO link in the model’s standard reforecast runs initialized on January 1, and find that the model reproduces the observed MJO-QBO connection, with stronger MJO events during the easterly QBO compared to the westerly QBO. However, in these model runs the stratospheric influence on the MJO is already present in the model initial conditions, such that it is unclear what role the stratosphere plays in directly modulating MJO convection in the model. To explore this direct stratospheric impact more explicitly, we conduct a series of experiments altering the conditions in the stratosphere to prescribe QBO easterly and westerly states which persist during the model integrations. We then examine how the MJO responds, looking in particular at how changes to the MJO depend on its amplitude and phase, as well as on the strength of the QBO forcing.