Influence of quasi‐biennial oscillation on the boreal winter extratropical stratosphere in QBOi experiments

Abstract This study investigates the Holton–Tan (HT) relationship based on re‐analyses and co‐ordinated experiments for present‐day and warming scenarios within the SPARC Quasi‐Biennial Oscillation initiative (QBOi) using Meteorological Research Institute Earth System Model version 2.0 (MRI‐ESM2.0). The aim is to explore to what extent global warming modulates the HT mechanism in the effect of the QBO on the polar vortex. The simulations of the present‐day conditions reproduce the zonal wind composite differences in the re‐analysis; however, there are some discrepancies. In regions near the QBO (i.e. 20–40°N), there is good agreement between the simulations and the re‐analysis. Examples include the midlatitude (∼30°N) lower (middle) stratosphere Eliassen–Palm (EP) flux convergence (divergence) anomalies between the easterly phase and the westerly phase of the QBO, which are common in the re‐analysis and in the present‐day and warming simulations. The influence of the QBO on the midlatitude lower stratosphere is associated with an effective waveguide for enhanced upward‐propagating Rossby waves near the subtropical jet. At high latitudes, the significant (less significant) convergence anomalies of significant horizontal EP fluxes in the middle stratosphere are dominant in the re‐analysis (the present‐day simulations). In the warming scenarios, the QBO signatures of the zonal wind are strengthened with the significant EP flux convergence anomalies of less significant upward‐propagating waves inside the polar vortex above 20 hPa. These results suggest that fundamental mechanisms behind the HT effect in regions near the QBO can be explained by enhanced upward‐propagating waves associated with an effective waveguide (by inhibiting equatorward‐refracting upward waves) in the lower‐to‐mid stratosphere. In the warming scenarios, however, HT mechanisms, especially inside the polar vortex, may have another pathway, different from that of the present‐day climate.