QBO-MJO interactions in E3SM with enhanced vertical resolution

Observational evidence indicates that the Madden-Julian oscillation (MJO), the leading mode of subseasonal variability in the tropical troposphere, is modulated by the quasi-biennial oscillation (QBO), the leading mode of variability in the tropical stratosphere. Although a consensus understanding of QBO-MJO connection has not yet been achieved, recent studies suggest that QBO phase-dependent changes to lower stratospheric static stability and cloud-radiative feedbacks may be contributing factors to MJO modulation. Other research has demonstrated that, of 30 models participating in the Coupled Model Intercomparison Project 6, none exhibited the correct QBO-MJO relationship. This absence of QBO-MJO connection precludes the use of a potentially valuable source of subseasonal to seasonal prediction of weather patterns and extremes. In this study, we explore the impacts that enhancing lower stratospheric vertical grid resolution has on the QBO and QBO-MJO connection in the Energy Exascale Earth System Model (E3SM). A development configuration of E3SM version 3 with 72 vertical levels produces a reasonable MJO but a very weak QBO amplitude. An 80-level version of the model, with increased vertical resolution imposed only in the lower stratosphere, dramatically improves QBO amplitude. Characteristics of the improved QBO in the modified E3SM are reviewed, QBO-MJO connections are examined, and plausible contributing mechanisms are discussed.