The Madden-Julian Oscillation (MJO) and stratospheric Quasi-Biennial Oscillation (QBO) are two atmospheric phenomena that are potentially the base for prediction beyond two weeks. Observations have shown that the MJO can be modulated by the QBO. Recent studies also suggested that the MJO can be better predicted in easterly phases than westerly phases of the QBO.
Kim et al.  contradict previous results by showing that MJO prediction skills of eight prediction systems do not significantly depend on the QBO when the lead time is greater than 20 days. This controversy touches upon a critical issue of whether and how prediction beyond two weeks—known as subseasonal-to-seasonal (S2S) prediction—should be improved.
This study points out that to adequately address this issue, prediction systems must accurately reproduce the structures and variations of both MJO and QBO, the physical mechanisms for the observed QBO-MJO connection must be well understood (which are not currently), and a large sample size of observations is needed. Another factor to consider is how a prediction system is initialized (using its own data assimilation product or not). Different skill assessment metrics may also bring additional perspectives to the problem.
Citation: Kim, H., Richter, J. H., & Martin, Z. . Insignificant QBO‐MJO prediction skill relationship in the SubX and S2S subseasonal reforecasts. Journal of Geophysical Research: Atmospheres, 124, 12655– 12666. https://doi.org/10.1029/2019JD031416
—Chidong Zhang, Editor, JGR: Atmospheres