Cooperative effects of QBO and ENSO on controlling the favorableness of the MJO realization

A mechanism for the interannual variability of the Madden–Julian Oscillation (MJO) realization frequency is examined. Based on the number of active days of MJO events detected using the tracking method for the Real-time Multivariate MJO Index, we quantify the year-to-year variability in the initiation and propagation of boreal-winter MJOs. Active years of MJO realization (MJO-A) are characterized by more frequent MJO initiation, leading to complete propagation into the western Pacific (WP), whereas this is less common in inactive years (MJO-IA) due to stronger advective drying and the resultant hindrance of column moistening over the WP. This contrast is linked to differences in boreal-winter mean convection and circulations: MJO-A (MJO-IA) years are characterized by enhanced and suppressed (suppressed and enhanced) convection over the WP/IO and Maritime Continent (MC), respectively. This modulation is driven by the combined effects of the El Niño-Southern Oscillation (ENSO) and the quasi-biennial oscillation (QBO). During moderate-to-strong El Niño events, MJO realization manifests actively regardless of QBO phase or amplitude, unless additional convective suppression occurs in the eastern Indian Ocean and/or MC due to other forcings, such as a positive Indian Ocean Dipole. In contrast, during ENSO-neutral and La Niña conditions, stronger QBO easterly phases tend to favor MJO realization, independent of ENSO. This QBO–MJO connection (except during El Niño conditions) is due to the zonal heterogeneity of QBO impacts; changes in the seasonal mean static stability near the tropopause over the WP modify the mean convective activity in that region. The zonal heterogeneity and ENSO phase-dependency of QBO impacts are interpreted by focusing on the vertical propagation of the Kelvin wave structure over the MC, influenced by both QBO winds and background Walker circulations.