Since the MJO is the anomalous disturbances upon the basic state, the mean state in the Indo-Pacific warm pool is important to propagation and maintenance of the MJO.

Previous studies attempted to understand the interannual variability of the MJO focused on the influence of the El Niño–Southern Oscillation (ENSO, Gualdi et al. 1999; Hendon et al. 1999; Slingo et al. 1999; Waliser et al. 2002; Pohl and Matthews 2007; Tang and Yu 2008) and that of the Indian Ocean Dipole (IOD, Benedict et al. 2015; Wilson et al. 2013). Recent studies also reported a notable relationship of seasonal MJO activity with the Quasi-Biennial Oscillation (QBO, Son et al. 2017; Yoo and Son 2016; Zhang and Zhang 2018; Nishimoto and Yoden 2017; Hendon and Abhik 2018). Their results showed a marked change in the MJO activity, however, the mechanism behind the QBO-MJO relationship has remained elusive.