Deterministic chaotic behavior in the propagation of the Madden-Julian oscillation

The Madden–Julian oscillation (MJO) is a planetary-scale tropical weather disturbance marked by eastward propagating cumulus cloud clusters over the Indo-Pacific region, causing severe weather and climate events worldwide. The mechanism and predictability of MJO propagation remain elusive, partly because relevant multi-scale processes are poorly understood. Here, we reveal chaotic MJO propagation arising from cross-scale nonlinear interactions, based on 4,000-member ensemble simulations of two MJO events in November and December, 2018, using the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) at 14-km horizontal resolutions. Against conventional linear MJO theories, multiple regimes with distinct timings of MJO propagation emerge under a single atmosphere-ocean background in December. The emergence of regime bifurcation depends critically on the equatorial asymmetry of climatological sea surface temperature, mainly regulated by the seasonal march. Selection of the bifurcated regimes is probabilistic, influenced by whether tropical-extratropical interplay promotes moistening associated with westward-propagating tropical waves over the western Pacific. Specifically, this regime distinction is rooted in differences in MJO-related upper-tropospheric westerly strengths over the western Pacific when MJO convection is located in the Indian Ocean, affecting the degree of the extratropical Rossby-wave refraction that can interefere with the tropical waves. These results contribute to a more complete MJO conceptual model and help foresee when coherent MJO propagation emerges.