Propagation of the Madden-Julian oscillation as a deterministic chaotic phenomenon

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 multiscale processes are poorly understood. Here, we reveal chaotic MJO propagation arising from cross-scale nonlinear interactions, based on 4000-member ensemble simulations of two MJO events with a global cloud-system-resolving model. Against conventional linear thinking, multiple regimes with distinct timings of MJO propagation emerge under a single atmosphere-ocean background. The emergence of regime bifurcation depends critically on the equatorial asymmetry of climatological sea surface temperature. 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. These results contribute to a more complete MJO conceptual model and help foresee when coherent MJO propagation emerges.