Changes in MJO Propagation Speed under Global Warming

The Madden–Julian Oscillation (MJO) has widespread impacts on global weather and climate. The global warming could induce changes in MJO and how it interacts with global weather and climate. It has been found that MJO exhibits increased eastward propagation speed under warming, yet there is no universally accepted mechanism. This study examines the impact of global warming on MJO’s propagating speed via 15 selected CMIP6 models, and explores the mechanism in a synthetical way by considering the changes in MJO’s circulation structure and background states. The results show that the MJO propagation speed increases by 19.6% at the end of the 21st century. Corresponding to the increased phase speed, MJO exhibits larger zonal scales, stronger Kelvin wave response, and more backward tilted vertical structure. Moisture budget analysis indicates that the global warming can accelerate MJO propagation via two pathways. First, global warming leads to amplification of meridional moisture gradient over the southern Maritime Continent, resulting in enhanced horizontal moisture advection that accelerates the propagation of MJO. Second, the eastward expansion of the Indo-Pacific warm pool under global warming enlarges the MJO zonal scale and strengthens the Kelvin-wave response to the east of the MJO convection center, which enhances pre-moistening of the lower troposphere though amplifying boundary-layer moisture convergence, leading to a more tilted vertical structure that favors faster eastward propagation of the MJO. The results of this work have a clear implication for understanding the dynamics of MJO propagation.