Madden Julian Oscillation Moves Faster as the Meridional Moisture Gradient Intensifies in a Warming World

The eastward-moving large-scale convective system associated with the Madden-Julian Oscillation (MJO) significantly impact global weather and climate. Recent decades have seen notable changes in the MJO's lifecycle due to non-uniform tropical ocean warming, with the roles of natural climate variability and anthropogenic influence still requiring quantification. This study examines observed and projected long-term changes in the MJO phase speed using four twentieth-century reanalyses and CMIP6 simulations. We find a substantial increase in MJO phase speed in three reanalyses during the twentieth century (0.6–1.2 m s⁻¹ century⁻¹) and further increase in MJO phase speed during the twenty-first century (0.3–1.5 m s⁻¹ century⁻¹), with notable multidecadal fluctuations. We attribute the overall acceleration of the MJO to the global warming-driven increase in the meridional moisture gradient around the warm pool while attributing the multidecadal variability in the MJO phase speed to changes in the zonal moisture gradient associated with the Pacific Decadal Oscillation.