Madden Julian Oscillation moves faster as the meridional moisture gradient intensifies in a warming world

Abstract Madden Julian Oscillation (MJO), characterized by slow eastward moving (at an average speed of 5 m s -1 ) largescale cloud bands and associated circulations, is a dominant tropical phenomenon that impacts global weather and climate. Recent studies have reported significant changes in the MJO lifecycle during the post-satellite period (1979 onwards) due to the non-uniform tropical ocean warming. However, the relative roles of natural climate variability and anthropogenic impact in the observed MJO lifecycle change remain unclear and are yet to be determined. In this study, we investigate the observed and projected long-term changes in the MJO lifecycle during the 20 th and 21 st centuries using four 20 th century reanalyses (NCEP-20CRV3, NCEP-20CRV2c, ERA-20C, and CERA-20C) and climate model simulations from the Coupled Model Intercomparison Project phase 6 (CMIP6) under different future emission scenarios. Here, we show for the first time a significant increase in the MJO phase speed by almost 1 m s -1 during the 20 th century (1900–2014), potentially caused by the steepening of the meridional moisture distribution due to the increased moisture in the tropics compared to the extra-tropics. We also observe a prominent multidecadal variability of the MJO phase speed due to the changes in the zonal moisture distribution associated with the Pacific Decadal Oscillation (PDO). Future climate projections under shared socioeconomic pathways (SSPs) from CMIP6 indicate an increase in the MJO phase speed by 0.10 (SSP1—sustainable path, low emission scenario), 0.68 (SSP2—intermediate emissions) and 1.18 (SSP5—high emission scenario) m s -1 during the 21 st century (2015–2100), in response to further steepening of the meridional moisture distribution over the Indo-Pacific warm pool. The increasing MJO phase speed can potentially impact the MJO-related teleconnections and predictability of global weather and climate, and hence need to be monitored closely.