As a dominant mode of tropical intraseasonal variability, the Madden-Julian Oscillation (MJO) significantly influences global weather and climate. MJO initiation and propagation display evident interannual variations, which have been mostly attributed to modulation effects of El Nino-Southern Oscillation (ENSO) in previous studies. Nevertheless, whether the Indian Ocean (IO) warming independently of ENSO modulates the MJO is seldom examined. Here, we exclude boreal summer seasons of ENSO but consider those when basin-scale warming phenomena occur over the Indian Ocean (IOBM hereafter). We find that the MJO deep convection tends to initiate more westward and only propagate confined over the IO under warm IOBM conditions. However, under cold IOBM conditions, the MJO usually starts from the central-eastern IO and can propagate smoothly across the Maritime Continent (MC) and re-develop over the western Pacific (WP). We try to understand mechanisms by first examining background mean states. Compared to the cold IOBM, the warm IOBM is featured by a westward shift of warm waters (29℃) over the IO, thus causing more westward-initiated MJO. Additionally, the warm IOBM induces anomalous anticyclone circulation over the north WP, which inhibits MJO eastward propagation. Secondly, under warm IOBM conditions, the anomalous suppressed convection and thus low-level easterly wind anomalies over MC-WP regions leading the MJO deep convection become weaker. Consequently, the premoistening from high-frequency synoptic-scale meridional advection is reduced, leading to a nonpropagating MJO. We also perform CESM model experiments to validate these observational findings. Our results emphasize that one could attribute interannual variations of MJO by considering independent modulation effects of the IO. One may further use mechanisms refined here to focus on decadal IO warming to fully understand how global warming could affect MJO features.

Madden-Julian Oscillation,Indian Ocean basin-wide SST,CESM,moisture,convection