MJO-equatorial Rossby wave interferences in the tropical intraseasonal oscillation

A better understanding and simulation of the tropical intraseasonal oscillation (ISO) is the cornerstone of subseasonal-to-seasonal predictions. Here, we have revealed crucial roles of interference effects between Madden–Julian Oscillation (MJO) and low-frequency equatorial Rossby (ER) waves on the intensity, structure and initiation of the ISO. Over where ER waves are sufficiently strong, the ISO convection usually manifests localized strengthening or weakening due to constructive or destructive MJO-ER interferences. For the ISO interannual variability, though the strength is determined by the MJO, the area is largely controlled by ER waves. The Maritime Continent MJO (ER) varies synchronously (asynchronously) with El Niño-Southern Oscillation likely controlled by the meridional mean moisture gradient. Additionally, separating MJO and ER components helps explain the northwest-southeast tilted structure of boreal summer ISOs, which dramatically arises from decreased phase speeds of the MJO away from the equator. Moreover, the considerable damping of ISO deep convection over the Maritime Continent partly arises from the further weakened MJO aloft due to the dry intrusion of ER waves triggered from the central-eastern Pacific. Finally, a primary ISO event with jumping-like propagation behaviors is likely initiated over the Indian Ocean with the strengthening of preceding ER waves, implying a novel “initiation-propagation” linkage of the ISO. These ER signals may be detected approximately 20 days ahead from the western Pacific and are thus potentially useful for monitoring and predicting ISO initiation early. Taking together, the findings here highlight the importance of ER waves in understanding the dynamics and predictability of the ISO/MJO.