AbstractMadden–Julian oscillations (MJOs) are the dominant mode of intraseasonal variability (ISV) in the atmosphere acting as a bridge between weather and climate. During boreal winter, many MJO events are detoured southward while propagating across the Maritime Continent. Although MJO simulations have been greatly improved in recent years, the mechanism and simulation of MJO detouring near the Maritime Continent are still a great scientific challenge. Several mechanisms have been proposed based on atmospheric dynamics and thermodynamics. In this study, the oceanic role in MJO detouring is diagnosed using observations and reanalysis products. It is found that warm sea surface temperature (SST) anomalies occur over the southeastern Indian Ocean that induce a cyclone in the lower troposphere. Due to the westerly background winds, westerly winds are strengthened (weakened) to the north (south) of warm SST anomalies. As a result, the latent heat flux (LHF) is enhanced, and convection is reinforced to the north of warm SST anomalies. In contrast, the LHF is reduced, and SSTs warm to the south of pre-existing warm SST anomalies. Hence, the warm SST anomalies and convection system shift the MJOs southward before they reach the Maritime Continent. The identification of the oceanic influence on the MJO detouring deepens our understanding of the mechanism of their detour and elicits the role of the ocean. It is expected to brighten the prospects for better simulation and forecast of MJOs over the Maritime Continent. The oceanic ISV in the southeastern Indian Ocean is subject to many forcings, such as intraseasonal atmospheric forcing, the Indonesian Throughflow, local oceanic instability, and coastal Kelvin waves along Sumatra. Determining the mechanism of ISV in the southeastern Indian Ocean requires further dedicated studies.