The Impact of the Madden‐Julian Oscillation on the Formation of the Arabian Sea Monsoon Onset Vortex.

During certain years, a synoptic scale vortex called the monsoon onset vortex (MOV) forms within the northward advancing zone of precipitating convection over the Arabian Sea. The MOV does not form each year and the reason is unclear. Since the Madden‐Julian Oscillation (MJO) is known to modulate convection and tropical cyclones in the tropics, we examined its role in the formation of the MOV. While the convective and transition phases of the MJO do not always lead to MOV formation, the suppressed phase of the MJO hinders the formation of the MOV more consistently. This asymmetric relationship between the MJO and MOV can be partially explained by the modulation of the large‐scale environment, measured by a tropical cyclone genesis index. It also suggests that the Arabian Sea is generally near a critical state that is favorable for MOV formation during the monsoon onset period. Plain Language Summary: The monsoon onset vortex (MOV) is a cyclonic vortex, which forms in the Arabian Sea in some years during the onset of the Indian summer monsoon. It often intensifies into a tropical cyclone. The MJO is an eastward‐moving band of clouds and rainfall near the equatorial regions, having a cycle of 30–60 days. The MJO enhances the formation of tropical depressions and tropical cyclones worldwide. This study shows that the wet phase of the MJO is neither a necessary nor a sufficient condition for the MOV to form over the Arabian Sea. Additionally, the peak dry phase of the MJO is least likely to witness the formation of a MOV. Key Points: The monsoon onset vortex (MOV)'s response to the Madden‐Julian Oscillation (MJO) phases is asymmetricA convectively active MJO is neither a necessary nor a sufficient condition for the formation of the MOVThe genesis potential index is a useful metric for studying MOV formation [ABSTRACT FROM AUTHOR]