During some 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 non-linear 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.

We examined the Cyclone Global Navigation Satellite System (CYGNSS) retrievals of surface winds and enthalpy fluxes in African easterly waves that led to the formation of 31 Atlantic tropical cyclones from 2018–2021. Lag composites show a cyclonic proto-vortex as early as 3 days prior to tropical cyclogenesis. The distribution of enthalpy fluxes within the proto-vortex does not vary substantially prior to cyclogenesis, but subsequently, there is an increase in the upper extreme values. A negative radial gradient of enthalpy fluxes becomes apparent as early as 2 days before cyclogenesis. These results—based on a novel data blending satellite retrievals and global reanalysis—are consistent with recent studies that have found that tropical cyclone spin-up is associated with a shift of peak convection towards the vortex-core and a radially inward increase of enthalpy fluxes. They provide additional evidence for the importance of surface enthalpy fluxes and their radial structure for tropical cyclogenesis.