The Role of Precursor Disturbances on the Modulation of Western Pacific Tropical Cyclogenesis by the Madden-Julian Oscillation

The present study considers tropical cyclogenesis as a multi-stage process in which pre-cursor disturbances develop first and a fraction of them further strengthens to become a tropical cyclone (TC). Using this framework, we analyze the impact of Madden-Julian oscillation (MJO) associated large-scale environmental conditions on i) the triggering of tropical convective clusters (TCCs) – a type of pre-cursor disturbances – and ii) the TCC-to-TC transition in the western Pacific. We find that, within the MJO’s lifecycle, the modulation of the TCC frequency by the MJO drives TC genesis anomalies earlier than the TCC-to-TC transition rate. Also, the fluctuation of the TCC frequency is most strongly associated with the MJO's large scale ascent and relative humidity assomalies, while that of the transition of TCCs to a TC is mainly associated with the MJO’s vorticity anomalies. Our results suggest distinct roles of large-scale environmental variables on different stages of tropical cyclogenesis.