The Role of Precursor Disturbances in the Modulation of Western Pacific Tropical Cyclogenesis on Intraseasonal Timescales

Abstract

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