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

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, we 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. We find 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. Our results suggest the distinct roles of large-scale environmental variables in different stages of tropical cyclogenesis. The frequency of tropical cyclones (TCs, also called hurricanes and typhoons) varies on many different timescales, including interannual (year to year) and subseasonal (weeks to months) timescales. The study of subseasonal variability in the frequency of TCs is often studied by attempting to understand why existing smaller-scale storms organize into a TC. However, this approach does not consider the possibility that the frequency of the smaller-scale storms may also vary. This study shows that the frequency of the smaller-than-TC storms is modulated on subseasonal timescales by a semi-oscillatory phenomenon in the tropics known as the Madden-Julian oscillation (MJO) in the western Pacific. The MJO does this by changing the amount of rising air and moisture throughout the region. This finding helps us better understand the processes that lead to the formation of a TC. The Madden Julian oscillation (MJO) influences western Pacific tropical cyclogenesis in part through the modulation of the frequency of the tropical cloud clusters The frequency of tropical cloud clusters varies most closely with large-scale ascent and relative humidity anomalies of the MJO The rate of transition from a tropical cloud cluster to a tropical cyclone is most correlated with large-scale vorticity anomalies associated with the MJO