Long-term Statistical Features of Tropical Cyclone Formation Associated with Monsoon- and Easterly-related Environments

Tropical cyclone (TC) formations in different environments have significantly varied features. This study analyzes the statistical features of development from tropical cloud clusters (TCCs) to TCs associated with monsoon- and easterly-related environments over a long term using cluster analysis. Five significant environmental types associated with TC formation are classified in the western North Pacific (accounting for 41% of the total), with three belonging to monsoon-related environments (monsoon types), and two, to easterly-related environments (easterly types). Comparing features between monsoon and easterly types shows that the TCCs associated with easterly-related environments develop into TCs faster than those associated with monsoon-related environments. Moreover, the features of mesoscale convective system (MCS) and angular momentum flux (AMF) associated with TC formation are also analyzed. Results show that for easterly types, the number of MCS is less but the distribution of MCS is more concentrated to the TCC center, and has stronger short-term scale (less than 10 days) inward AMF at mid-level. For monsoon types, although the initial conditions are more favorable, the number of MCS is more but the distribution of MCS is not concentrated to the TCC center, and the significant short-term scale inward AMF develops from the low-level to mid- and high-level. These different features result in different development times of TCCs into TCs between the easterly-type and the monsoon-type TC formation.