An Analysis of Tropical Cyclone Formations in the South China Sea during the Late Season

Abstract

This study examines tropical cyclone (TC) formations in the South China Sea (SCS) associated with the strong northeasterly monsoon during the late season. Results show that the percentage of all vortices that develop to TC intensity is lower in the late season when compared to that associated with the mei-yu front in May and June. But the average formation time for the late-season cases is significantly shorter than that for the mei-yu frontal cases. Composite analyses show that the formation cases in the late season have larger low-level vorticity and upper-level divergence as well as higher 700-hPa relative humidity when compared to the nonformation cases. Another major difference between the formation and the nonformation cases is the low-level northeasterlies to the north (or upstream) of the incipient disturbance, which weakens right before the pre-TC disturbance reaching 25 kt (~13 m s−1). The weakening of the northeasterlies may be important to TC formation because it decreases the vertical wind shear and prevents the shift of the disturbance center from over the southern SCS to near the Borneo landmass. Furthermore, it reduces the stabilizing effect associated with the cold- and dry-air intrusion. Previous studies have shown that stronger equatorial westerlies during the active Madden–Julian oscillation (MJO) period may produce stronger cyclonic shear vorticity; thus, favorable for triggering more convection activities and more vortex formations. However, more vortices or cloud clusters are not necessarily more favorable for an individual vortex to organize into a TC. Nonetheless the initial setup of favorable synoptic environment appears to be important for the further development of the incipient vortex.