An analysis of tropical cyclone formations in the South China Sea during the late season

This study attempts to examine tropical cyclone (TC) formations in the South China Sea (SCS) associated with the strong northeasterly monsoons during the late season. Results show that the percentage of all incipient lows that develop to TC intensity is lower in the late season when compared to that of the typical frontal-type formations associated with the mei-yu front (Lee et al. 2006). But the averaged formation time for the late season cases is significantly shorter than that for the mei-yu frontal cases. The composite analyses show that the formation cases in the late season have larger low-level vorticity and upper-level divergence compared to the nonformation cases. Another major difference between the formation and the nonformation cases is the low-level northeasterly to the north of the incipient disturbance which shows a weakening right before the pre-TC disturbance reaching 25 kt. The weakening of the northeasterly might be a critical factor for TC formation because it decreases the vertical wind shear and prevents the shift of the low-level circulation center of the pre-TC disturbance from over the southern SCS to near the Borneo landmass. Furthermore it could reduce the stabilizing effect associated with the cold and dry air intrusion. Previous studies have shown that the stronger equatorial westerly during the active Madden-Julian oscillation (MJO) period would produce stronger cyclonic shear vorticity thus is favorable for triggering more convection activities and more incipient vortex formations. However, more vortices or cloud clusters are not necessarily more favorable for an incipient vortex to organize into a TC. Nonetheless the setup of favorable synoptic environment appears to be important for the further development of the incipient vortex.