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

Observations have shown that circulation patterns in the South China Sea (SCS) are relatively more favorable for tropical cyclone (TC) formations during the mei-yu (May-June) and the late (November-January) seasons compared to those in the western North Pacific (WNP). This study attempts to examine TC formation in the SCS associated with the strong northeasterly monsoons during the late season. Results show that the percentage of all incipient lows (with surface closed isobar lasted for more than 24 hours) that develop to TC intensity (maximum wind speed of 25 kt) is lower during the late season when compared to the typical frontal-type formations associated with the mei-yu front (Lee et al. 2006). But the formation time (from 25 kt to tropical storm intensity) for the late season cases is significantly shorter than that for the mei-yu front cases.

The composite analyses show that the formation cases during the late season have larger low-level vorticity and upper-level divergence compared to the nonformation cases (defined as the cases with surface closed isobar lasted for more than 24 hours but was not classified as tropical cyclones). Another major difference between the formation and the nonformation cases is the low-level northeasterly to the north of the incipient disturbance. The temporary weakening of the northeasterly in formation cases might be a critical factor that prevents the shift of the low-level circulation center of the formation cases from the southern SCS to a location near the Borneo landmass. Furthermore, the decreasing trend in the vertical wind shear for the formation cases appears to be more favorable for development when compared to that of the typical frontal-type formation whose vertical wind shear is 2-4 m s-1 smaller.

The stronger equatorial westerly during the active MJO period would produce stronger cyclonic shear vorticity thus is favorable for triggering more convection activity and more vortex formations. However, more vortices or cloud clusters is not necessarily more favorable for an incipient vortex to organize into a TC. Therefore, the probability for an incipient vortex to become a TC is actually higher during the non-MJO period in the SCS during the late season. Such feature suggests that the TC formation in the SCS during the late season is more like a stochastic process.