Tropical Cyclone Formations in the South China Sea

In the past 34 years (1972-2005), there were about one thousand tropical cyclones (TCs) formed in the western North Pacific (WNP). During the same period, 131 TCs formed in the South China Sea (SCS) with an annual average of 3.9. Statistics shows that among all SCS TC formations, 18.3% of them occur in the mei-yu season (May-June), while 16% occur in the late season (October–January). These two numbers are significantly larger than those in the WNP indicating that circulation patterns in the SCS during the mei-yu and late seasons are relatively more favorable for TC formations. This study attempts to study TC formation in the SCS during these two seasons, especially those associated with the weak baroclinic environment of mei-yu fronts or the strong northeasterly monsoons.

Results show that the typical mei-yu frontal-type TC formation in the SCS consists of three essential steps. First, an incipient low-level disturbance that originates over land moves eastward along the semi-stationary mei-yu front. Second, the low-level circulation center with the relative vorticity maximum moves to the open ocean with the semi-stationary front. Finally, with strengthened northeasterlies cyclonic shear vorticity continues to increase in the SCS, and after detaching from the stationary front, the system becomes a tropical depression (TD). Results from MM5 simulations of Noguri (2002) and Russ (1994) indicate that the strengthening of the northeasterly flow which is associated with an eastward-moving anticyclone helps concentrate the vorticity and creates strong vorticity patches to the northeast of the low center. The merging of strong vorticity patches in the lower troposphere plays an important role during TC formations and the vorticity maximum become more axis-symmetric in the later stage of formation.

Results also show that the percentage of all incipient lows (which have surface closed isobar for at least 48 hours) that develop to TC intensity is lower during the late season when compared to that associated with the mei-yu front. The composite analyses show that the major difference between the formation and non-formation cases during these two seasons is the northeasterly to the north of the incipient disturbance. In addition, during the late season the non-formation cases are located closer to the Borneo coast when compared to the formation cases thus have higher chance to move inland and experience no further development.