Tropical cyclone formations in the South China Sea associated with the strong northeasterly monsoons

Observations show that the percentage of tropical cyclone (TC) that formed in the South China Sea (SCS) in November and December is 16% of the total number of TCs in the SCS. This number is significantly higher than that in the western North Pacific. Therefore, it is worth the effort to examine the TC formation process in the SCS during the late season when the northeasterly monsoon prevails. Lin and Lee (2011) shows that the strong northeasterly jet provides background vorticity favorable for vortex formation but not TC formation. They argues that the weakening of the upstream northeasterlies might be a critical factor for TC formation to occur because it decreases the vertical wind shear and reduces the stabilizing effect associated with the cold and dry air intrusion. In this study, the Weather Research and Forecasting (WRF) model is used to simulate the formation and non-formation cases to address the important processes in this type of TC formation. Sensitivity tests are also performed to help understand the importance of the strength of upstream low-level northereast wind and its changes to TC formation. Two formation (29W and TY Vamei in 2001) and four non-formation (20021230, 20030109, 20051209 and 20051227) cases are simulated. Results of simulations and sensitivicty tests show that for a formation (non-formation) case, an increase (decrease) in the upstream low-level northeasterlies during the later period of the developing process would increase (decrease) the cold advection, leading to the changes of the convective instability. Consequently, the intensity of the model system will be weaker (stronger) or even become a non-formation (formation) case. These results are consistent with those of composites, and support the argument that the change in the upstream low-level northeasterlies is the key factor to dictate TC formation under such environment.